HPAVC/HPAVC CD-ROM.iso/pc/FAQSYS18.ZIP/FAQS.DAT/RAYSTUFF.TXT

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            "Light Makes Right"

              October 1, 1990
            Volume 3, Number 4

Compiled by Eric Haines, 3D/Eye Inc, 2359 Triphammer Rd, Ithaca, NY 14850
    wrath.cs.cornell.edu!eye!erich or [e-mail]
All contents are US copyright (c) 1990 by the individual authors
Archive locations: anonymous FTP at cs.uoregon.edu [128.223.4.13], /pub/RTNews,
           weedeater.math.yale.edu [130.132.23.17], /pub/RTNews, and
           freedom.graphics.cornell.edu [128.84.247.85], /pub/RTNews.
UUCP access: Vol 3, No 1 or write Kory Hamzeh (quad.com!avatar!kory) for info.

Contents:
    Introduction
    New People, Address Changes, etc
    Photorealism, and the color Pink (TM), from Andrew Glassner
    DKBTrace v2.0 and Ray Tracing BBS Announcement, by David Buck, Aaron Collins
    Article Summaries from Eurographics Workshop, by Pete Shirley
    Convex Polyhedron Intersection via Kay & Kajiya, by Eric Haines
    New Radiosity Bibliography Available, by Eric Haines
    A Suggestion for Speeding Up Shadow Testing Using Voxels, by Andrew Pearce
    Real3d, passed on by Juhana Kouhia, "Zap" Andersson
    Utah Raster Toolkit Patch, by Spencer Thomas
    NFF Shell Database, by Thierry Leconte
    FTP list update and New Software, by Eric Haines, George Kyriazis
    ======== USENET cullings follow ========
    Humorous Anecdotes, by J. Eric Townsend, Greg Richter, Michael McCool,
    Eric Haines
    Graphics Interface '91 CFP
    Parametric Surface Reference, by Spencer Thomas
    Solid Light Sources Reference, by Steve Hollasch, Philip Schneider
    Graphics Gems Source Code Available, by Andrew Glassner, David Hook
    Graphics Gems Volume 2 CFP, by Sari Kalin
    Foley, Van Dam, Feiner and Hughes "Computer Graphics" Bug Reports,
    by Steve Feiner
    Radiosity via Ray Tracing, by Pete Shirley
    Algorithm Order Discussion, by Masataka Ohta, Pete Shirley
    Point in Polygon, One More Time..., by Mark Vandewettering, Eric Haines,
    Edward John Kalenda, Richard Parent, Sam Uselton, "Zap" Andersson,
    and Bruce Holloway
    Quadrant Counting Polygon In/Out Testing, by Craig Kolb, Ken McElvain
    Computer Graphics Journals, by Juhana Kouhia

-------------------------------------------------------------------------------

Introduction

Well, SIGGRAPH has come and gone, and as usual I ran out of time.  There's
just too many people to meet and too much to see and do.  Highlights for me
included the SIGGRAPH Trivia Bowl, meeting in a cemetery for lunch with other
renderers after an all night session of Hurtenflurst (don't ask), and getting
my head turned into data at Failure Analysis Associates (still haven't heard
from them, though.  Does anyone have the phone number or address for this
company?  I'd like to call and find out what is happening with them).  At
SIGBowl our Ithaca team was able to lock in third place [of 3 teams] early on
and held it against all comers.  Nonetheless, a great time:  one of many
favorite moments was:

    Pike (the moderator): "Who's biographical sketch reads as follows-"

        BUZZ!

    Team 1 (with no other information): "Turner Whitted!?"

    Pike: "No, wrong.  Now let me read the question" and proceeds to read
        some biographical data.

        BUZZ!

    Team 2: "Is it Turner Whitted!?"

    Pike: "Still wrong."

Team 3, sadly, did not guess "Turner Whitted?", but, rather, declined to answer.

--------

Does anyone have a nice, compact version of the Teapot?  That is, just the
spline description (which I do have) and a tessellator which will turn it into
quads/triangles (which I have, but it's deeply buried in some scary code)?
This would be a nice addition to SPD 3.0, but I don't have one easily
accessible.  Here's your chance to get your code distributed in the next
release.  I'm willing to massage your code into outputting NFF format.

I'll also announce that there is a new version of the ray tracing bibliography
which I've finished as of today.  Many of the new articles listed are due to
the untiring work of Tom Wilson, who says the librarians now hate him.  This
new version should be available via FTP by October 5th on
freedom.graphics.cornell.edu and cs.uoregon.edu (see the FTP site list later
in this issue for more information).

Important dates:
    Graphics Interface papers due October 31st
    Graphics Gems, Volume 2 submissions due November 1st

-------------------------------------------------------------------------------

New People, Address Changes, etc


# Tom Wilson - parallel ray tracing, spatial subdivision
# Center for Parallel Computation
# Department of Computer Science (407-275-2341)
# University of Central Florida
# P.O. Box 25000
# Orlando, FL 32816-0362
alias    tom_wilson    [e-mail]

I am a Ph.D. student at UCF.  I am working in the area of parallel processing
with ray tracing as an application.  I am working on a nonuniform spatial
subdivision scheme that will eventually be implemented on our BBN Butterfly
GP1000.

--------

# Steve Hollasch - efficiency, algorithms, ray-tracing
# Master's Student in C.S., Arizona State University
# 1437 West 6th Street
# Tempe, AZ  85281-3205
# [phone]
alias   steve_hollasch   [e-mail]

    I'm working on completing my thesis in December 1990 or January 1991.  The
research basically covers viewing 4D objects (as physical entities, not 3D
scalar fields).  I'm working on a 4D raytracer right now that handles
hyperspheres and tetrahedrons, and I hope to extend it to hyperellipsoids,
hypercones, hypercylinders and so on.  I'd also like to learn how to tessellate
4D solids with tetrahedrons to be able to view arbitrary 4D solids.  Practical
uses?  I'm still trying to think off something...  =^)

--------

# Christophe Vedel - improving radiosity
# LIENS, Ecole Normale Superieure
# 45, rue d'Ulm
# 75230 PARIS Cedex 05 FRANCE
# (33) 1 43 26 59 74
e-mail  [e-mail]
    [e-mail]

I'm interested in improving the radiosity method for finer results or larger
scenes.  Spatial subdivision techniques developed for ray tracing should help
to achieve this goal.  I'm also working on interactivity with lighting
simulation programs.

--------

# Frederic Asensio - behavioral animation, radiosity
# LIENS, Ecole Normale Superieure
# 45, rue d'Ulm
# 75230 PARIS Cedex 05 FRANCE
# (33) 1 43 26 59 74
e-mail  [e-mail]
    [e-mail]

I am interested in interactive radiosity methods, which could be used in the
design process and in stochastic methods for light sampling with rays.

--------

# Scott Senften - efficiency, scientific visualization
# McDonnell Douglas Corp.
# McDonnell Aircraft Co.
# Mail Code 1065485
# St. Louis, MO 63166
# [phone]
alias scott_senften      [e-mail]
alias senften_tu     [e-mail]

Presently I'm doing research in neural nets.  There is a real need in the NN
society for some good scientific visualization.  I'm presently doing work in
speech recognition, diagnostics, flight reconfiguration, and signal
processing, as well as graphic visualization of neural networks.  My graduate
work was in Ray Tracing efficiency.

--------

Sam Uselton

Internet:
[e-mail]

    Work             Home
Phone:
[phone]               [phone]

USMail:
Samuel P. Uselton, PhD           Sam Uselton
Computer Sciences Corp.
M/S T 045-1               1216 Austin St.
NASA Ames Research Center      Fremont, CA 94539
Moffett Field, CA 94035


Ray Tracing interests:
efficiency, accuracy, distributed ray tracing, parallel implementations


At NASA I'm working on volume visualization for computational fluid dynamics.
I've used this as an excuse ( :-) ) to write a ray caster (no bounces) that
integrates through the volume, accelerated by taking advantage of the special
grids, as well as an item buffer-like initial hit finder.

I'm also working with Redner & Lee on extending our distributed ray tracing
work to handle diffuse illumination, caustics, wavelength dependent
refraction, and other realistic effects.

Once these problems are completely solved ( :-) :-) ), there are several
parallel machines here that I would like to wring out.

Oh yes, we (Lee, Steve Stepoway, Scott Senften and myself) have yet another
Acceleration technique we like better than any of the popular ones.  (Actually
it can combine with bounding boxes.)  It is somewhere between regular space
subdivision (a la medical volume folks and Japanese SEADS..)  and the bounding
slabs idea (Kaplan?  [no, Kay]).  It was written and submitted for pub once,
but we keep thinking of new tweaks, so it is being re-written.

--------

# Juhana Kouhia
# Ylioppilaankatu 10 C 15
# 33720 Tampere
# Finland
alias    juhana_kouhia    [e-mail]

I have done my wireframe, scanline and ray tracing programs with Jim Blinn's
modeling language.

[Another renderer with a last name starting with K!  There's Kajiya, Kaplan,
Kay, Kirk, Kolb, Kuchkuda, Kyriazis, just to mention a few. --EAH]

--------

# Michael L. Kaufman  - speed, ray tracing on a PC, ray tracing fractal images
# Northwestern University
# 2247 Ridge Ave #2k
# Evanston IL, 60201
# (708) 864- 7916
# [e-mail]

I am just getting started in Ray-Tracing.  I am interested in coming up with
algorithms that are fast enough to do on a 386 PC.  I am also interested in
using Ray-Tracing techniques as a new way of looking at certain types of
fractal images.

--------

Stephen Boyd Gowing - light transmission, motion, art
B.App.Sc (Computing) Undergraduate.
University of Technology, Sydney

4/10 Short St, Glebe 2037 AUSTRALIA (home)

alias    stephen_gowing    [e-mail]

I'm currently half way through a project to study the effect of light passing
through sphere(s).  The eventual aim is to produce a short animation showing a
single sphere inverting an image which breaks (unmerges?)  into two spheres
one behind the other and study the "flow" of the image through them.  At the
moment I am still rewriting the tracer I used for my 2nd-last graphics
assignment last year.  I'd like to do it in C++ but this would make things
awkward as we don't have access to a compiler.  I'd also like to implement
some sort of shading language into the image description - probably using
something like forth.

--------

# Rick Brownrigg  --  efficiency, texturing, visualization, DTM displays
# Kansas Geological Survey
# University of Kansas
# 1930 Constant Avenue
# Lawrence, KS  USA  66046
# 913 864 4991

-------------------------------------------------------------------------------

Mail, from Andrew Woo, George Kyriazis, Zap Andersson


from Andrew Woo:

I have gone through many changes in the last year, from graduating with a
M.Sc. at University of Toronto, to working at SAS Institute, to working at
Alias Research - which is my current working address.  My e-mail at Alias is
[e-mail].

I checked out your latest RT News.  I think David Jevans was a little too hard
on you about not knowing about the ray id for voxel traversal.  In fact, in GI
90, one of the authors was unaware of it, too (in the "Approximate Ray
Tracing" paper).  So you are not alone.  In fact, there is an explicit
mentioning of ray id in an early paper that David did not mention:

    John Amanatides, "A Fast Voxel Traversal Algorithm for Ray Tracing",
    EuroGraphics, 1987, pp. 1-10.

On the claim that David Jevans made about wasting time on more intersection
culling schemes, I tend to agree.  In fact, I have stopped looking for more
clever ways to cull intersections.  But I intend to check out another route to
ray tracing savings.  Once I get some promising results, I will share the idea
with you (to avoid any embarrassments in case the idea backfires).

On renaming DIstributed Ray Tracing, I am more of a fan of coming up with
amusing acronyms for the algorithms.  For Cook's, I thought DiRT might be
interesting.  I also came up with an acronym for my paper in GI 90 - Voxel
Occlusion Method as an Intersection-culling Technique, which spells VOMIT.

--------

from George Kyriazis:

I was reading the last RTN issue I received.  In there it states about a
program that converts sculpt4d to NFF, and it also commented that Sculpt4D is
an exceptional modeler.  Well, I have my doubts.  Last spring I was working on
a program to raytrace Scultpt4D files and animations on a Pixel Machine.
Well, the file format is far from being strait-forward.  We had to call Byte
by Byte for a few questions and stayed on the phone for more than an hour.
One of the Sculpt4D unwanted 'features' was that it normalized the intensity
of all displayed pixels wrt the brightest.  The result on the Pixel Machine
was disastrous!  Images were either overexposed or totally black!  :-) Well,
this is one of the problems faced, but I don't want to list more...

-------------------------------------------------------------------------------

Photorealism, and the color Pink (TM), from Andrew Glassner

From a Tektronix ad in this month's Advanced Imaging, we learn that it is now
possible to turn real photographs into photorealistic images (presumably
thereby increasing realism):

"Traditional workstations can't handle reality.  They create a simulated
world...  With Dynamic Visualization, reality and simulation are brought
together by TekImaging (TM), powerful image-processing software...  [and now,
the big one -ag] TekImaging lets you transform real-world images into
photo-realistic scenes, complete with light, shadow, and texture."

So now I can take a picture of myself, and using their software make it look
photo-realistic and indistinguishable from a real picture - what a
breakthrough!

--------

Beware of the colors you pick for your images.  Here's a footnote in an
Owens-Corning Fiberglas ad in this month's [August?] Popular Science (pg 21):

"* The color Pink is a trademark of Owens-Corning Fiberglas Corp."

I think I'll get a trademark on red before all the good colors are taken...

-------------------------------------------------------------------------------

DKBTrace v2.0 and Ray Tracing BBS Announcement, by David Buck, Aaron Collins

I recently completed version 2.0 of my freely distributable raytracer called
DKBTrace.  This raytracer is a project I've been working on (actually, off and
on) for the past five years.  The raytracer features:

   - primitives for spheres, planes, triangles, and quadrics
   - constructive solid geometry
   - solid texturing using a Perlin-style noise function
   - image mapping (IFF and GIF formats)
   - hierarchical bounding boxes
   - transparency
   - fog
   - 24 bit output files
   - adaptive anti-aliasing using jittered supersampling
   - "quick" previewing options

The raytracer was developed on an Amiga computer, but it is easily portable to
other platforms.  The interface to the raytracer raytracer is a scripting
language (sorry, no GUI).  The language is quite readable compared to many
other input file formats.

The source and Amiga executables are available by anonymous FTP from
alfred.ccs.carleton.ca [134.117.1.1].  The source and executables are freely
distributable.

As with any raytracer, there are still things that need to be done, added, or
improved with DKBTrace.  The image mapping was added at the last minute, so it
only uses a simple projection scheme.  This means that the mappings look
bizarre when viewed from the sides or the back.  For the most part, however,
the solid textures are quite powerful and can create some very interesting
effects.

To implement transparency, I decided to add the transparency value value to
the color structure.  A color contains red, green, blue, and alpha components.
This makes it much easier to use in textures.

The textures also allow you to define your own color map and to have the
colors automatically interpolated from one range to the next.  Interpolating
the colours makes the images look much better than just a simple step
function.

Questions and comments concerning the raytracer can be directed to
[e-mail] on BITNET.  The IBM port (and several enhancements)
were done by Aaron Collins.  Aaron can be reached on Compuserve.

--------

DKBtrace dox info:

Aaron Collins can be reached on the following BBS'es

Lattice BBS              [phone]
The Information Exchange BBS  [phone]
Stillwaters BBS              [phone]

AAC:  As of July of 1990, there will be a Ray-Trace specific BBS in the (708)
Area Code (Chicago suburbia) for all you Traceaholics out there.  The phone
number of this new BBS is [phone].  I will be Co-Sysop of that board.
There is also a new Ray-Trace and Computer-Generated Art specific SIG on
Compuserve, GO COMART.  And now, back to the DOCS...

-------------------------------------------------------------------------------

Article Summaries from Eurographics Workshop, by Pete Shirley
    ([e-mail])

[What follows are summaries of ray tracing related articles given during the
Eurographics Workshop on Photosimulation, Realism and Physics in Computer
Graphics in Rennes, France, June 1990. - EAH]

INCREMENTAL RAY TRACING by Murakami and Hirota (Fujitsu)
A extension of Parameterized ray tracing that allowed moving some scene
objects in addition to changing material properties.  Used tables of
changed voxels to determine if a ray's interaction with geometry has
changed.  Included implementation on multiple CPUs.  Also includes
reference to a paper in Japanese by Matsumoto on Octree ray tracing in
1983!

PARAMETRIC SURFACES AND RAY TRACING by Luc Biard (IMAG, France)
Like most parametric patch papers, this one went over my head, but
it seems to be an interesting paper, and includes some implementation results.

A PROGRESSIVE RAY TRACING BASED RADIOSITY WITH GENERAL REFLECTANCE FUNCTIONS
by Le Saec and Schlick (France)
A proposal for general BRDF radiosity (very similar to what I propose in the
paper above).  Also includes method for interactive display - display only
non-mirrors until viewer stops, then ray trace (UNC style).

LIGHT SOURCES IN A RAY TRACING ENVIRONMENT by Roelens et al. (France)
A method for showing 'cone of light' effect when there is not very
dense participating media in a room (primary scattering only).

METHODS FOR EFFICIENT SAMPLING OF ARBITRARILY DISTRIBUTED VOLUME DENSITIES
by Hass and Sakas (FRG)
Methods of sampling a volume density along a ray.

-------------------------------------------------------------------------------

Convex Polyhedron Intersection via Kay & Kajiya, by Eric Haines

I like Kay and Kajiya's bounding slabs method [SIGGRAPH 86] a lot.  By
thinking about how this algorithm actually works, we can derive a method for
intersecting convex polyhedra.  A working definition of a convex polyhedron is
a polyhedron which can have at most two intersection points for any ray.

To review:
Their idea is to form a bounding volume around an object by making slabs.  A
slab is the volume between two parallel planes.  The intersection of all slabs
is the bounding volume (i.e. the volume which is inside all slabs).  The
intersection method works by intersecting each slab and checking out
intersection conditions for it alone, then comparing these results to the
answer for previous slabs.  That is, first we get the near and far
intersection points for the ray with the slab.  If the far hit is behind the
ray's origin (i.e. negative distance) or the near hit is beyond the maximum
distance (i.e. you might limit how far the ray can travel by keeping track of
the closest object hit so far, or the distance to the light if this is a
shadow ray), then the ray cannot hit the bounding volume.

If the near and far hits are within bounds, then we essentially do a set
operation of this line segment and any previous slab line segments formed.
For example, if this is the second slab tested, we try to find the overlap of
the first slab's hits and this slab's hits.  If the two segments do not
overlap, then there is no point along the ray that is inside both slabs.  In
other words, there is no point inside the bounding volume, since this volume
is the logical intersection of all slabs.  To get the logical intersection of
the slab segments, we merely store the largest of the near hits and the
smallest of the far hits.  If this largest near hit is greater than this
smallest far hit, there was no overlap between segments, so the bounding
volume is missed.  If near is less than far, this new high-near, low-far
segment is used against successive slab segments.  If the ray survives all
segment tests, then the resulting near and far values are the distance along
the ray of the near and far hits on the bounding volume itself.

What's interesting about this test is that it is essentially doing
Constructive Solid Geometry operations between slabs, similar to [Roth 82].
This simple idea can be extended further, allowing you to quickly intersect
convex polyhedra.  Another good feature is that the polyhedra can be defined
entirely by the plane equations for the faces, so no vertices have to be
stored.

Imagine that you have defined a convex polyhedron by specifying each face
using a plane equation, Ax + By + Cz + D = 0.  Have each face's normal {A,B,C}
point outwards.

To intersect this object with a ray, simply test the ray against each face:
if you hit a plane that faces towards the ray (i.e. the dot product of the
normal and the ray direction is negative), then use this as a hit for near;
if you hit a plane facing away, then use this as a hit for far.   Now, instead
of having a line segment for each slab, you have an unbounded ray for each
plane.  For example, if you have a hit for a near distance (i.e. you hit a
forward facing plane), then you would first check if this near is beyond your
current maximum distance.  If not, then you would store the maximum of this
near value and the previous (if any) near value.  If at this point the stored
near value is greater than the stored far value, the ray has missed.  This
process continues with each plane until you miss or you run out of planes.
The near and far points are then your intersection points with the polyhedron.

Essentially, this process is an extension of Kay & Kajiya: the ray is tested
against each face and the intersection point is used to form a segment which
is unbounded (infinite) on one end.  This segment is tested for validity
against the ray's segment and the polyhedron's current intersection segment.
If the polyhedron's segment is valid at the end of the process, then the ray
hit the polyhedron.  Basically, each face of the polyhedron defines a
half-space in which the inside of the polyhedron must lay.  The intersection of
these half-spaces is the polyhedron itself.

One minor problem is what happens when the ray is parallel to the plane.  You
could use Kay & Kajiya's suggestion of using some epsilon for the divisor,
but I prefer handling this special case separately.  If parallel, then we
want to check if the ray origin is inside the half-space:  if it is not, then
the ray cannot ever hit the polyhedron (since the ray does not cross into the
volume of valid intersections formed by this half-space).  In other words, if
we substitute the origin into the plane equation, the result of the expression
must be negative for the point to be inside the half-space; if not, the ray
must miss the polyhedron.

The pseudo-code:

    Maxdist = maximum distance allowed along ray
    Tnear = -MAXFLOAT, Tfar = MAXFLOAT
    For each plane in the polyhedron:
    Compute intersection point T and sidedness
    dp = ray.direction DOT plane.normal
    do = (ray.origin DOT plane.normal) - plane.d
    If ( dp == 0 )
        /* ray is parallel to plane - check if ray origin is inside plane's
           half-space */
        If ( do > 0 )
        /* ray is outside half-space */
        return MISSED
    Else
        /* ray not parallel */
        T = do / dp
        If ( dp < 0 )
        /* front face - T is a near point */
        If ( T > Maxdist ) return MISSED
        If ( T > Tnear )
            Tnear = T
            Normal = plane.normal
        Else
        /* back face - T is a far point */
        If ( T < 0.0 ) return MISSED
        If ( T < Tfar ) Tfar = T
        If ( Tnear > Tfar ) return MISSED
    endfor
    return HIT

At the end, Tnear is the intersection distance and Normal is the surface
normal.  Tfar is the exit distance, if needed.

That's it:  instead of laborious inside/outside testing of the polygon on each
face (and storing all those vertices), we have a quick plane test for each
face.  If the number of planes is large, it might have been better to store
the polygons and use an efficiency scheme.  However, the method above is
certainly simpler to code up and is pretty efficient, compact, and robust:
for example, there are no special edge intersection conditions, as there are
no edges!

An aside:
One thing that I hadn't mentioned is how we can better initialize the near and
far hit distances before the slab tests.  It turns out that when testing
bounding volumes we can set Tnear (the near distance) to 0 and Tfar to the
maximum ray distance (if any - else set it to "infinity").  This corresponds
to the segment formed by the ray:  we consider points between 0 and the
maximum ray distance to be valid points on the ray, and so want to find slab
segments that overlap this segment.  Note that these initial conditions gets
rid of some complication for the algorithm:  we now don't have to test our
slab segment against the ray's segment and the previous bounding volume
segment, but rather are always testing against a single segment which
represents the intersection of both of these.  This idea is not in Kay &
Kajiya's presentation, by the way.

Note that there is one change that occurs if you initialize the near and far
distances in this way:  the near and far distances computed when a volume is
hit will not yield the true surface intersections, but rather will give the
first and last points inside the volume.  This is useful for bounding volumes,
since we usually just want to know if we hit them and have some idea of the
distance.

-------------------------------------------------------------------------------

New Radiosity Bibliography Available, by Eric Haines

A bibliography of publications related to radiosity is now available at:

    freedom.graphics.cornell.edu [128.84.247.85]: /pub/Radiosity

It's a compressed package using "refer" format.  Articles related to radiosity
or "non-classical" rendering (soft shadows, caustics, etc) are included here.
This version is significantly improved from the version I posted some months
ago.  Many thanks to all who helped update it.

-------------------------------------------------------------------------------

A Suggestion for Speeding Up Shadow Testing Using Voxels, by Andrew Pearce

Requirements:
------------
This method is applicable to any type of spatial subdivision.  It is probably
best suited to those of us who tessellate our objects to ray trace them.

Method:
------
I've expanded on Eric Haines' method of storing the last object hit by a
shadow ray with each light source, I now save a pointer to the voxel which
contains the last object hit (or at least the voxel the intersection occurred
in if the object spans multiple voxels).  My rationale is that if the shadow
ray does NOT intersect the "last object" which shadowed that light source,
then the likelihood of it hitting something in the neighborhood of that same
object is pretty good.  If we save the voxel which the shadowing occurred in
for the previous ray, we can examine the other objects in that voxel for
possible light source occlusion WITHOUT the ray startup and voxel traversal
costs.  Now this assumption is likely untrue if you're just tracing spheres
and checker boards (some slight intended :^) but it works quite well for
tessellated objects (NURBS patches in my case).

I NULL my pointers to both the last object and last voxel if no shadow
intersection was found on the last shadow ray to this light.

I store a ray_id with every object to insure that any given ray is tested for
intersection with an object ONLY ONCE even if it spans multiple voxels.  Each
ray has a unique id.  (I thought, as did David Jevans, that this was a well
known technique).  So, even if the shadow ray misses all of the objects in
"last voxel" and must be traced like a regular shadow ray, we are likely not
losing much since if the shadow ray enters the "last voxel" during it's
traversal of the voxels, the ray will see that it has already been intersected
with all of the objects in that voxel, and that voxel will be skipped
relatively quickly (slightly slower than an empty voxel; the time it takes to
compare the ray ID against the ray ID stored with each object).

Pseudo-code:
-----------
/****************************************************************************/
/* Obviously we cannot use the "last object hit" for transparent        */
/* objects since multiple objects may be involved in the shadowing process. */
/* The code outlined below assumes that we only store the "last object" and */
/* "last voxel" for shadow rays generated from a primary ray intersection.  */
/* What we really should have is a tree indicating what was hit at each     */
/* level of recursion. Ie. What object shadowed the intersection point      */
/* generated by the last refraction ray, generated from a reflection ray    */
/* generated by a primary ray?                            */
/****************************************************************************/
float check_shadowing(ray, light)
RAY_REC   ray;   /* ray from shading point to light source */
LIGHT_REC light; /* the light source we are interested in */
{
if (light.last_object != NULL) {

    /* intersect_object() marks object as having been */
    /* intersected by this ray. */
    hit = intersect_object( ray, light.last_object, &object);

    if (hit) {
        return(1.0); /* full shadowing */
    }

    if (light.last_voxel != NULL) { /* implied !hit */

        /* intersect_object_in_voxel_for_shadows() returns hit = TRUE */
        /* on first affirmed intersection with a non-transparent
         * object. */
        /* It ignores transparent objects altogether. */
        hit = intersect_objects_in_voxel_for_shadows( ray,
                                  light.last_voxel,
                                  &object);
        if (hit) {
            light.last_object = object;
            return(1.0);
        }
    }
}

/* traverse_voxels_for_shadows() DOES intersect transparent objects and */
/* sorts the intersections for proper attenuation of the light intensity. */
/* If it hits multiple objects, the object returned is the transparent one. */
hit = traverse_voxels_for_shadows(ray, &object, &voxel, &shadow_percent);

if (!hit) {
    light.last_object = light.last_voxel = NULL;
    return(0.0);
}
if (object.transparency_value > 0.0) {
    /* the object is transparent */
    light.last_object = light.last_voxel = NULL;
}
else {
    light.last_object = object;
    light.last_voxel  = voxel;
}
return ( shadow_percent );
}


Results:
-------
(For the discussion below, "positive occlusion" means we have guaranteed that
the point we are shading is shadowed from the light source.)

The "last object hit" method provided a positive occlusion 52% of the time,
and if the "last object hit" method did not provide positive occlusion, the
"last voxel" method provided a positive occlusion 76% of the time.

I performed a "pixie" of the code with and without this opt. on an SGI
Personal Iris with no other code changes or scene changes, there is ONE light
source in the scene which is casting shadows.  The ray tracer with the "last
voxel" optimization used 2% fewer cycles.  (Actual system times vary wildly
based on load, but the last voxel version did run about 10% faster using the
UNIX "times()" system call, but I don't trust "times()").  Two percent
doesn't seem like an awful lot, but this is just a quick and dirty hack, and I
would expect to save 2% on EACH light source which casts shadows.

The test scene I'm using is a snare drum with a shadow casting spotlight on
it.  See IEEE Computer Graphics & Applications, Sept.  1988, the cover story
includes a tiny reproduction of the image should you wish to see what I'm
playing with, although the image in CG&A was done with a 2D projective
renderer (ray casting), not ray tracing.  The reflections in the floor and on
the chrome in that image were realised using two separate cubic environment
maps, the shadows were done with a depth map, the wallpaper is a simple
parametric map and the floor boards have 6 separate solid wood textures
randomly assigned to them.

The test scene contains approximately 60,000 triangles and I'm rendering at
512x512 resolution with no anti-aliasing, and a limit of one recursion level
for both shadow and reflection rays for a total of 638,182 rays.  There is
only one light in the scene which casts shadows.

I'll be doing tests on more scenes with various levels of voxel subdivision,
and object distribution.  I'll let you know the results, even if they're
negative!  (The above results did surprise me a little).

Additional Note:  I urge anyone doing ray/triangle intersections to use Didier
Badouel's "An Efficient Ray-Polygon Intersection" (Graphics Gems pp.
390-393).  I have implemented both Badouel's method and Snyder/Barr's
barycentric method, and Badouel's method is about 19% faster (I optimized the
snot out of my implementation of the barycentric method, but I used most of
those same opts in Badouel's method as well).  This result is from comparing
the same scene ray traced with the two versions.

_________________________________________________________________________

[A second article followed some days later - EAH]

More results from the voxel/shadow optimization:
-----------------------------------------------

One thing I neglected to mention in the previous message was that you should
be sure to NULL out your last object and last voxel hit pointers at the end of
each scanline.

NEW TEST SCENES:
---------------
The test scenes producing the results below are 40x40x40 arrays of polygons
(each polygon is broken down into 8 triangles).  The polygons are distributed
at unit distances from each other, and then their orientations are jittered
(rotations) and their positions are jittered (translate of -1.  to +1.).  Each
polygon is roughly a unit in width and height.  The polygons are inside of a
larger box (the room) with 15 shadow casting lights in a 5x3 array just below
the "ceiling".  There were no reflection or refraction rays generated.  All
images were computed at 645x486 pixels with 4 supersamples per pixel.  Every
intersection generated 15 shadow rays.  The "sparse" scene had every polygon
scaled by 0.2 in all dimensions.  The resulting sparse image looks like an
exploding particle system (due mainly to the 145 degree field of view).  In
the dense image, almost no background can be seen.

CHART DESCRIPTION:
-----------------
"Last Voxel" speed up refers to the percentage fewer cycles the "last voxel"
method took to compute the same image.  Since this percentage is calculated
based on the number of cycles the entire ray trace took, it is an exact
measure of the speed up.  Negative numbers mean that the "last voxel" method
was slower.  It is important to note that file read, tessellation and spatial
subdivision time is included in the count of the cycles, so the actual speed
up to the ray tracing alone may be greater than is stated, depending on how
you want to measure things.

Average Hits Per Ray is included as a general measure of the density of the
scene, it is the number of confirmed ray/triangle intersections divided by the
total number of rays (shadow rays included).  In the sparse scene it is less
than one since most of the shadow rays made it to the light sources without
hitting anything.  The dense scene is greater than one because some confirmed
intersections are abandoned due to nearer intersections being found in the
same voxel.

Average Hits Per Primary Ray is the number of confirmed ray/triangle
intersections divided by the number of primary (eye) rays.

--------------+-----------------+-----------------+
Scene         | 64,000 jittered | 64,000 jittered |
Description   | polygons (0.2)  | polygons (1.0)  |
          |   (sparse)      |   (dense)       |
--------------+-----------------+-----------------+
Number of     |   551,408       |  551,408        |
Triangles     |                 |                 |
--------------+-----------------+-----------------+
Number of     |                 |                 |
Shadow Casting|       15        |     15          |
Lights        |                 |                 |
--------------+-----------------+-----------------+
Number of Rays|   11,324,318    |  8,427,904      |
--------------+-----------------+-----------------+
"Last Object" |                 |                 |
Success Rate  |       50.7%     |  90.9%          |
--------------+-----------------+-----------------+
"Last Voxel"  |                 |                 |
Success Rate  |       23.4%     |  39.3%          |
--------------+-----------------+-----------------+
"Last Voxel"  |                 |                 |
Speed Up      |       1.04%     |  3.6%           |
--------------+-----------------+-----------------+
Average Hits  |                 |                 |
Per Ray       |       0.265     |  1.001          |
--------------+-----------------+-----------------+
Average Hits  |                 |                 |
Per Primary   |       2.363     |  6.638          |
Ray           |                 |                 |
--------------+-----------------+-----------------+

It is encouraging that there is a speedup even in very sparse scenes (however
slight a speed-up it is).  These "random" scenes are not very indicative of
the type of scenes we are generally interested in ray tracing.  (Really, these
scenes look like particle systems, I can think of much better ways to render
them with to produce similar images :^).  So, here's the same chart for the
snare drum with increasing numbers of lights.  The extra lights are scattered
around the "room" and all point towards "Spanky" the snare drum.

--------------+---------+---------+---------+
Scene         | Snare 1 | Snare 3 | Snare 6 |
Description   | Shadow  | Shadow  | Shadow  |
          | Light   | Lights  | Lights  |
--------------+---------+---------+---------+
Number of     |  59,692 |  59,692 |  59,692 |
Triangles     |         |         |         |
--------------+---------+---------+---------+
Number of     |         |         |         |
Shadow Casting|    1    |    3    |    6    |
Lights        |         |         |         |
--------------+---------+---------+---------+
Number of Rays| 638,182 |1,097,569|1,737,021|
--------------+---------+---------+---------+
"Last Object" |         |         |         |
Success Rate  |  52.6%  |  89.0%  |  88.7%  |
--------------+---------+---------+---------+
"Last Voxel"  |         |         |         |
Success Rate  |  76.3%  |  77.0%  |  76.9%  |
--------------+---------+---------+---------+
"Last Voxel"  |         |         |         |
Speed Up      |  1.97%  |  3.37%  |  4.39%  |
--------------+---------+---------+---------+
Average Hits  |         |         |         |
Per Ray       |  0.75   |  0.67   |  0.59   |
--------------+---------+---------+---------+
Average Hits  |         |         |         |
Per Primary   |  1.84   |  2.84   |  3.94   |
Ray           |         |         |         |
--------------+---------+---------+---------+

Well, the speed-up isn't quite 2% per light as I said in my previous article,
but it is there.  The "last voxel" trick has not slowed down the ray tracing
process in any of these tests which is quite encouraging.

------------------------------------------------

Another helpful hint if you are ray tracing tessellated or planar surfaces:
In general when spawning a shadow ray, one must be careful to avoid
intersecting the object just struck.  Usually this is done by adding some
small epsilon to the origin of the shadow ray along it's direction of travel.
However, if you store a ray id with every object (triangle) to record if that
object has been tested against the current ray, then you can use that id to
avoid testing your shadow ray against the intersected object which generated
the shadow ray.  Before spawning the shadow ray, place the id number of the
shadow ray into the ray id field of the object which has just been
intersected.  This method won't work for objects which can self shadow (eg.
parametric or implicit surfaces), but it works fine for planar surfaces (eg.
triangles from surface tessellations).

--------------------------------------------------------

- Andrew Pearce, Alias Research, Toronto, like Downtown
- [e-mail]   |   [e-mail]

-------------------------------------------------------------------------------

Real3d, passed on by Juhana Kouhia ([e-mail]) and "Zap" Andersson

[This was an advertisement on some amiga board.]

    Real 3D FEATURES
    ----------------

Real 3D is design and animation program for producing high quality, realistic
pictures of three dimensional objects.  It provides an impressive set of
advanced features including:

Ray Tracing        A ray tracing of Real 3D is strongly based on the
            physical reality of the real world. Real 3D
            produces pictures by simulating the laws of physics,
            and consequently they represent reality with
            astonishing accuracy.

Speed            Innovative methods and new ray tracing algorithms
            make Real 3D really fast. When using fastest ray tracing
            mode, rendering time is typically from 1 to 15 minutes.

Hierarchical        With Real 3D you can create hierarchical objects.
Object            This means that objects you create can be made of
Oriented        subobjects, and these subobjects may have their
Construction        own substructure and so on. This kind of a tree
of Objects        structure is well known in the context of disk
            operating systems, in which you can create directories
            inside directories. In Real 3D counterparts of these
            directories are used to collect objects into logical
            groups.
            This kind of approach makes for example object
            modifications extremely easy, because it is possible
            to perform operations to logical entities. If you
            want to copy a DOS directory, you don't have to
            take care of the files and directories inside it.
            In the same manner, you can stretch a complex object in
            Real 3D as easily as one part of it.

True Solid        Real 3D includes a true solid modeler. Solid model is the
Model            most sophisticated way to represent three dimensional
            objects. This modeling technique requires much computing
            power and therefore it has earlier been used only in
            environments, which are many times faster than Amiga.
            Now it is possible to Amiga owners to reach all the
            advantages of solid model, thanks to ultimate
            optimizations carried out when developing Real 3D.

Smoothly Curved     In addition to plane surfaces, Real 3D includes several
Surfaces        curved surfaces, such as ball, cylinder, cone and
            hyperboloid. This means that no matter how much you
            enlarge a ball created by Real 3D, you don't find
            any edges or corners on its surface. Furthermore,
            this makes the program much faster. And what is most
            important, the produced pictures look really good.

Boolean            Solid model allows Boolean operations between objects.
Operations        It is possible, for example, to split an object into
            two pieces and move the pieces apart so that the inner
            structure of the object is revealed.
            Operations can also be done so that the properties of
            the material of the target object are changed. By using
            a brilliant cylinder one can drill a brilliant hole into
            a matt object. Operations are a powerful way to create
            and modify objects. Especially in modeling technical
            objects these Boolean operations are indispensable.

Properties of       A user of Real 3D is not restricted to use some basic
Surfaces        surface brilliancies such as matt or shiny. Instead,
            the light reflection properties can be freely adjusted
            from absolute matt to totally mirrorlike, precisely
            to the desired level.

Properties of       Due to solid model, it is possible to create objects
Materials        from different materials, which have suitable physical
            properties. Just as surface brilliancy, also transparency
            of a material can be adjusted without any restrictions.
            Even light refraction properties are freely adjustable
            so that it is possible to create optical devices from
            glass lenses. These devices act precisely as their
            real counterparts: a magnifying glass in Real 3D world
            really magnifies!

Texture Mapping     The texture mapping properties of Real 3D are not
            restricted to a typical chequered pattern: Any IFF
            picture can be used to paint objects. You can create
            pictures with your favorite painting program as well
            as with a video digitizer or a scanner. For example, by
            digitizing a wood filament pattern, it is easy to create
            wooden objects looking very realistic.
            Pictures can be located precisely to desired places,
            with desired sizes and directions.
            Real 3D offers as many as seven texture mapping methods,
            including parallel, cylinder, ball and spiral projections.

Light Sources       Unlimited number of light sources of desired color and
            brightness. The only restriction is amount of memory.

Animation        As well as you can create single pictures, you can
Support            create series of pictures, animations. Real 3D includes
            also software for representing these animations
            interactively. Animation representation can be directed
            by a script language from ascii files or even from
            keyboard. Instead of looping animations you can define
            infinitely many ways to represent your pictures. Therefore
            you can create animations from a small number of pictures
            by displaying them various ways.

Rendering        Real 3D includes several different rendering techniques:
Techniques        a real time wireframe model, a hidden line wireframe model,
            a high speed one light source ray tracing model,
            a non-shadow-casting ray tracing model and a perfect ray
            tracing model. You can select either a HAM display mode
            with 4096 colors or a grey scale display mode offering
            higher resolution. Also version with 16 million color
            rendering (24 bits output) will become available during
            November 1990.

Availability        When writing this document (6.9.1990), marketing of
            Real 3D is already started in Europe with a little
            lower price than that of Sculpt 4D. The distribution
            in the USA is not yet arranged. For further information
            of Real 3D, please contact:

            REALSOFT KY
            KP 9, SF-35700 VILPPULA
            FINLAND

            Tel. [phone]

--------

from "Zap" Andersson:

REAL3d ('member that?) is available (in Sweden at least) from:
KARLBERG & KARLBERG AB
FLADIE KYRKOVAEG
S-23700 BJARRED
SWEDEN
Phone: [phone]
Phax:  [phone]

-------------------------------------------------------------------------------

Utah Raster Toolkit Patch, by Spencer Thomas

The first patch for the Utah Raster Toolkit version 3.0 is now available.  The
patch file is urt-3.0.patch1, and is currently available from cs.utah.edu and
freebie.engin.umich.edu, and will soon be available from our other archive
sites (depending on how quickly the archive maintainers grab the patch file).
There are also slight changes to the files urt-img.tar and urt-doc.tar (in
particular, if you had trouble printing doc/rle.ps, this is fixed).

[p.s. there was also a fix to a getx11 bug for the Sun 4, which is
pub/urt-SUNOS4.1-patch.tar.Z on freebie and weedeater. --EAH]

Here is the description from the patch file:
    Fixed core dump in rletogif, compile warnings in giftorle.
    Minor update bug in getx11 fixed.
    getx11 now exits if all its input files are bogus.
    New program: rlestereo, combines two images (left and right eye)
    into a red-blue (or green) stereo pair.
    Configuration file for Interactive Systems 386/ix.
    Minor fix to rleskel: ignore trailing garbage in an input image.

And the list of the current archive sites, from the urt.README file in
the ftp directory:

  North America
     East coast
    weedeater.math.yale.edu    130.132.23.17    (pub/*)
     Midwest
    freebie.engin.umich.edu    35.2.68.23    (pub/*)
     West
    cs.utah.edu        128.110.4.21    (pub/*)
  Europe
     Sweden
    alcazar.cd.chalmers.se    129.16.48.100    (pub/urt/urt-3.0.tar.Z)
    maeglin.mt.luth.se    130.240.0.25    (pub/Utah-raster/*)
  Australia
         ftp.adelaide.edu.au    129.127.40.3    (pub/URT/*)
    or, if you know what this means:
        Fetchfile:     sirius.ua.oz in URT

=Spencer W. Thomas         EECS Dept, U of Michigan, Ann Arbor, MI 48109
[e-mail]        313-936-2616 (8-6 E[SD]T M-F)

-------------------------------------------------------------------------------

NFF Shell Database, by Thierry Leconte ([e-mail])

[Below is Thierry Leconte's code for an NFF version of the seashell generator
I listed last issue.  He added some reasonable lights and a view (which I was
too lazy to do).  I'll probably add it to the 3.0 version of the SPD.  Setting
"steps" is similar to an SPD SIZE_FACTOR type control.  You'll need the SPD to
compile and link this program.  -- EAH]

#include <stdio.h>
#include <math.h>
#include "def.h"
#include "lib.h"

main(argc,argv)
int argc;  char *argv[];
{
static  double  gamma = 1.0 ;   /* 0.01 to 3 */
static  double  alpha = 1.1 ;   /* > 1 */
static  double  beta = -2.0 ;   /* ~ -2 */
static  int     steps = 600 ;   /* ~number of spheres generated */
static  double  a = 0.15 ;      /* exponent constant */
static  double  k = 1.0 ;       /* relative size */
double  r,x,y,z,rad,angle ;
int     i ;
COORD4  back_color, obj_color ;
COORD4  center, light ;
COORD4  from, at, up ;
COORD4  sphere;

#define OUTPUT_FORMAT    OUTPUT_CURVES

/* output viewpoint */
SET_COORD( from, 6, 60, 35 ) ;
SET_COORD( at, 0.0, 8.0, -15.0 ) ;
SET_COORD( up, 0.0, 0.0, 1.0 ) ;
lib_output_viewpoint( &from, &at, &up, 45.0, 0.5, 512, 512 ) ;

/* output background color - UNC sky blue */
SET_COORD( back_color, 0.078, 0.361, 0.753 ) ;
lib_output_background_color( &back_color ) ;

/* output light sources */
SET_COORD( light, -100.0, -100.0, 100.0 ) ;
lib_output_light( &light ) ;

/* set up sphere color */
SET_COORD( obj_color, 1.0, 0.8, 0.4 ) ;
lib_output_color( &obj_color, 0.8, 0.2, 100.0, 0.0, 1.0 ) ;

    for ( i = -steps*2/3; i <= steps/3 ; ++i ) {
    angle = 3.0 * 6.0 * M_PI * (double)i / (double)steps ;
    r = k * exp( a * angle ) ;
    sphere.x = r * sin( angle ) ;
    sphere.y = r * cos( angle ) ;
    /* alternate formula: z = alpha * angle */
    sphere.z = beta * r ;
    sphere.w = r / gamma ;
    lib_output_sphere( &sphere, OUTPUT_FORMAT ) ;
    }
}

-------------------------------------------------------------------------------

FTP list update and New Software, by Eric Haines, George Kyriazis

I posted my FTP site list for ray tracing related stuff, and a few people were
nice enough to write and update this list.  The new list is posted after this
note from George Kyriazis at RPI (his site has the friendliest login I've ever
seen):

--------

iear.arts.rpi.edu [128.113.6.10]:

There was an article in IEEE CG&A a while ago from Sandia National Labs (the
guy's name was Mareda I think) that uses fourier transforms and digital
filters to create wave height fields from white noise.  What I have in the
directory is an implementation of this algorithm and a program that raytraces
it on the AT&T Pixel Machine.

A list of what exists in there follows:

graphics-gems:    source code to Glassner's Graphics Gems book.
ray-tracing-news:What else?
wave:        Rendering of ocean waves using fft. (Mareda, et. al.)
coldith:    conversion from my image format to sun rasterfiles
        and dithering from 32 or 24 bit -> 8 bit rasterfiles.
drt:        A ray-tracer from the Netherlands by Marinko Laban
gk:        A distributed raytracer by me.
microray:    DBW_uRAY by David Wecker
mtv:        Well, you know what this is.  It probably is an old version.
non-completed-OO-modeler:
        Something I was working on.  It barely works, but I put
        it out there just for the fun of it.
ohta:        Masataka Ohta's constant time ray-tracer;
        with a few improvements.
pxm-and-vmpxm-ray.etc:
        Two raytracers for the AT&T Pixel Machine.  The second
        one uses some virtual memory code to store more objects.
        The VM source is included also.
qrt:        Well, QRT!
rayshade:    Rayshade 2.21 by Craig Kolb.

I hope I'll have a few anonymous ftp sessions after this.. :-)

--------

Corrected FTP site list for ray tracing related material:

weedeater.math.yale.edu [130.132.23.17]:  /pub - *Rayshade 3.0 ray tracer*,
    *color quantization code*, RT News, *new Utah raster toolkit*, newer
    FBM, *Graphics Gems code*.  Craig Kolb <[e-mail]>

cs.uoregon.edu [128.223.4.13]:  /pub - *MTV ray tracer*, *RT News*, *RT
    bibliography*, other raytracers (including RayShade, QRT, VM_pRAY),
    SPD/NFF, OFF objects, musgrave papers, some Netlib polyhedra, Roy Hall
    book source code, Hershey fonts, old FBM.  Mark VandeWettering
    <[e-mail]>

hanauma.stanford.edu [36.51.0.16]: /pub/graphics/Comp.graphics - best of
    comp.graphics (very extensive), ray-tracers - DBW, MTV, QRT, and more.
    Joe Dellinger <[e-mail]>

freedom.graphics.cornell.edu [128.84.247.85]:  *RT News back issues, source
    code from Roy Hall's book "Illumination and Color in Computer
    Generated Imagery", SPD package, Heckbert/Haines ray tracing article
    bibliography, Muuss timing papers.  [CURRENTLY NOT AVAILABLE]

alfred.ccs.carleton.ca [134.117.1.1]:  /pub/dkbtrace - *DKB ray tracer*.
    David Buck <[e-mail]>

uunet.uu.net [192.48.96.2]:  /graphics - RT News back issues (not complete),
    other graphics related material.

iear.arts.rpi.edu [128.113.6.10]:  /pub - *Kyriazis stochastic Ray Tracer*.
    qrt, Ohta's ray tracer, other RT's (including one for the AT&T Pixel
    Machine), RT News, Graphics Gems, wave ray tracing using digital
    filter method.  George Kyriazis <[e-mail]>

life.pawl.rpi.edu [128.113.10.2]: /pub/ray - *Kyriazis stochastic Ray Tracer*.
    George Kyriazis <[e-mail]>

xanth.cs.odu.edu [128.82.8.1]:  /amiga/dbw.zoo - DBW Render for the Amiga (zoo
    format).  Tad Guy <[e-mail]>

munnari.oz.au [128.250.1.21]:  */pub/graphics/vort.tar.Z - CSG and algebraic
    surface ray tracer*, /pub - DBW, pbmplus.  David Hook
    <[e-mail]>

cs.utah.edu [128.110.4.21]: /pub - *Utah raster toolkit*.  Spencer Thomas
    <[e-mail]>

gatekeeper.dec.com [16.1.0.2]: /pub/DEC/off.tar.Z - *OFF objects*,
    /pub/misc/graf-bib - *graphics bibliographies (incomplete)*.  Randi
    Rost <[e-mail]>

abcfd20.larc.nasa.gov [128.155.23.64]: /amiga - DBW,
    /usenet/comp.{sources|binaries}.amiga/volume90/applications -
    DKBTrace 2.01.  Tad Guy <[e-mail]>

expo.lcs.mit.edu [18.30.0.212]:  contrib - *pbm.tar.Z portable bitmap
    package*, *poskbitmaptars bitmap collection*, *Raveling Img*,
    xloadimage.  Jef Poskanzer <[e-mail]>

venera.isi.edu [128.9.0.32]:  */pub/Img.tar.z and img.tar.z - some image
    manipulation*, /pub/images - RGB separation photos.  Paul Raveling
    <[e-mail]>

ftp.ee.lbl.gov [128.3.254.68]: *pbmplus.tar.Z*.

ucsd.edu [128.54.16.1]: /graphics - utah rle toolkit, pbmplus, fbm, databases,
    MTV, DBW and other ray tracers, world map, other stuff.  Not updated
    much recently.

okeeffe.berkeley.edu [128.32.130.3]:  /pub - TIFF software and pics.  Sam
    Leffler <[e-mail]>

irisa.fr [131.254.2.3]:  */iPSC2/VM_pRAY ray tracer*, /NFF - many non-SPD NFF
    format scenes.  Didier Badouel <[e-mail]>

surya.waterloo.edu [129.97.129.72]: /graphics - FBM, ray tracers

vega.hut.fi [128.214.3.82]: /graphics - RTN archive, ray tracers (MTV, QRT,
    others), NFF, some models

netlib automatic mail replier:  UUCP - research!netlib, Internet -
    [e-mail].  *SPD package*, *polyhedra databases*.  Send one
    line message "send index" for more info.

UUCP archive: avatar - RT News back issues.  For details, write Kory Hamzeh
    <[e-mail]>

======== USENET cullings follow ===============================================

Humorous Anecdotes, by J. Eric Townsend, Greg Richter, Michael McCool,
    Eric Haines


from J. Eric Townsend ([e-mail]):

Went to lunch with a good (but non-technical) friend of mine the other day.
On the way home, we were talking about what a nice day it was for lying around
and reading.  She mentioned that she'd gotten a Joyce Carol Oates book I was
looking for and was going to read it.

I said, "I wish I had time to read something fun, but I've got to read some
more ray tracing today for my special problems class."

She said, "Ray Tracing?  Who's that.  I don't think I've heard of him."

She thought it was funny only after I stopped laughing and took a few minutes
to explain it to her.

Tomorrow, I think I'm going to add a second name to my door (we have slots for
two as most RA's share offices):  "Ray Tracing".

--------

from Greg Richter ({emory,gatech}!bluemtn!greg)

We had a similar incident with a new secretary who informed me that a salesman
had called asking about our Gator Rays.  She wanted to know if we were
involved in animal testing.

--------

from Michael McCool ([e-mail])

>She said, "Ray Tracing?  Who's that.  I don't think I've heard of him."

For some reason the term seems to collect fun.  At SIGGRAPH in Dallas this
year the Ray-tracing course organizers [Glassner & Arvo] kept bringing up the
"related fields" of rat-racing and tray-racing.  Last SIGGRAPH there was a
"tutorial video" on tray-racing that was shown; it involved sliding trays
down stairs, and gave examples of acceleration techniques, etc.  One wonders.

--------

from Eric Haines:

And don't forget the t-shirt (a number of Canadians, Calgarians I think, had
them on):  There's a picture of a nerdy guy holding a pencil and kneeling over
a man laying on the ground covered with a sheet of paper.  The entire figure
is labeled "Ray Tracing".  The caption:  "Okee dokee, Ray, here comes Mr.
Pencil."

John Wallace also mentioned to me seeing a poster in Denver of jewels with the
words "Ray Tracy" below, as this was the jeweller's name.

For those who've moved from graduate work to the world of business, a slogan:
"Once I was a ray-tracer, now I'm a rat racer."

I received the "Spherical Aberration" award from Andrew Glassner & Jim Arvo
for helping organize various ray tracing things.  The award was a glass sphere
on a wooden base.  It sat on my office windowsill until I moved to a new
office one day.  Looking at the award, I noticed black marks at the base.  The
sphere had acted like a (crummy, luckily) magnifying glass and burnt grooves
in the wood!  It was interesting to see how the marks seemed to correspond to
the movement of the sun and the seasons.  Anyway, the moral is "Caustics can
be dangerous" (after all, why do you think they call them "caustic"?).

-------------------------------------------------------------------------------

G r a p h i c s   I n t e r f a c e   ' 9 1
    Calgary, Alberta, Canada
    3-7 June 1991

CALL FOR PARTICIPATION

Graphics Interface '91 is the seventeenth Canadian Conference devoted to
computer graphics and interactive techniques, and is the oldest regularly
scheduled computer graphics conference in the world.  Now an annual
conference, film festival, and tutorials, Graphics Interface has
established a reputation for a high-quality technical program.  The 1991
conference will be held in Calgary, Alberta, June 3-7 1991 in conjunction
with Vision Interface '91. Graphics Interface '91 is sponsored by the
Canadian Man-Computer Communications Society.

IMPORTANT DATES:

Four copies of a Full Paper due:    31 Oct. 1990    *** N O T E ***
Tutorial Proposals due:            15 Nov. 1990
Authors Notified:             1 Feb. 1991
Cover Submissions due:             1 Feb. 1991
Final Paper due:            29 Mar. 1991
Electronic Theatre Submissions due:     1 May  1991

TOPICS:

Contributions are solicited describing unpublished research results and
applications experience in graphics, including but not restricted to the
following areas:

Image Synthesis & Realism        User Interface
Shading & Rendering Algorithms        Windowing Systems
Geometric Modeling            Computer Cartography
Computer Animation            Image Processing
Interactive Techniques            Medical Graphics
Graphics for CAD/CAM            Graphics in Education
Computer-Aided Building Design        Graphics & the Arts
Industrial & Robotics Applications    Visualization
Graphics in Business            Graphics in Simulation

Four copies of full papers should be submitted to the Program Chairman
before Oct.31 1990. Include with the paper full names, addresses, phone
numbers, fax numbers and electronic mail addresses of all the authors. One
author should be designated "contact author"; all subsequent correspondence
regarding the paper will be directed to the contact author. The other
addresses are required for follow-up conference mailings, including the
preliminary program.

FOR GENERAL INFORMATION:        SUBMIT PAPERS TO:

Wayne A. Davis                Brian Wyvill
GI '91 General Chairman            GI '91 Program Chairman
Department of Computing Science        Department of Computer Science
University of Alberta            University of Calgary
Edmonton, Alberta, Canada        Calgary, Alberta, Canada
T6G 2H1                    T2N 1N1

Tel:    [phone]            Tel:    [phone]
EMail:    [e-mail]    EMail:    [e-mail]

[There are often a fair number of papers on ray tracing at this conference
{vs.  maybe one at SIGGRAPH}, so it is a good place to consider submitting RT
research.  --EAH]

-------------------------------------------------------------------------------

Parametric Surface Reference, by Spencer Thomas

In article <[e-mail]> [e-mail] writes:
> Can someone please tell me where I can find an algorithm for
> finding the intersection of a ray and a Bezier and/or B-Spline
> patch.

You might look at

Lischinski and Gonczarowski, "Improved techniques for ray tracing
parametric surfaces," Visual Computer, Vol 6, No 3, June 1990, pp
134-152.

Besides having an interesting technique, they refer to most of the
other relevant work.

[This entire issue is dedicated to ray tracing, and is worth checking out.
-EAH]

--
=Spencer W. Thomas         EECS Dept, U of Michigan, Ann Arbor, MI 48109
[e-mail]        313-936-2616 (8-6 E[SD]T M-F)

-------------------------------------------------------------------------------

Solid Light Sources Reference, by Steve Hollasch, Philip Schneider
from: comp.graphics

Steve Hollasch ([e-mail]) writes:

    How do raytracers make light sources out of arbitrary objects?  I
thought a while back that one approach would be to find the direction to
the object from the illuminated point, fire a random cone of rays at the
object, and assign some fraction of the object's light to the point for
each unobstructed ray.

    The main drawback of this approach, as I see it, is that it would
yield a mottled illuminated area, and the mottling would vary in a
random manner.

    About five minutes ago I had an idea for another approach:

    -  Find the 2D bounding box (from the illuminated point's view) of the
       illuminating object.

    -  From this box, get the two orthogonal basis vectors.

    -  Now subdivide this bounding box (using the basis vectors), just
       as you would the original raytrace grid.

    -  For each light ray fired, determine if the ray intersects the
       illuminating object.  If it does, increment the `silhouette'
       counter.  If the light ray intersects no other object, then
       increment the `light' counter.

    -  Once done, the light that shines on the illuminated point is
       (light_counter/silhouette_counter) * object_light.

    This technique would also lend itself to numerous optimizations.
For example, if you assume that all light objects cast a convex
silhouette, then you could use binary search techniques to locate the
edges of the silhouette.  That is, you can assume that all scan lines
will be runs of space-silhouette-space intersections, hone in on the
edges, and then multiply the resulting silhouette width by the scanline
height to get the relative area.

    Is there a better way to do this?  I haven't come across this
problem in any of the graphics texts I've read.

--------

Philip Schneider ([e-mail]) replies:

    Get in touch with the University of Washington Department of Computer
Science.  Two or three years ago Dan O'Donnell wrote an M.S. thesis
on what he called "solid light sources".  (Sorry, my copy is in a box
right now, so I don't recall the exact title :-(

    Real nice work, as I recall, and the resulting pictures were pretty
interesting -- one of them featured a coffee mug, with steam rising from it
that turned into a glowing "neon sign" light formed into the shape of
the word "Espresso" (of course, I'm biased from having worked alongside him
at the UW graphics lab :-)

Philip J. Schneider
DEC Advanced Technology Development

[Has anyone seen this thesis?  Could you give me an exact reference (for the
Ray Tracing Bibliography)?  --EAH]

-------------------------------------------------------------------------------

Graphics Gems Source Code Available, by Andrew Glassner, David Hook

As many readers of the usenet are aware, at Siggraph '90 Academic Press
released a new book, "Graphics Gems" (edited by Andrew Glassner, published by
Academic Press, Cambridge MA, 864 pp, $49.95, ISBN 0-12-286165-5).  The book
is a compilation of many people's work, showing how they solved important
problems in computer graphics.  Many of the Gems are realized with
ready-to-run C implementations, presented in two appendices.

The authors and the publisher are pleased to release this source code to the
public domain:  neither the authors nor publisher hold any copyright
restrictions on any of these files.  The code is freely available to the
entire computer graphics community for study, use, and modification.  We do
request that the comment at the top of each file, identifying the original
author and the program's original publication in the book Graphics Gems, be
retained in all programs that use these files.

Each Gem is made available on an as-is basis; although considerable effort has
been expended to check the programs as originally designed and their current
release in electronic form, the authors and the publisher make no guarantees
about the correctness of any of these programs or algorithms.

All source files in the book are now available via anonymous ftp from site
'weedeater.math.yale.edu'.  To download the files, connect to this site with
an ftp program.  For user name type the word 'anonymous'; for password enter
your last name.  When you are logged in, type 'cd pub/GraphicsGems/src'.  Each
program from the book is stored in its own plaintext file.  I suggest you
first download the file README (type 'get README', then quit ftp and open the
file with any text editor); among other things it describes how to download
the rest of the directory, identifies the administrator of the site (who will
collect bug reports, etc.), and provides a table of contents so you can
identify the source files with their corresponding Gems.

We have enjoyed putting this book together.  It was a pleasure for me to work
with the many talented people who contributed to the success of this project.
A central theme of the book's philosophy was for the results to be practical
and useful - public release of the source code is a happy result of this
philosophy, shared by the authors, editor, and publisher.

We all hope this free source code is a useful resource for programmers
everywhere.

--------

From David Hook:

AARnet/ACSnet sites can now obtain GraphicGems source code from
gondwana.ecr.mu.oz.au (128.250.1.63) via anonymous ftp in pub/GraphicsGems or
through fetchfile, (for a general info file do a "fetchfile
-dgondwana.ecr.mu.oz pub/GraphicsGems/GEMS.INFO" and for a general listing
"fetchfile -dgondwana.ecr.mu.oz -LV pub".

Please note this is a clone site of the GraphicsGems code at
weedeater.math.yale.edu, and bug reports, etc...  should still be forward to
the administrators there.  Their addresses are listed in the GEMS.INFO and
README files.

--------

[I felt the following was a good way to summarize much of what "Graphics Gems"
has in it.  It's excellent, highly recommended (no, I don't have anything in
it).  Andrew Woo's trick for a quick rejection test on polygons gave me a few
percent speedup on intersecting these, for example.  Oddly enough, I had tried
his idea earlier (Kells Elmquist independently discovered it here), but didn't
get much speedup.  Seeing it in print made me try it again, but this time on a
variety of databases.  This time it gave some speed-up - the first time I
tried it on a database particularly ill-suited for performance improvement!
Finding out that someone else had used it successfully encouraged me to
explore further.  What's his trick?  You'll have to see the book...  - EAH]

The table below gives the correspondence between each source
file in this directory and the name of the Gem it implements.
Each implementation illustrates one way to realize the
techniques described by the accompanying Gem in the book.
The files here contain only the source code for that
realization.  For a more complete description of the
algorithms and their applications see the Gems of the same
name in the first 11 Chapters of the book.

---------- header files ----------
GraphicsGems.h           / Graphics Gems C Header File

----------    C code    ----------
2DClip               / Two-Dimensional Clipping:
             A Vector-Based Approach
AALines                / Rendering Anti-Aliased Lines
AAPolyScan.c           / Fast Anti-Aliasing Polygon
             Scan Conversion
Albers.c               / Albers Equal-Area Conic Map
             Projection
BinRec.c               / Recording Animation in Binary Order
             For Progressive Temporal Refinement
BoundSphere.c          / An Efficient Bounding Sphere
BoxSphere.c            / A Simple Method for Box-Sphere
             Intersection Checking
CircleRect.c           / Fast Circle-Rectangle Intersection
             Checking
ConcaveScan.c          / Concave Polygon Scan Conversion
DigitalLine.c          / Digital Line Drawing
Dissolve.c            / A Digital "Dissolve" Effect
DoubleLine.c           / Symmetric Double Step Line Algorithm
FastJitter.c           / Efficient Generation of Sampling
             Jitter Using Look-up Tables
FitCurves.c            / An Algorithm for Automatically
             Fitting Digitized Curves
FixedTrig.c            / Fixed-Point Trigonometry with
             CORDIC Iterations
Forms.c                / Forms, Vectors, and Transforms
GGVecLib.c             / 2D And 3D Vector C Library
HSLtoRGB.c             / A Fast HSL-to-RGB Transform
Hash3D.c               / 3D Grid Hashing Function
HypotApprox.c          / A Fast Approximation to
             the Hypotenuse
Interleave.c           / Bit Interleaving for Quad-
             or Octrees
Label.c                / Nice Numbers for Graph Labels
LineEdge.c             / Fast Line-Edge Intersections On
             A Uniform Grid
MatrixInvert.c         / Matrix Inversion
MatrixOrtho.c          / Matrix Orthogonalization
MatrixPost.c           / Efficient Post-Concatenation of
             Transformation Matrices
Median.c               / Median Finding on a 3x3 Grid
NearestPoint.c         / Solving the
             Nearest-Point-On-Curve Problem
             and
             A Bezier Curve-Based Root-Finder
OrderDither.c          / Ordered Dithering
PixelInteger.c         / Proper Treatment of Pixels
             As Integers
PntOnLine.c            / A Fast 2D Point-On-Line Test
PolyScan               / Generic Convex Polygon
             Scan Conversion and Clipping
Quaternions.c          / Using Quaternions for Coding
             3D Transformations
RGBTo4Bits.c           / Mapping RGB Triples Onto Four Bits
RayBox.c               / Fast Ray-Box Intersection
RayPolygon.c           / An Efficient Ray-Polygon
             Intersection
Roots3And4.c           / Cubic and Quartic Roots
SeedFill.c             / A Seed Fill Algorithm
SquareRoot.c           / A High-Speed, Low-Precision
             Square Root
Sturm                  / Using Sturm Sequences to Bracket
             Real Roots of Polynomial Equations
TransBox.c             / Transforming Axis-Aligned
             Bounding Boxes
TriPoints.c            / Generating Random Points
             In Triangles
ViewTrans.c            / 3D Viewing and Rotation Using
             Orthonormal Bases

-------------------------------------------------------------------------------

Graphics Gems Volume 2 CFP, by Sari Kalin

This is a quick note to let everyone know that Academic Press will be
publishing a sequel to the book GRAPHICS GEMS, edited by Andrew Glassner.
Since Andrew decided to take a breather from editing books, I'll be doing the
honors this time around.  So, if you're interested in contributing a clever
graphics algorithm or insight (i.e.  a "Gem") to the new volume, contact Sari
Kalin (the editor at Academic Press) and she'll send you an author's packet.
Here's the address:

    Sari Kalin
    Academic Press
    955 Massachusetts Ave.
    Cambridge, MA  02139

    tel:   [phone]
    email: cdp![e-mail]

AP wants to get the book out by SIGGRAPH `91, so that means the schedule is
tight.  The submission deadline for first drafts is November 1, 1990, so don't
delay!  Also, I'm sure AP would appreciate hearing any comments you might have
about the first volume.

-------------------------------------------------------------------------------

Foley, Van Dam, Feiner and Hughes "Computer Graphics" Bug Reports,
    by Steve Feiner

Sender: [e-mail] (Steven Feiner)
Organization: Columbia University Dept. of CS

We're in the process of setting up a separate email account to make it easy to
report bugs, suggest changes, and obtain a copy of the bug list for Computer
Graphics:  Principles and Practice, 2nd Ed. by Foley, van Dam, Feiner, and
Hughes.  Since Dave Sklar, the person who is setting up the account, is also
busting to get the versions of the book's SRGP and SPHIGS graphics packages
ready for their SIGGRAPH demos, the email account won't be available until
after SIGGRAPH.  We'll post details as soon as it's up.

Meanwhile, here are fixes for some dangling references and a missing exercise,
all of which had been devoured by a hungry gremlin:

1) Dangling references:

FIUM89 Fiume, E.L. The Mathematical Structure of Raster Graphics, Academic
Press, San Diego, 1989.

FOUR88 Fournier, A. and D. Fussell,
``On the Power of the Frame Buffer,'' ACM TOG, 7(2), April 1988,
103-128.

HUBS82 Hubschman, H. and S.W. Zucker, ``Frame-To-Frame Coherence and the Hidden
Surface Computation: Constraints for a Convex World,'' ACM TOG, 1(2),
April 1982, 129-162.  [The bibliography includes HUBS81, the SIGGRAPH 81 paper
on which HUBS82 was based.]

SNYD87 Snyder, J.M. and A.H. Barr, ``Ray Tracing Complex Models Containing
Surface Tessellations,'' SIGGRAPH 87, 119-128.

TAMM82 Tamminen, M. and R. Sulonen.  ``The EXCELL Method for Efficient
Geometric Access to Data,'' in Proc. 19th ACM IEEE Design Automation
Conf., Las Vegas, June 14-16, 1982, 345-351.

2) A missing exercise:

15.25 If you have implemented the z-buffer algorithm, then add hit detection
to it by extending the pick-window approach described in Section 7.12.2 to
take visible-surface determination into account.  You will need a SetPickMode
procedure that is passed a mode flag, indicating whether objects are to be
drawn (drawing mode) or instead tested for hits (pick mode).  A SetPickWindow
procedure will let the user set a rectangular pick window.  The z-buffer must
already have been filled (by drawing all objects) for pick mode to work.  When
in pick mode, neither the frame-buffer nor the z-buffer is updated, but the
z-value of each of the primitive's pixels that falls inside the pick window is
compared with the corresponding value in the z-buffer.  If the new value would
have caused the primitive to be drawn in drawing mode, then a flag is set.
The flag can be inquired by calling InquirePick, which then resets the flag.
If InquirePick is called after each primitive's routine is called in pick
mode, picking can be done on a per-primitive basis.  Show how you can use
InquirePick to determine which object is actually visible at a pixel.

Steve Feiner
[e-mail]

-------------------------------------------------------------------------------

Radiosity via Ray Tracing, by Pete Shirley ([e-mail])
from: comp.graphics


[e-mail] (Michael L. Kaufman) writes:

>This may be a stupid question, but why can't radiosity be handled by ray
>tracers?  Also, are there any archives that contain code/papers on radiosity
>that I can learn from?
>Michael

Ray Tracers can indeed do radiosity.  Check out the Sillion and Puech paper
or the Wallace et al. paper in Siggraph '89.  Also the Airey et al. paper
in proceedings of the symposium of Interactive 3D graphics (Computer
Graphics 24 (2)), and my Graphics Interface 90 paper.  Also Heckbert's
Siggraph 90 paper.

If all you want is a brute force radiosity code (and this code works pretty
well), then start with N polygons.  Each polygon will have reflectivity Ri
and will emit power Ei.   Assume you want to send R rays on the first pass.
We will now do what amounts to physical simulation:

T = 0           // T is total emitted power
For i = 1 to N
    Ui = Ei    // Ui is unsent power
    Pi = Ei    // Pi is total power estimate
    T = T + Ei

dP = T / R       // dP is the approximate power carried by each ray
For b = 1 to NumReflections
   For i = 1 to N
    r = int(Ui / dP)  // patch i will send power Ui in r rays
    dPi = Ui / r
    Ui = 0
    for j = 1 to r
        choose random direction with cosine weighting and
         send ray with until it hits polygon p (see Ward et al.
        Siggraph 88 paper equation 2 p 87)

        Up = Up + Rp * dPi
        Pp = Pp + Rp * dPi


// Once this is done, we will have the power Pi coming from each surface,
// Now we should convert to radiance (see my GI 90 paper or Kajiya's
// Course notes in Siggraph 90 Advanced RT notes)

For i = 1 to N
    Li = Pi / (pi * Ai)  // Li is radiance, pi is 3.14..., Ai is area


This ignores interpolation to polygon vertices to give a smooth image
(see Cohen & Greenberg siggraph 85 figure 8A).


You might also want to check out Ward et al.'s Siggraph 88 paper.  Not true
radiosity, but is ray tracing based and has some of the best features
of ray tracing methods.  If your scene is all diffuse, that method may be
the way to go.

-------------------------------------------------------------------------------

Algorithm Order Discussion, by Masataka Ohta, Pete Shirley


In article <[e-mail]> [e-mail] (Adrian McCarthy) writes:

>Rather than using recursive or hierarchical
>spatial subdivision techniques to reduce ray-object intersection tests
>(which are of O(log n) algorithms)

Can you prove it? I think (but can't prove) hierarchical spatial
subdivision is only O(n**(1/3)).

>many instances can use a surface map for
>a single bounding volume as a lookup table to immediately determine a small
>subset of objects to test (which is truly of O(1)). (Small subset here is
>roughly equivalent to the set of objects in the smallest volume in a
>comparable hierarchical scheme.)

I already published an O(1) ray tracing algorithm (See "An introduction
to RAY TRACING" edited by Andrew S. Glassner, Academic Press, 6.3 The Ray
Coherence Algorithm, page 232-234, or, "Computer Graphics 1987 (Proc.
of CG International '87)", page 303-314, Ray Coherence Theorem and
constant time ray tracing algorithm).

Judging from the above brief description of your algorithm, it may be
similar or identical to mine.

>It's *not* general,

Hmmm, mine is general.

--------

Kaplan also has claimed a constant time algorithm.  I don't believe that
his is constant time - it (like an octree) is a divide and conquer
search, so it will AT BEST be O(logN)  (it takes O(logN) time to travel
down the height of a tree).

I don't really see how we can even say what the big-O of a ray intersection
is without precisely stating what rays are allowed on what geometry.
For example, suppose I set up N epsilon radius spheres in random locations
within a cube, where epsilon is small.  In the typical case a ray might
come close to many spheres.  If an octree is used, the candidate sets
of many leaves might be checked (worse than O(logN)).  If all of the spheres
have the same center, how can any scheme get a candidate set for a ray
through the center that doesn't include all spheres?  This would be
O(NlogN) for an octree and O(N) optimally.  How would your method be O(1)?
It seems that often we have good algorithm behavior in practice with
pathological cases giving terrible big-Os.  Perhaps we can bound the set
of scenes somehow?

--------

Ohta replies:

>Kaplan also has claimed a constant time algorithm.  I don't believe that
>his is constant time--

I agree with you. I know what is O(1).

>I don't really see how we can even say what the big-O of a ray intersection
>is without precisely stating what rays are allowed on what geometry.

Well, it is assumed that, ray object intersection calculation takes only
constant time. That is, to ray trace a surface defined by N-th degree
polynominal in constant time regardless to N, is just impossible.

>For example, suppose I set up N epsilon radius spheres in random locations
>within a cube, where epsilon is small.  In the typical case a ray might
>come close to many spheres.  If an octree is used, the candidate sets
>of many leaves might be checked (worse than O(logN)).  If all of the spheres
>have the same center, how can any scheme get a candidate set for a ray
>through ythe center that doesn't include all spheres?  This would be
>O(NlogN) for an octree and O(N) optimally.  How would your method be O(1)?

Good question.  But, first, we should make it clear what is constant time ray
tracing.  As for the per-scene complexity, no algorithm (including your first
example) can be better than O(N) just because of input complexity (reading all
the input takes O(N) time).  So, we should talk about complexity to trace a
single ray.

My algorithm may consume possibly long and computationally complex
pre-processing time to analyze a scene.  But, the important fact is that the
time for pre-processing is dependent only on the scene and not on the number
of rays.

And, after pre-precessing, all rays can be traced in constant time.

With your example of co-centric spheres (assuming epsilon radius is different
on each sphere, or the problem is reduced to too trivial to trace a single
sphere), each sphere should further be subdivided into smaller pieces to
obtain constant time property.  With such a finer subdivision, even octree
method may be able to do better than O(NlogN).

>It seems that often we have good algorithm behavior in practice with
>pathological cases giving terrible big-Os.  Perhaps we can bound the set
>of scenes somehow?

It may be reasonable to define complexity of a scene by the complexity of
pre-processing to obtain constant-timeness.

--------

In a separate note, Ohta writes:

>Can you prove it? I think (but can't prove) hierarchical spatial
>subdivision is only O(n**(1/3)).

I have found it is obvious.  All spatial subdivision is at most O(n**(1/3)) if

    1) Objects are tiny
    2) Objects are uniformly scattered in space

1) means that the possibility of intersection is negligible.
2) means at least O(n**(1/3)) subspace must be traversed.

-------------------------------------------------------------------------------

Point in Polygon, One More Time..., by Mark Vandewettering, Eric Haines,
    Edward John Kalenda, Richard Parent, Sam Uselton, "Zap" Andersson, and
    Bruce Holloway


Mark VandeWettering writes about the point in polygon test:

The solution proposed by [e-mail] was based upon the Jordan Curve
theorem: any ray from inside a polygon crosses an odd number of edges on its
way to infinity.  He chose a random ray that began at the point in question,
and counted the number of intersections.  Problem: if you intersected a vertex
you were kind of clueless.  Solution, fire another ray.

You solve these problems by simplifying everything.  The ray you shoot should
go to the positive X axis.  Assume without loss of generality that your
point is the origin.  Now: if you are going to intersect a vertex, its because
the y component of an edge endpoint is == 0.  Well, decide whether you want
to count this as positive or negative.  Assume positive (I always do).  It
turns out you get the right answer anyway.  For example

      origin         ^
             |
    o         o            counts as one intersection
             |
             v

      origin         ^ ^
             |/
    o         o            counts as zero or two intersections

      origin

    o         o            counts as zero or two intersections
             |\
             v v


Voila!  C'est explique!


[and in a later note:]

Hardly my own idea.  It was shown to me by Eric Haines originally, but I don't
think he claims credit for it either.  Any takers?  Is it patented by Unisys
as well :-)?

--------

Eric Haines [the crazy fool!] replies:

    Anyway, having an ego and all that, I do indeed claim to be the
inventor of the "consider all points on the line to be above the line"
solution, which gets rid of those pesky special cases where vertices are on the
test ray.  I started with some very complicated special case code in 1986
which worried about whether the points were on the ray.  Months went by...
One day, looking at the code, I noticed another stupid special case that I
didn't handle correctly (something like "if the last point is on the ray and
the first point is on the ray...").  I looked at the problem again and
realized that the only property of the ray that I really cared about was that
it was a divider, not that it was a set of points forming a ray.  Eureka,
Arkansas!  Get rid of those points, and so define away the problem - no points
can lie on the ray, if we define the ray as having no points.  O happy day, it
even worked.  Anyway, it's all written up in my section of "An Introduction to
Ray Tracing", edited by Andrew Glassner, Academic Press.

    Historical note:  the earliest reference I have to the point in
polygon test is the classic "A Characterization of Ten Hidden-Surface
Algorithms" by Ivan Sutherland et al, 1974, ACM Computing Surveys v6 n1.  This
has both the ray crossing test and the sum of the angles test, plus the convex
polygon test (check if the point is inside all lines by substitution into each
edge's 2D line equation).

    Computational geometry fiends noted that the method has the problem of
being indeterminate on edges:  if your intersection point is exactly on an
edge, it's arbitrary whether the point is determined to be inside or outside
the polygon (that is, do you consider an edge crossing the point to be to the
right or left of the point, or do you want a third category, such as "on"?).

    However, there is a general consistency to the ray crossing test for
ray tracing.  If the point being tested is along the edge joining two visible
polygons (which both face the same general direction, i.e. not a silhouette
edge) and the polygons are projected consistently upon the plane perpendicular
to the ray, the point is guaranteed to be considered to be inside one and only
one of the polygons.  That is, no point will be considered within any two
non-overlapping polygons, nor will it "fall in the crack" between polygons.

    Think about it this way:  if points on the left edges of the polygon
are considered outside, then the points on the right edges will be considered
inside.  This is because we would then consider an edge crossing the x axis at
exactly 0 as being to the right of the test point.  Since one polygon's left
edge is another's right edge, it all balances out to make each point be inside
only one polygon in a polygon mesh.  Horizontal edges are treated the same
way:  if a point is actually on such an edge, when tested, the point will be
categorized as "below" the edge consistently.  This will lead it to be inside
one and only one polygon in a mesh.

    In reality, when ray tracing you very rarely get points that are
exactly on an edge, so this is something of a non-problem.  But if you really
really care about this possibility, make sure to always cast your polygons
onto the plane perpendicular to the ray (this plane is also good for
consistency if you want to get edges for Gouraud RGB interpolation, Phong
normal interpolation, etc).

    Finally, for you incredibly demonic CompGeom types: yes, indeed,
points on silhouette edges are still inconsistent.

P.S.  As our patent on the 90 degree angle was successful, the pending patent
on the Jordan Curve Theorem should come through any day now... ;-)

--------

Edward John Kalenda replies:

Sorry to be a poop Eric, but my boss at Calma, Steve Hoffman, told me
about the "points on the ray are above the ray" thing in 1981. He claimed
someone told HIM several years before.

I think it's one of those things that just need to be attributed to the
anonymous coder.

--------

I reply:

    Oh, well, at least I can claim to be the first to publish!  Sadly
enough, the word still hasn't fully percolated out.  The latest Foley & Van
Dam (& Feiner & Hughes) says on page 34:  "Next, choose a ray that starts at
the test point and extends infinitely in any direction, and that does not pass
through any vertices".  Page 339 makes reference to "intersections at vertices"
being a "special case".  Passing through a vertex is still considered to be a
problem (even though it isn't).

    It's this kind of thing that makes me happy to see books like
"Graphics Gems" come out.  Letting the world know about the little tricks of
the trade saves us all a lot of time and replication of effort (I sure wish I
had known about the "above the ray" trick in 1986 - it would have saved me
hours of struggle).

--------

Richard Parent ([e-mail]) writes:

With respect to 'consider all the points on the line to be above the line',
both Steve Levine (Ph.D. from Stanford, I believe) and myself discussed
this as a solution we used in our respective problems; mine was implementing
Lourtrel's hidden line routine and I imagine his had to do with the
hierarchical clipping he was doing.  This was at one of the Siggraph's in the
mid to late seventies, '76, if I had to guess.  It's been around for awhile.

--------

I reply:

    I guess I could compare myself to Leibniz vs. Newton (both
independently discovering calculus), but I'm probably more in the class of the
guy that discovers that Wintergreen Life Savers give off sparks when chewed in
the dark.

--------

Sam Uselton writes:

I've been off news for a few days and just saw your posting of sept 6.  Isn't
the "consider all points on the line to be above the line" the same as
"shortening an edge to prevent the vertex lying on the scan line" as suggested
in Foley & Van Dam (the old one) when describing polygon scan conversion?
That's not the first place I heard this idea either, it was just the easiest
reference to grab off the shelf.

I think this is one of those solutions that is just subtle enough that most
people don't think of it themselves and think it is neat when they see it but
just obvious enough that a few people every year re-invent it, and go showing
it around to others.  Just the kind of thing IMHO that makes Glassner's
collection and the RTNews such useful things, because it probably couldn't get
published "traditionally" .

Trying VERY hard to pull up archival storage, I'm pretty sure I saw this first
while still at UTD in grad school, which means Henry Fuchs was probably
explaining it.  Whether he is one of the many originators or he got it from a
fellow Utah grad student, I couldn't guess.  (Oh, yeah.  Time?  1975-1979).

--------

I note:

For scan conversion it is still a bit tricky:  you have to think a bit about
where edges begin and end (you want edges with vertices exactly on the scan
line to be handled correctly, e.g. if the edge's maximum y ends on the scan
line, delete this edge just before checking scan line).  This kind of problem
goes away for ray tracing, since you don't have to worry about active edges,
etc.

--------

Sam replies:

After a weekend of R&R old memory is more easily accessible.  I seem to
associate my first connection with point in polygon/ ray intersection/ Jordan
curve theorem/etc etc to an informal seminar, led by Henry Fuchs and Zvi
Kedem, attended by Alain Fournier, Don Fussell, Jose Barros, myself, and I
think Bruce Naylor, in which we read and studied early Michael Shamos papers
(Shamos & Hoey, Bentley & Shamos...).  Must have been about 1978 or so.

--------

I reply:

    Thanks for the info.  What I find amazing about all this is that
somewhere, somewhere this was not mentioned in the ray tracing literature
before "An Intro to RT".  Until you (and others) pointed out that this problem
and solution is analogous to the scan-line problem, I honestly hadn't made the
connection!  Nor, it seems, had anyone else in print.  Sedgewick's
"Algorithms" talks about this problem but doesn't give a "points above the
ray" solution, Berlin in 1985 SIGGRAPH course notes gives a few solutions to
the problem, but says that test rays through vertices are a special case & are
very nasty.  Certainly Cornell's ray tracer didn't solve this problem.
Glassner used the sum of the angles method, so he didn't know about it, so UNC
didn't know about it.  Sounds like somewhere out west was the point of origin,
but it never made it east of the Mississippi.  Curious.

    Sutherland et al's 1974 paper "A Characterization of Ten Hidden
Surface Algorithms" mentions the problem as a problem ("care is required to
get consistent results").  Anyway, sounds like there probably wasn't a
solution before them (if anyone would know, I would think it would be them).
Interesting puzzle to try to figure out the identity of this anonymous
programmer.

--------

From "Zap" Andersson:

As I think I posted earlier, this is somewhat similar to my own approach in
getting rid of special cases: Assume we are testing for a ray along X axis...
For all line segments, swap 'em, so it's "first" point is upwards (Have a lower
Y coordinate). Then, for each segment, test if the point's Y > firstY. If not,
discard (This, in essence, is a version of Eric's idea, only mine :-) then,
check if Y <= lastY. If not, discard. IMPORTANT ISSUE HERE is that I actually
CARE about the 'second' point of the line, and include it in the check, that's
MY way of getting rid of problem of a corner pointing downwards: It will yield
2 intersections, Eric's method yields none. No big deal, really.... after
this simple boundstest, it's time for the dreary math on checking if the
intersection is on POSITIVE X, i.e. if X > firstX && X > lastX, discard, and
last but not least, check the real intersection with some math (Left as an
exercise for the reader :-).

OBTW, fr'got! FIRST, you check if (LastY - Y) * (FirstY - Y) is > 0. If so,
discard.

--------

Bruce Holloway posts:

In article <[e-mail]> [e-mail] writes:
>
>In article <[e-mail]>, [e-mail] (Peter
>da Silva) writes:
>|> In article <[e-mail]> [e-mail] (Edward
>John Kalenda) writes:
>|> > Sorry to be a poop Eric, but my boss at Calma, Steve Hoffman, told me
>|> > about the "points on the ray are above the ray" thing in 1981. He claimed
>|> > someone told HIM several years before.
>|> >
>|> > I think it's one of those things that just need to be attributed to the
>|> > anonymous coder.
>|>
>|> Another of those obvious techniques like the XOR-cursor. Good thing nobody
>|> patented *this* one...
>
>I didn't see any smiley's after this one Peter.  I'm sure that
>many readers don't realize that the XOR-cursor in hardware *IS* patented.
>
>... and that's not the only obvious technique that this particular
>company has a patent on.

This thread is a riot.  My old boss at Calma, Joe Sukonick, patented the
XOR-cursor technique based on work he did around 1974, when I went to work
there.  I remember meeting Jim Clark for the first time around 1979 in front
of Stacey's bookstore in Palo Alto before he became famous for the Geometry
Engine.  He was married to a friend of mine from Calma (whom we were all in
love with!) & said he didn't think Joe's patent was valid because it wasn't
original.  I agree with him.

One of the things the patent says is that XOR works because it is "idempotent".
Joe had a Ph.D. in math from MIT & liked to use words like that, but I thought
AND & OR were idempotent & XOR worked because it wasn't!  What you need is
any operation that can be undone, or inverted, like ADD, say.  (What is XOR
but an ADD without carry?)  Anyway, the patent is now owned by Cadtrak, a
corporate shell whose charter is to sue everyone under the sun over that
patent.  Last I heard, Western Digital was going to take them to court to
overturn it.

Now as to the "points on the ray are above the ray", this is really the same
as the idea of half-open intervals which has broad usage in computer graphics.
When you point-sample a polygon, this is how you make sure that rectangles
of area m*n hit exactly m*n pixels, even if they lie on your sample grid.
When I was just a kid at Calma, I wrote two programs that used that idea.
One was a program to cross-hatch concave (and convex) polygons for making
plots of overlapping layers on ICs.  Another was a program which intersected
arbitrary closed polygons with each other.  This was an application program
written in Calma's GPL which was used to calculate the capacitance between
layers of a chip, by finding the areas of all the overlaps.  Both of these
algorithms needed to use the half-open idea to take care of the corner cases.

I left Calma in 1976, too inexperienced to realize what a singular place it
had been.  After the ownership changed, most everyone scattered to the four
corners of the globe.  Later, I actually had a contract job working for
Cal & Irma Hefte, the founders of Calma.  Cal passed away some years ago--
he seemed to me to be a lot like Willy Loman in "Death of a Salesman".
He told me "people are the most complicated & interesting things in the
world".  Mrs. Hefte and her daughters have a flower shop on Blossom Hill Rd.
A genuine Silicon Valley story!  (There's a lot more.)

-------------------------------------------------------------------------------

Quadrant Counting Polygon In/Out Testing, by Craig Kolb, Ken McElvain
from: comp.graphics

[After all that, I thought it was worth posting some real live code, but
instead for a quadrant counting method that is almost the same as the point in
polygon ray test.  The "infinite ray" test is really counting quadrants (in
"An Intro to RT" I show how you can get the winding number using it), but does
not bother to evaluate the exact quadrant unless need be.  For example, if
both endpoints of an edge are in the +Y space, we have no need to evaluate
whether we're in +X or -X.  -EAH]

In article <[e-mail]> [e-mail]
(Raymond Chen) writes:

>Nobody yet has mentioned my favorite method for point-in-polygon
>detection:  Using the winding number.

[...]

>/* Point in polygon detection.  Original author unknown.
> * Except where noted, I have NOT edited the code in any way.
> * I didn't even fix the misspellings.  (And no, I don't code
> * in this style.)
> */

[...]

>    return(quad);    /**rjc**/ /* for some reason, the original
>                     author left out this crucial line! */
>}
>--
>A remark:  The whichquad function is not perfect.  It doesn't handle
>the boundary cases right (but as I said, that's only a problem
>if the point lies on the polygon).

(After a long-overdue search through my archives...)

The code was posted to comp.graphics in February of '88 by Ken McElvain
(decwrl!sci!kenm).  Regarding the typos and the boundary cases, Ken wrote:

> I pulled this out of some other code and hopefully didn't break it in
> doing so.  There is some ambiguity in this version as to whether a point
> lying on the polygon is inside or out.  This can be fairly easily
> detected though, so you can do what you want in that case.

I once did a number of tests on Ken's winding number code and an implementation
of the 'ray' algorithm, using a ray tracer (rayshade).  For the test scenes
I rendered, I found that the winding number method was approximately 10% faster
than the ray method.  Your mileage may vary.

[It surprises me that this algorithm is ever faster:  the infinite ray
solution is essentially the same idea as the quadrant idea, except that it
avoids x compares when the signs of the y components are the same.  Maybe I'll
hack rayshade & prove it someday...  -- EAH]

From: [e-mail] (Ken McElvain)
Organization: Silicon Compilers Systems Corp. San Jose, Ca

In article <[e-mail]>, [e-mail] (W. Harry Plantinga) writes:
> In article <[e-mail]> [e-mail] (Robert Beaty) writes:
> >In article <[e-mail]> [e-mail] (Rick Dorris) writes:
> >>In article <[e-mail]> [e-mail] (Jim @ MAC) writes:
> >>>Have a nice one here:
> >>>Have a boundary defined on the screen. Boundary is composed of points
> >>>joined by lines... Now some random points are generated and I want to check
> >>>if a given point is within or outside the existing boundary.. Any algorithm for
> >> <rick suggests the infinite ray/intersection counting algorithm>
> >
> >I have seen this type of algorithm before and the one I thought up
> >in an afternoon is FAR superior and vastly simpler to code up.
> >
> ><robert beaty suggests an algorithm that measures sums the angles of
> >the lines connecting the points and the test point>
>
> Haven't I seen this discussion of point-in-polygon algorithms about 15
> times before? :-)
>
> Robert, your algorithm is simpler only in the kind of problems it
> handles.  It will only work for convex polygons, whereas the other
> works for any polygons.  It's not even much simpler; measuring angles
> and determining whether line segments intersect are of comparable
> difficulty.
>
> Maybe you should have said that although your algorithm is far
> inferior, it's not any easier to code?


Robert's suggestion is a good one.  The sum of the angles about the
test point is known as the winding number.  For non intersecting
polygons if the winding number is non-zero then the test point is
inside the polygon.  It works just fine for convex and concave
polygon's.  Intersecting polygon's give reasonable results too.
A figure 8  will give a negative winding number for a test point
in one of the loops and a positive winding number for the other loop,
with all points outside having a winding number of 0.  Other advantages
of the winding number calculation are that it does not suffer from
the confusion of the infinite ray algorithm when the ray crosses a vertex
or is colinear with an edge.

Here is my version of a point in poly routine using a quadrant
granularity winding number.  The basic idea is to total the angle
changes for a wiper arm with its origin at the point and whos end
follows the polygon points.  If the angle change is 0 then you are
outside, otherwise you are in some sense inside.  It is not necessary to
compute exact angles, resolution to a quadrant is sufficient, greatly
simplifying the calculations.

I pulled this out of some other code and hopefully didn't break it in
doing so.  There is some ambiguity in this version as to whether a point
lying on the polygon is inside or out.  This can be fairly easily
detected though, so you can do what you want in that case.


--------
/*
 * Quadrants:
 *    1 | 0
 *    -----
 *    2 | 3
 */

typedef struct  {
    int x,y;
} point;

pointinpoly(pt,cnt,polypts)
point pt;       /* point to check */
int cnt;    /* number of points in poly */
point *polypts; /* array of points, */
        /* last edge from polypts[cnt-1] to polypts[0] */
{
    int oldquad,newquad;
    point thispt,lastpt;
    int a,b;
    int wind;       /* current winding number */

    wind = 0;
    lastpt = polypts[cnt-1];
    oldquad = whichquad(lastpt,pt); /* get starting angle */
    for(i=0;i<cnt;i++) { /* for each point in the polygon */
        thispt = polypts[i];
        newquad = whichquad(thispt,pt);
        if(oldquad!=newquad) { /* adjust wind */
            /*
             * use mod 4 comparisons to see if we have
             * advanced or backed up one quadrant
             */
            if(((oldquad+1)&3)==newquad) wind++;
            else if((((newquad+1)&3)==oldquad) wind--;
            else {
                /*
                 * upper left to lower right, or
                 * upper right to lower left. Determine
                 * direction of winding  by intersection
                 *  with x==0.
                 */
                a = lastpt.y - thispt.y;
                a *= (pt.x - lastpt.x);
                b = lastpt.x - thispt.x;
                a += lastpt.y * b;
                b *= pt.y;

                if(a > b) wind += 2;
                else wind -= 2;
            }
        }
        lastpt = thispt;
    }
    return(wind); /* non zero means point in poly */
}

/*
 * Figure out which quadrant pt is in with respect to orig
 */
int whichquad(pt,orig)
point pt;
point orig;
{
    if(pt.x < orig.x) {
        if(pt.y < orig.y) quad = 2;
        else quad = 1;
    } else {
        if(pt.y < orig.y) quad = 3;
        else quad = 0;
    }
    return ( quad ) ;    /* [I added the fix - EAH] */
}


Ken McElvain
decwrl!sci!kenm

-------------------------------------------------------------------------------

Computer Graphics Journals, by Juhana Kouhia ([e-mail])

Here is a list, not as good as it could be.  I would like to see a short
descriptions about each journals on it.  I would like to see more information
about geometry related journals or journals that covers this newsgroup
subjects.  End of this list has list of journals, which was suggested to me,
but I didn't find enough information about them or I was too lazy to find
anything.  If somebody has suggestions what to do with them, please let me
know.

Additions, corrections, etc. are welcome to the list.  Thank you all who
helped me.


Computer Graphics, Geometry, Image Processing Journals
======================================================

ACM Transactions On Graphics
----------------------------
   -Quarterly
   -Available from:
    Association For Computing Machinery
    11 West 42 Street
    New York, NY 10036
    USA

Computer Aided Design
---------------------
   -10 issues / year
   -Available from:
    Butterworth Scientific Ltd.
    88 Kingsway
    London WC2 6AB
    England

Computer Graphics
-----------------
   -Includes SIGGRAPH conference proceeding
   -Includes SIGGRAPH panel proceeding
   -Available from:
    Association For Computing Machinery
    11 West 42 Street
    New York, NY 10036
    USA
   -Also available from:
    Academic Press

Computer Graphics Forum
-----------------------
   -Journal of the European Association of Computer Graphics
    (EuroGraphics)
   -Available from:
    Elsevier Science Publishers B.V.
    Journals Department
    P.O. Box 211
    1000 AE Amsterdam
    The Netherlands

Computer Graphics World
-----------------------
   -Professional graphical application and industrial use
   -Monthly
   -Available from:
    PennWell Publishing Company
    119 Russell Street
    P.O. Box 457
    Littleton, Massachusetts 01460-0457
    USA

Computer Images International
-----------------------------
   -Professional graphical application and industrial use
   -Monthly
   -Available from:
    P.O. Box 109
    Maclaren House
    Scarbrook Road
    Croydon CR9 1QH
    England
    Phone: [phone]
    Telex: 946665
    Fax:   [phone]

Computers & Graphics
--------------------
   -Available from:
    Oxword Books (GB)

Computer Vision, Graphics and Image Processing
----------------------------------------------
   -Available from:
    Academic Press
---> [split 1991]

Computer Vision, Graphics and Image Processing:
 Graphical Models and Image Processing
-----------------------------------------------
   -Available from:
    Academic Press

Computer Vision, Graphics and Image Processing:
 Image Understanding
-----------------------------------------------
   -Available from:
    Academic Press

Graphics Interface
------------------
   -Proceedings of Canadian computer graphics convention
   -Available from in Canada:
    Canadian Information Processing Society
    243 College Street, 5th Floor
    Toronto, Ontario
    M5T 2Y1
    (416)-593-4040
   -Available from in USA:
    Morgan Kaufmann Publishers
    Order Fulfillment Center
    PO Box 50490
    Palo Alto, CA 94303
    USA
    (415)-965-4081

IEEE Computer Graphics & Applications
-------------------------------------
   -Monthly
   -Available from:
    IEEE Computer Society
    Circulation Department
    P.O. Box 3014
    Los Alamitos, CA 90720-9804
    USA

Image and Vision Computing
--------------------------
   -Image analysis
   -Available from:
    Butterworth Scientific Ltd.
    88 Kingsway
    London WC2 6AB
    England

Pixel
-----
   -Available from:
    Pixel Communications, Inc.
    245 Henry St., Suite 2-G
    Brooklyn, NY 11201
    USA
    Phone: [phone]

Signal Processing: Image Communication
--------------------------------------
   -A publication of the European Association for Signal Processing
    (EURASIP)
    EURASIP
    P.O. Box 134
    CH-1000 Lausanne 13
    Switzerland
   -Also available from:
    Elsevier Science Publishers B.V.
    Journals Department
    P.O. Box 211
    1000 AE Amsterdam
    The Netherlands

Visual Computer
---------------
   -Available from:
    Springer-Verlag
    Heidelberger Platz 3
    D-1000 Berlin 33
    Germany
   -Also available from:
    Springer-Verlag New York, Inc.
    Journal Fulfillment Services, Dept. J
    P.O. Box 2485
    Secaucus, N.J. 07094
    USA

******************************************************************************

Miscellaneous Journals
======================

Leonardo
--------
   -Journal of the International Society for the Arts, Sciences &
    Technology
   -SIGGRAPH art show catalogue
   -Available from:
    Pergamon

******************************************************************************

What?
=====

IEEE Trans. on Pattern Analysis and Machine Intelligence,

IEEE Trans. on Systems, Man, and Cybernetics

Journal of the Discrete and Computational Geometry

Machine Vision and Applications

Pattern Recognition
   -Available from:
    Pergamon

Pattern Recognition Letters
   -Available from:
    North-Holland

Symposium on Computational Geometry

Symposium on Foundation of CS
   -One or two papers a year on geometry

Symposium on Theory of Computing

-------------------------------------------------------------------------------
END OF RTNEWS