A Lightwave to Open Inventor 3D object converter
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This directory contains a fairly simplistic 3D object conversion program to
convert from the Lightwave 3D object file format into Open Inventor 2.0 files.
It was developed on an SGI, and the source will only compile on a machine with
C++ and the Open Inventor developer's option installed. For those people
without C++ or the Open Inventor developer's option, I have included an
executable that should run on any SGI machine running IRIX 5.3 that supports
OpenGL and Open Inventor.
USAGE
The Lightwave object file name is read from the command line, and the
Open Inventor file is output to stdout.
So to create an ascii Open Inventor object from an existing Lightwave file,
just type something like:
lwtoiv lightwave_file >inventor_file
I created this converter so I could use my SGI at work to preview many of
the lightwave objects I download from various ftp sites on the internet. So
to quickly preview a Lightwave object, try this:
lwtoiv lightwave_file | ivview
or
lwtoiv lightwave_file | /usr/demos/Inventor/gview
Both of these commands will pipe the output to an Open Inventor viewer that
will allow you to translate, rotate, or zoom in and out of the object. The
second command will also show you the hierarchical node structure of the
resulting Open Inventor object, and allow you to edit some parts of the
structure or individual fields in each node of the structure.
LIMITATIONS / PROBLEMS / BUGS
As I mentioned earlier, this is just a simplistic converter used mainly for
previewing Lightwave objects before I take them home to my trusty old Amiga.
For that reason, many features found in a Lightwave file are ignored or are
just not converted correctly into Open Inventor format. Here is a list of
most of the limitations of this converter.
All textures and reflection maps are ignored. Maybe someday I'll add the
ability to handle reflection maps and color textures.
The shading model used by Lightwave is slightly different than that used
by Inventor. The converter uses some of my own guesses as to how to
convert between the color, diffuse, specular, glossiness, luminance, and
transparency parameters of Lightwave to the diffuse, ambient, specular,
emissive, shininess, and transparency parameters of Inventor.
For some objects, the default smoothing angle of Lightwave (89.5 degrees
which is 1.56207 radians) causes objects to look strange (too smoothed??)
when used as Inventor's crease angle. You can often change the crease
angle in the Inventor nodes to improve the appearance of smoothed objects.
The program gview (mentioned above in the USAGE section) is an excellent
tool for experimenting with different crease angles (look for the
creaseAngle field in the ShapeHints node). Remember, the crease angle
in Inventor should be specified in radians.
Inventor does not handle the wide variety of polygon shapes allowed by
Lightwave. By default, OpenGL (the software interface upon which the Open
Inventor toolkit is based) assumes polygons must be convex. OpenGL does
provide some utility routines that Open Inventor uses to try and tesselate
concave poygons into groups of convex poygons, but these tesselation
routines still can not cope with all the polygon shapes allowed in Lightwave.
Here is a brief rundown on how I handle (or ignore) these problems:
a) I test for repeated vertices in a polygon, and if found, create a
polyline node instead of a polygon and output a message to stderr that
informs the user as to which surface(s) contains polylines. I output
this message so that it is easy for an experienced Inventor user to
search for the polyline occurences in the Inventor file, and then
edit the file by hand to try and rectify this situation (for most
occurences of this problem I have only had to exert a small amount
of manual effort to convert the SoIndexedLineSet node back into the
appropriate SoIndexedFaceSet node). The following diagram depicts
a common situation in Lightwave objects in which this occurs (a
polygon with a hole):
***see the readme file for the illustration***
b) The other situation that Inventor does not like is when a vertex lies
on an existing edge of the polygon (self-intersecting polygon). I do
nothing to test for this situation, and Inventor outputs its own error
message, then does not create the polygon. Here are 2 diagrams of
polygons in which I have seen this problem:
***see the readme file for the illustration***
These problems could be solved by an experienced Inventor user in the
same way as problem a), but it was not as simple for me to test for
this problem. Since I do not test for this situation, Inventor finds
the problem and just skips over the offending polygon altogether, no
polyline node is created to help rectify the situation. The offending
polygon still exists in the Inventor file, so if it can be found it
can be corrected (its just not as easy to find the offending polygon
as it is when it is seperated into its own polyline node).
Well, that's the major problems with this converter, I'm sure there are others
I don't know about or just forgot to mention here. If you find other problems
or just have any suggestions about the converter, please feel free to contact
me. I am always glad to hear any suggestions or criticisms, just realize that
I may not have the time or the inclination to do anything else to this code,
since it does the majority of the things I created it for.
Marvin Landis
marvinl@amber.rc.arizona.edu