Graphics Plus/Graphics Plus.iso/formats/hdf/hdfs.apa

Text File | 1980-02-06 | 19.3 KB | 713 lines

Appendix  A    NCSA HDF Tags




Overview

This appendix contains a complete description of HDF tags that
have been assigned at NCSA as of February, 1989. Most of these
tags are represented by two- or three-character uppercase names,
such as ND or FID. In the NCSA software, the identifiers that refer
to these names are preceded by DFTAG_ and are defined in the
files df.h and dfF.h, which are listed in Appendix B.

These listings are grouped approximately according to the roles
that the tags play under the headings Utility Tags, Raster-8 Tags,
General Raster Image Tags, and so forth. These groupings imply
a general context for the use of each tag, but are not meant to restrict
the use of the tags to any particular context.


Utility Tags

ND
No data
0 bytes
(xxx)

This tag is used for place holding and to fill empty portions of the
data descriptor block. The length and offset fields of a ND DD must
be equal to zero.


FID
File identifier
string
(100)

This tag points to a string which the user wants to associate with
this file. The string is intended to be a user-supplied title for the
file.



FD
File descriptor
text
(101)

This tag points to a block of text describing the overall file
contents. The text can be any length, in the standard text format for
HDF. Text is intended to be user-supplied comments about the file.


TID
Tag identifier
string
(102)

The data for this tag is a string that identifies the functionality of
the tag indicated in the reference number. For example, the tag
identifer for tag identifier would point to data that reads "tag
identifier". The reference number for the tag identifier is another
tag number.

Many tags are identified in the HDF specification, so it is usually
unnecessary to include their identifiers in the HDF file. But with
user-defined tags or special-purpose tags, the only way for a
human reader to diagnose what kind of data is stored in a file is to
read tag identifiers. Use tag descriptors to define even more detail
about your user-defined tags.

Note that with this tag you may make use of the user-defined tags to
check for consistency. Although two persons may use the same
user-defined tag, they probably will not use the same tag identifier.


TD
Tag descriptor
text
(103)

The data for this tag is a text block which describes in relative
detail the functionality and format of the tag which is indicated in
the reference number. This tag is mainly intended to be used with
user-defined tags and provides a medium for users to exchange
files that include human-readable descriptions of the data.

It is important to provide everything that a programmer might
need to know to read the data from your user-defined tag. At the
minimum, you should specify everything you would need to know
in order to retrieve your data at a later date if the original program
were lost.



DIL
Data identifier label
string
(104)

The data for this tag is a data identifier, made up of a tag and
reference number, followed by a string that the user wants to place
in the file. The purpose of this tag is to associate the string with the
data identifier as a label for whatever that data identifier points to
in turn.

With DIL, any data identifier can be labeled. Each data identifier
points to some data in the file. By including DILs, you can give
any piece of data a label for future reference. For example, DIL is
often used to give titles to images.


DIA
Data identifier annotation
text
(105)

The data for this tag is a data identifier, which is made up of a tag
and a reference number, followed by a text block that the user
wants to place in the file. Its purpose is to associate the text block
with the data identifier as annotation for whatever that data
identifier points to in turn.

With DIA, any data identifier can have a lengthy, user-written
description of why that data is in the file. This will be used to
include user comments about images, data sets, source code, and so
forth.


NT
Number type
4 bytes
(106)

NT consists of four fields of 1 byte each, as shown in Table A.1.

Table A.1    Number Type Fields
Field    Contents
VERSION    version number of NT information, currently=1
TYPE    unsigned int, signed int, unsigned char, char, float,
double
WIDTH    number of bits (assumed all significant)
CLASS    a generic value, with different interpretations
depending on type:  floating point, integer, or
character


Some possible values that may be included for each of the three
types in the field CLASS are listed in Table A.2.

Table A.2    Number Type
Values
Type    Possible Values
floats     IEEE floating point, VAX floating point, CRAY
floating point
ints    VAX byte order, Intel byte order, Motorola byte order
chars    ASCII, EBCDIC


The number type flag is used by any other element in the file to
indicate specifically what a numeric value looks like. Other tag
types should contain a reference number pointer to an NT tag
instead of containing their own number type definitions.

If an MT element is present in the file, then all NTs can be
assumed to be of the appropriate default types for that machine,
unless required by definition.

The definition of NT has brought up a unique situation where the
definition of a generic, always applicable, set of fields is
considered too difficult for implementation in the early versions.
Therefore, we are defining version 1 of the NT tag which contains
only four fields. We have a draft of the version 2 tag definition
which starts with the same four fields and adds fields that can
define types of numbers not currently listed.

Version 1 NT implementations should check the version number
field only to confirm it, and must always write a 1 into this field.
Version 2 implementations will have to be backward compatible in
as many cases as possible.


MT
Machine type
0 bytes
(107)

The MT tag specifies that all unconstrained or partially
constrained values in this HDF file are of the default type for that
hardware. When the MT tag is set to VAX, for example, all
integers will be assumed to be in VAX byte order unless
specifically defined otherwise with an NT tag. Note that all of the
headers and many tags, the whole raster-8 set for example, are
defined with bit-wise precision and will not be overidden by the
MT setting.

For MT, the reference field itself is the encoding of the MT
information. The reference field is 16 bits, taken as four groups of
four bits, specifying the types for unsigned char, unsigned int,
float, and double respectively. This allows 16 generic
specifications for each type.

To the user, these will be defined constants in df.h, specifying the
proper descriptive numbers for Sun, VAX, CRAY, Alliant, and
other computer systems. If there is no MT tag in a file, the
application may assume that the data in the file has been written on
the local machine╤assuming any portability problems are taken
care of by the user. For this reason, we recommend that all HDF
files contain an MT tag for maximum portability.

Possible machine types are shown in Table A.3.

Table A.3    Possible Machine
Types
Type    Possible Machines
floats    IEEE32, VAX32, CRAY64
ints    VAX32, Intel16, Intel32, Motorola32, CRAY64
chars    ASCII, EBCDIC
double    IEEE64, VAX64, CRAY128

Obviously, each of these is extensible as we find a need for new
types.


RLE
Run length encoded data
0 bytes
(11)

This tag is used in the ID compression field and other places to
indicate that an image or section of data is encoded with a run-
length encoding scheme. The RLE method used is byte-wise. The
low seven bits of the count byte indicate the number of bytes (n).
The high bit of the count byte indicates whether the next byte should
be replicated n times (high bit=1), or whether the next n bytes should
be included as is (high bit=0).

See also:  RIG (Raster Image Group)


IMC
IMCOMP compressed data
0 bytes
(12)

This tag is used in the ID compression field and other places to
indicate that an image or section of data is encoded with an
IMCOMP encoding scheme. This scheme is a 4:1 aerial averaging
method which is easy to decompress. It counts color frequencies in
4x4 squares to optimize color sampling.

See also:  RIG (Raster Image Group)



Raster-8 (8-Bit Only) Tags

ID8
Image dimension-8
4 bytes
(200)

The data for this tag consists of two 16-bit integers representing the
width and height of an 8-bit raster image in bytes.


IP8
Image palette-8
768 bytes
(201)

The data for this tag consists of 256 triples of three bytes. The bytes
are for the red, green, and blue elements of the 256-byte palette
respectively. The first triple is palette entry 0 and the last is palette
entry 255.


RI8
Raster image-8
mxn bytes
(202)

The data for this tag is a row-wise representation of the elementary
8-bit image data. The data is stored width-first (hence row-wise)
and is 8-bits per pixel. The first byte of data represents the pixel in
the upper-left hand corner of the image.


CI8
Compressed image-8
? bytes
(203)

The data for this tag is a row-wise representation of the elementary
8-bit image data. Each row is compressed using the following run-
length encoding where n is the low seven bits of the byte. The high
bit represents whether the following n character will be reproduced
exactly (high bit=0) or whether the following character will be
reproduced n times (high bit=1). Since CI8 and RI8 are basically
interchangeable, it is suggested that you not have a CI8 and a RI8
that have the same reference number.


II8
IMCOMP image-8
? bytes
(xxx)

The data for this tag is a 4:1 compressed 8-bit image, using the
IMCOMP compression scheme.


General Raster Image Tags

RIG
Raster image group
n*4 bytes
(306)

The raster image group (RIG) data is a list of data identifiers
(tag/ref) that describe a raster image. All of the members of the
group are required to display the image correctly. Application
programs that deal with RIGs should read all the elements of a RIG
and process those identifiers which it can display correctly. Even
if the application cannot process all of the tags, the tags that it can
process will be displayable.

Tag types that may appear in a RIG are listed in Table A.4.

Table A.4    Possible Tag Types
in an RIG
Tag    Description
ID    image dimension
RI    raster image
XYP    X-Y position
LD    LUT dimension
LUT    color lookup table
MD    matte channel dimension
MA    matte channel
CCN    color correction
CFM    color format
AR    aspect ratio
MTO    machine-type override


Example
ID,RI,LD,LUT
An image dimension record, the raster image, an LUT dimension
and the LUT go together. The application reads the image
dimensions, then reads the image with those dimensions. It also
reads the lookup table according to its dimensions and displays the
corresponding image.


ID, LD, MD
Image dimension
20 bytes
(300)

LUT dimension
20 bytes
(307)

Matte dimension
20 bytes
(308)

These three dimension records have exactly the same format.
They define the dimensions of the 2D array to which each refers.
ID specifies the dimensions of an RI tag, LD specifies the
dimensions of an LUT tag, and MD specifies the dimensions of a
MA tag. The fields are defined as shown in Table A.5.

Table A.5    Dimension Record
Fields
Field    Definition
X-dimension (width)    32 bit integer
Y-dimension (height)    32 bit integer
NT/ref (element type)    tag and ref=32 bits
Elements per node    16 bit integer
Interlace scheme    16 bit integer (0,1 or 2)
Compression tag    tag and ref=32 bits


For example, a 512x256 row-wise 24-bit raster image with each
pixel stored as RGB bytes would have the following values:

X:512, Y:256

In this case, NT specifies an 8-bit integer. There are three
elements per node╤one each for red, green, and blue.

Interlace=0 indicates that the RGB values are not separated.
Compression=0 means no compression scheme is used.


RI
Raster image
x*y bytes
(302)

This tag points to raster image data. It must be stored as specified
in an ID tag.

Interlace=0 means each pixel is contiguous
Interlace=1 means each element is grouped by scan lines
Interlace=2 means each element is grouped by planes


LUT
Lookup table
? bytes
(301)

The LUT, sometimes called a palette, is used by many kinds of
hardware to assign RGB colors or HSV colors to data values.
When a raster image consists of data values which are going to be
interpreted through hardware with a LUT capability, the LUT
should be loaded along with the image.

The most common lookup table will have X dimension=256 and Y
dimension=1 with three elements per entry, one each for red,
green, and blue. The interlace will be either 0, where the LUT
values are given RGB, RGB, RGB . . . , or 1, where the LUT values
are given as 256 reds, 256 greens, 256 blues.



CCN
Color correction
52 bytes (usually)
(310)

Color correction specifies the Gamma correction for the image and
color primaries for the generation of the image. The fields, in
order, are shown in Table A.6.

Table A.6    Color Correction
Field Types
Correction    Field Type
Gamma    float
Red (X,Y,Z)    three floats
Green (X,Y,Z)    three floats
Blue (X,Y,Z)    three floats
White (X,Y,Z)    three floats
CFM
Color format
string
(311)

The color format is a clue to how each element of each pixel in a
raster image. It is defined to be a string which is in all caps, and is
one of the values shown in Table A.7.

Table A.7    Color Format String
Values
String    Description
VALUE    psuedo-color, or just a value associated with the
pixel
RGB    red, green, blue model
XYZ    color-space model
HSV    hue, saturation, value model
HSI    hue, saturation, intensity
SPECTRAL    spectral sampling method


AR
Aspect ratio
4 bytes
(312)

The data for this tag is the visual aspect ratio for this image. The
image should be visually correct if displayed on a screen with this
aspect ratio. The data consists of one floating point number which
represents width divided by height. An aspect ratio of 1.0 indicates
a display with perfectly square pixels. 1.33 is a standard aspect
ratio used by many monitors.



Composite Image Tag

DRAW
Draw
n*4 bytes
(400)

The data for this tag is a list of data identifiers (tag/ref) which
define a composite image. Each member of the DRAW data should
be displayed, in order, on the screen. This can be used to indicate
several RIGs which should be displayed simultaneously, or even
include vector overlays, like T14, which should be placed on top of a
RIG.

Some of the elements in a DRAW list will be instructions about
how images are to be composited (XOR, source put, anti-aliasing,
etc.). These are defined as individual tags.

See also:

RIG    (Raster image display)
T14    (Tektronix 4014 for overlay)


Composite Raster Image Tag

XYP
XY position
8 bytes
(500)

An XY position is used in composites and other groups to indicate
an XY position on the screen. For this, (0,0) is the lower left, X is the
number of pixels to the right along the horizontal axis and Y is the
number of pixels on the vertical axis. The X and Y pixel
dimensions are given as two 32-bit integers.

For example, if XYP is present inside an RIG, the XYP refers to the
position of the lower left corner of the raster image on the screen.

See also:  DRAW (Drawing from a list of elements)



Vector Image Tags

T14
Tektronix 4014
n bytes
(602)

This tag points to a Tektronix 4014 data stream. The bytes in the
data field, when read and sent to a Tektronix 4014 terminal, will
display a vector image. Only the low seven bits of each byte are
significant. There are no record markings or non-Tektronix
codes in the data.


T105
Tektronix 4105
n bytes
(603)

This tag points to a Tektronix 4105 data stream. The bytes in the
data field, when read and sent to a Tektronix 4105 terminal, will
be displayed as a vector image. Only the low seven bits of each byte
are significant. Some terminal emulators will not correctly
interpret every feature of the Tektronix 4105 terminal, so you may
wish to use only a subset of the possible Tektronix 4105 vector
commands.


Scientific Data Set Tags

SDG
Scientific data group
n*4 bytes
(700)

The scientific data group (SDG) data is a list of data identifiers
(tag/ref) that describe a scientific data set. All of the members of
the group provide information for correctly interpreting and
displaying the data. Application programs that deal with SDGs
should read all of the elements of a SDG and process those
identifiers which it can use. Even if an application cannot process
all of the tags, the tags that it can use will be displayable.

Tag types that may appear in a SDG are listed in Table A.8.

Figure A.8    Possible Tag Types
in an SDG
Tag    Description
SDD    scientific data dimension record (rank and
dimensions)
SD    scientific data
SDS    scales
SDL    labels
SDU    units
SDF    formats
SDM    maximum and minimum values
SDC    coordinate system
SDT    transposition


Example
SDD, SD, SDM
A dimension record, the scientific data, and the maximum and
minimum values of the data go together. The application reads the
rank and dimensions from the dimension record, then reads the
data array with those dimensions. If it needs maximum and
minimum, it also reads them.


SDD
Scientific data dimension record
16 + 4*rank bytes
(701)

This record defines the rank and dimensions of the array in the
scientific data set. The fields are defined as shown in Table A.9.

Table A.9    Scientific Data
Dimension Record
Fields
Field    Definition
Rank    16 bit integer
Dimension sizes    32 bit integers
Data NT (number type)    4 bytes
Scale NTs    rank*4 bytes

For example, an SDD for a 500x600x3 array of floating point
numbers would have the following values and components.

Rank:  3
Dimensions:  500, 600, and 3.
One data NT
Three scale NTs


SD
Scientific data
4*x*y*z*... bytes (x, y, z, etc. are the dimensions)
(702)

This tag points to an array of scientific data. The type of the data
may be specified by an NT tag included with the SDG. If there is no
NT tag, the type of the data is floating point in standard IEEE 32-bit
format. The rank and dimensions must be stored as specified in
the corresponding SDD tag.


SDS
Scientific data scales
n+4*(x+1+y+1+z+1+...) bytes
(703)

This tag points to the scales for the data set. The first n bytes
indicate whether there is a scale for the corresponding dimension
(1=yes, 0=no). This is followed by the scale values for each
dimension.


SDL
Scientific data labels
? bytes
(704)

This tag points to a list of labels for the data and each dimension of
the data set. Each label is a string terminated by a null byte.


SDU
Scientific data units
? bytes
(705)

This tag points to a list of strings specifying the units for the data
and each dimension of the data set. Each unit's string is
terminated by a null byte.


SDF
Scientific data format
? bytes
(706)

This tag points to a list of strings specifying an output format for
the data and each dimension of the data set. Each format string is
terminated by a null byte.


SDM
Scientific data max/min
8 bytes
(707)

This record contains the maximum and minimum data values in
the data set. It consists of two floating point numbers.


SDC
Scientific data coordinates
? bytes
(708)

This tag points to a string specifying the coordinate system for the
data set. The string is terminated by a null byte.
SDT
Scientific data transpose
0 bytes
(709)

The presence of this tag indicates that the data pointed to by the SD
tag is in column-major order, instead of the default row-major
order. No data is associated with this tag.

A.1    NCSA HDF Specifications

NCSA HDF Tags    A.1

National Center for Supercomputing Applications

March 1989



February 1989