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Appendix A NCSA HDF Tags

A.1 Overview

This appendix includes tables containing brief descriptions of most of the tags that have been assigned at NCSA for general use. This list will be expanded in future editions to include new tags as they are assigned. A more detailed description of the tags can be found in the HDF Specifications manual.

Each table contains a list of tags within one category. The titles of the tables, with a functional description of each table, are:

A.2 Tag Types and Descriptions

The following tables have five columns: the "Tag Name" column contains the abbreviated symbolic names of tags that are often used in an augmented form in HDF programs, the "Short Description" column contains a brief (four word maximum) description of the tag that is commonly used to describe to the tag in HDF manuals and in-line code documentation, the "Data Size" column describes the type of data that is associated with the tag and, where possible, lists the data size, the "Number" column list the numeric value of the tag symbol in the "hdf.h" header file, and the "Long Description" column contains a general description of the tag. The "Data Type" column contains a generalized description of the type of data referred to by the tag.

In the tables, any entry specified as "String" refers to a sequence of ASCII characters with the null byte possibly occurring at the end, but nowhere else. Any entry specified as "Text" also refers to a sequence of ASCII characters, but it may contain zero bytes anywhere in the sequence. An entry in the "Data Type" specified by an "n" refers to a data unit of variable-length. For a more detailed description of these units of data, refer to the appropriate tag entry in the HDF Specification Manual.

TABLE AA - The HDF Utility Tags

Tag Name
Short Description
Data Size
Tag Value
Long Description
DFTAG_NULL No Data

None

001
Used for place holding and filling up empty portions of the Data Descriptor Block.

DFTAG_VERSION Library Version Number

4 bytes + string

030
Specifies the latest version of the HDF library used to write to the file.

DFTAG_NT Number Type

4 bytes

106
Used by any other element in the file to specifically indicate what a numeric value looks like.

DFTAG_MT Machine Type

0 bytes

107
Specifies that all unconstrained or partially constrained values in this HDF file are of the default type for that hardware.

DFTAG_FID File Identifier

String

100
Points to a string that the user wants to associate with this file. This supports the inclusion of a user-supplied title for the file.

DFTAG_FD File Descriptor

Text

101
Points to a block of text describing the overall file contents. It is intended to be user-supplied comments about the file.

DFTAG_TID Tag Identifier

String

102
Provides a way to determine the meaning of a tag stored in the file.

DFTAG_TD Tag Descriptor

Text

103
Similar to DFTAG_TD, but allows more text to be included.

DFTAG_DIL Data Identifier Label

String

104
Associates the string with the Data Identifier as a label for whatever the identifier points to. By including DILs, any data element can be given a label for future reference. For example, this tag is often used to give titles to raster image data sets.

DFTAG_DIA Data Identifier Annotation

Text

105
Associates the text block with the Data Identifier as an annotation for whatever that Data Identifier points to. With DIAs, and Data Identifier can have a lengthy, user-provided description of why that particular data element is in the file.

DFTAG_RLE Run-length Encoding

0 bytes

011
Specifies that run-length encoding (RLE) is used to compress a raster image.

DFTAG_IMC IMCOMP

Compression

0 bytes

012
Specifies that IMCOMP compression is used to compress a raster image.

DFTAG_JPEG 24-bit JPEG

Compression

n bytes

013
Provides header information for 24-bit JPEG-compressed raster images.

DFTAG_GREYPEG 8-bit JPEG

Compression

n bytes

014
Provides header information for 8-bit JPEG-compressed raster images.

TABLE AB - The HDF General Raster Image Tags

Tag Name
Short Description
Data Size
Tag Value
Long Description
DFTAG_RIG Raster Image Group

n*4 bytes

306
Lists the Data Identifiers (tag/reference number pairs) that uniquely describe a raster image set.

DFTAG_ID Image Dimension

20 bytes

300
Defines the dimensions of the two-dimensional array the corresponding RI tag refers to.

DFTAG_LD LUT Dimension

20 bytes

307
Defines the dimensions of the two-dimensional array the corresponding LUT tag refers to.

DFTAG_MD Matte Dimension

20 bytes

308
Defines the dimensions of the two-dimensional array the corresponding MA tag refers to.

DFTAG_RI Raster Image

x*y bytes

302
Points to a raster image data set.

DFTAG_CI Compressed Image

n bytes

303
Points to a compressed raster image data set.

DFTAG_LUT Lookup Table

n bytes

301
Table to be used by the hardware for the purpose of assigning RGB or HSV colors to data values.

DFTAG_MA Matte Data

n bytes

309
Points to matte data.

DFTAG_CCN Color Correction

n bytes

310
Specifies the gamma correction for the raster image and color primaries used in the generation of the image.

DFTAG_CFM Color Format

String

311
Indicates the interpretation to be given to each element of each pixel in a raster image.

DFTAG_AR Aspect Ratio

4 bytes

312
Indicates the aspect ratio of the image.

DFTAG_XYP XY Position

8 bytes

500
Specifies the screen X-Y coordinate for raster image sets. (Also used for composite image sets - See the entry for DFTAG_XYP in Table 12.6)

TABLE AC - The HDF Composite Image Tags

Tag Name
Short Description
Data Size
Tag Value
Long Description
DFTAG_DRAW Draw

n*4 bytes

400

Specifies a list of Data Identifiers (tag/reference number pairs) which define a composite image.

DFTAG_XYP XY Position

8 bytes

500

Specifies the screen X-Y coordinate for composite image sets. (Also used for raster image sets - See the entry for DFTAG_XYP in Table 12.5)

DFTAG_RUN Run

n bytes

401

Identifies code that is to be executes as a program or script.

DFTAG_T14 Tektronix 4014

n bytes

602

Used as a vector image tag. Points to a Tektronix 4014 data. The bytes in the data field, when read and sent to a Tektronix 4014 terminal, will be displayed as a vector image.

DFTAG_T10S Tektronix 4015

n bytes

603

Used as a vector image tag. Points to a Tektronix 4015 data. The bytes in the data field, when read and sent to a Tektronix 4015 terminal, will be displayed as a vector image.

TABLE AD - The HDF Scientific Data Set Tags

Tag Name
Short Description
Data Size
Tag Value
Long Description
DFTAG_NDG Numeric Data Group

n*4 bytes

720
Lists the Data Identifiers (tag/reference number pairs) that describe a scientific data set. Supersedes DFTAG_SDG.

DFTAG_SDD SDS Dimension Record

n bytes

701
Defines the rank and dimensions of the array the corresponding SD refers to.

DFTAG_SD Scientific Data

Real Number

702
Points to scientific data.

DFTAG_SDS SCales

Real Number

703
Identifies the scales to be used when interpreting and displaying data.

DFTAG_SDL Labels

String

704
Labels all dimensions and data.

DFTAG_SDU Units

String

705
Displays units for all dimensions and data.

DFTAG_SDF Formats

String

706
Displays formats for axes and data.

DFTAG_SDM Maximum/minimum

2 Real Numbers

707
Displays the maximum and minimum values for the data.

DFTAG_SDC Coordinate system

String

708
Displays the coordinate system to be used in interpreting data.

DFTAG_SDLNK SDS Link

8 bytes

710
Links and old-style DFTAG_SDG and a DFTAG_NDG in cases where the DFTAG_NDG meets all criteria for a DFTAG_SDG.

DFTAG_CAL Calibration Information

36 bytes

731
The calibration record for the corresponding DFTAG.SD.

DFTAG_FV Fill Value

n bytes

732
The value which has been used to indicate unset values in the corresponding DFTAG_SD.

TABLE AE - The HDF Vset Tags

Tag Name
Short Description
Data Size
Tag Value
Long Description
DFTAG_VG Vgroup

14+n bytes

1965
Provides a general-purpose grouping structure.

DFTAG_VH Vdata Description

22+n bytes

1962
Provides information necessary to process a DFTAG_VS.

DFTAG_VS Vdata

n bytes

1963
Contains a block a data that is to be interpreted according to the information in the corresponding DFTAG_VH.

TABLE AF - The Obsolete HDF Tags

Tag Name
Short Description
Data Size
Tag Value
Long Description
DFTAG_IDS Image Dimension-8

4 bytes

200
Two 16-bit integers that represent the width and height of an 8-bit raster image in bytes.

DFTAG_IP8 Image Palette-8

768 bytes

201
A 256 x 3 byte array representing the red, green and blue elements of the 256-color palette respectively.

DFTAG_RI8 Raster Image-8

x*y bytes

202
A row-oriented representation of the elementary 8-bit image data.

DFTAG_CI8 Compressed Image-8

n bytes

203
A row-oriented representation of the elementary 8-bit raster image data, with each row compressed using a form of run-length encoding.

DFTAG_II8 IMCOMP Image-8

n bytes

204
A 4:1 8-bit raster image, compressed using the IMCOMP algorithm.

DFTAG_SDG Scientific Data Group

n*4 bytes

700
List the Data Identifiers (tag/reference number pairs) that uniquely describe a scientific data set.

DFTAG_SDT Transpose

0 bytes

709
Indicates that data is transposed in the file.



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