Appendices

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 Specification and Developer's Guide. Also see the HDF Specification and Developer's Guide for a description of extended tags, which are not discussed in this appendix.

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

Table A: The HDF Utility Tags. Used by the HDF utilties.
Table B: The HDF General Raster Image Tags. Used to describe aspects of raster image data.
Table C: The HDF Composite Image Tags. Used to describe aspects of composite image data.
Table D: The HDF Scientific Data Set Tags: Used to describe aspects of scientific data set (SDS) data.
Table E: The HDF Vset Tags. Used to describe aspects of HDF Vset data.
Table F: The Obsolete HDF Tags: Used to describe aspects of HDF data elements that have been replaced by newer tags or discontinued.

A.2 Tag Types and Descriptions

The following tables have five columns:

Tag Name contains the abbreviated symbolic names of tags that are often used in an augmented form in HDF programs.

Short Description 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.

Data Size describes the type of data that is associated with the tag and, where possible, lists the data size.

Tag Value lists the numeric value of the tag symbol in the hdf.h header file.

Long Description contains a general description of the tag.

In the tables, the term String refers to a sequence of ASCII characters with the null byte possibly occurring at the end, but nowhere else. The term Text also refers to a sequence of ASCII characters, but it may contain null characters anywhere in the sequence. An n in the Data Size column describes a data unit of variable-length. For more detailed descriptions of these units of data, refer to the appropriate tag entry in the HDF Specification and Developer's Guide.

TABLE 0A - 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.

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 0B - The HDF General Raster Image Tags

Tag Name Short Description Data Size Tag Value Long Description

DFTAG_RIG Raster Image Group

n4 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

xy 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)

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 0C - 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.

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 0D - 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.

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 0E - 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.

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 0F - 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

xy 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

n4 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.

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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HDF User's Guide - 05/19/99, NCSA HDF Development Group.