A.1 NCSA HDF Calling Interfaces and Utilities NCSA HDF Tags A.1 National Center for Supercomputing Applications March 1993 March 1993 Appendix A NCSA HDF Tags Overview This appendix includes a Table containing brief descriptions of most of the tags that have been assigned at NCSA for general use. The list is growing and will be expanded in future editions of the documentation. More detailed descriptions of the tags can be found in an appendix of the NCSA HDF Specifications. To obtain a draft of this document, contact the NCSA Software Development Group at the address listed on the README page at the beginning of this manual. Tag Types and Descriptions The following table has five columns. The Tag column gives the abbreviated symbolic name of the tag that is often used in an augmented form in HDF programs. The Name column gives a name commonly used in human-readable media. The Data Type column describes the type of data that is associated with the tag, and where possible, gives the amount of data. The Number column gives the actual numeric value that is stored in the file to represent the tag. The Use column describes the tag in brief and general terms. In the table, the term string refers to a sequence of ASCII characters, with a zero byte possibly occurring at the end, but nowhere else. The term text likewise refers to a sequence of ASCII characters, but it may contain zero bytes elsewhere than at the end. Table A.1 HDF Tags Tag Name Data type Number Use Utility Tags DFTAG_NULL No Data none 001 used for place holding and filling up empty portions of the Data Descriptor Block. DFTAG_VERSION Library 4 bytes 030 specifies version of latest version of version number + string HDF library used to write to the file. DFTAG_NT Number Type 4 bytes 106 used by any other element in the file to indicate specifically 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. It is intended to be 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 for a human reader to tell the meaning of a tag stored in a file. DFTAG_TD Tag Descriptor text 103 similar to TID, but allows more extensive text to be included. DFTAG_DIL Data Identifier Label string 104 associates the string with the Data Identifier as a label for whatever that Data Identifier points to in turn. By including DILs, any piece of data can be given a label for future reference. For example, this is often used to give titles to images. DFTAG_DIA Data Identifier text 105 associates the text block with the Data Annotation 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. DFTAG_RLE Run length encoding 0 bytes 011 specifies that run length encoding is used to compress an image. DFTAG_IMC IMCOMP 0 bytes 012 specifies that IMCOMP compression is Compression used to compress an image. Table A.1 HDF Tags (Continued) DFTAG_JPEG 24-bit JPEG ? bytes 013 provides header information for 24-bit Compression JPEG compressed images DFTAG_GREYPEG 8-bit JPEG ? bytes 014 provides header information for 8-bit Compression JPEG compressed images General Raster Image Tags DFTAG_RIG Raster Image Group n*4 bytes 306 lists the Data Identifiers (tag/ref) that describe a raster image set. DFTAG_ID Image Dimension 20 bytes 300 defines the dimensions of the 2D array that its corresponding RI tag refers to. DFTAG_LD LUT Dimension 20 bytes 307 defines the dimensions of the 2D array that its corresponding LUT tag refers to. DFTAG_MD Matte Dimension 20 bytes 308 defines the dimensions of the 2D array that its corresponding MA tag refers to. DFTAG_RI Raster Image x*y bytes 302 points to raster image data. DFTAG_CI Compressed Image n bytes 303 points to a compressed raster image. DFTAG_LUT Look Up Table n bytes 301 table to be used by hardware to assign RGB colors 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 image and color primaries for the generation of the image. DFTAG_CFM Color Format string 311 indicates what interpretation should be given to each element of each pixel in a raster image. DFTAG_AR Aspect Ratio 4 bytes 312 indicates the visual aspect ratio for this image. Composite Image Tag DFTAG_DRAW Draw n*4 bytes 400 identifies a list of Data Identifiers (tag/ref) which define a composite image. Tag for Composite or Raster Images DFTAG_XYP XY Position 8 bytes 500 indicates an XY position on the screen for composites and other groups. Table A.1 HDF Tags (Continued) DFTAG_RUN Run n bytes 401 identifies code that is to be executed as a program or script. Vector Image Tags DFTAG_T14 Tektronix 4014 n bytes 602 points to a Tek 4014 data stream. The bytes in the data field, when read and sent to a Tektronix 4014 terminal, will be displayed as a vector image. DFTAG_T105 Tektronix 4105 n bytes 603 points to a Tek 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. Scientific Data Set Tags DFTAG_NDG Numeric Data Group n*4 bytes 720 lists the Data Identifiers (tag/ref) that describe a scientific dataset. Supersedes DFTAG_SDG. DFTAG_SDD SDS n bytes 701 defines the rank and dimensions of the Dimension Record array that the corresponding SD refers to. DFTAG_SD Scientific Data reals 702 points to scientific data. DFTAG_SDS Scales reals 703 shows scales to be used when interpreting and displaying data. DFTAG_SDL Labels strings 704 labels for all dimensions and for the data. DFTAG_SDU Units strings 705 displays units for all dimensions and for the data. DFTAG_SDF Formats strings 706 formats for displaying axes and data. DFTAG_SDM Max/min 2 reals 707 shows maximum and minimum values for the data. DFTAG_SDC Coordinate system string 708 shows coordinate system to be used to interpret data. DFTAG_SDLNK SDS link 8 bytes 710 links together an old-style DFTAG_SDG and a DFTAG_NDG in cases where the NDG meets all criteria for an SDG. DFTAG_CAL Calibration info 36 bytes 731 a calibration record for the associated DFTAG_SD. Table A.1 HDF Tags (Continued) DFTAG_FV Fill value ? bytes 732 value which has been used to indicate unset values in the associated DFTAG_SD. VsetTags DFTAG_VG Vgroup 14+? bytes 1965 provides general purpose grouping structure DFTAG_VH Vdata description 22+? bytes 1962 provides information necessary to process a DFTAG_VS DFTAG_VS Vdata ? bytes 1963 contains a block of data that is to be interpreted according to the information in the corresponding DFTAG_VH Obsolete Tags DFTAG_ID8 Image Dimension-8 4 bytes 200 two 16-bit integers representing 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-wise representation of the elementary 8-bit image data. DFTAG_CI8 Compressed Image-8 n bytes 203 a row-wise representation of the elementary 8-bit image data. Each row is compressed using a form of run-length encoding. DFTAG_II8 IMCOMP Image-8 n bytes 204 a 4:1 compressed 8-bit image, using the IMCOMP compression scheme. DFTAG_SDG Scientific Data Group n*4 bytes 700 lists the Data Identifiers (tag/ref) that describe a scientific dataset. DFTAG_SDT Transpose 0 bytes 709 indicates that data is transposed in the file.