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GIFCHECK 1.11 DOCUMENTATION =========================== James W. Birdsall 03/19/93 0. CONTENTS ----------- 0. CONTENTS 1. INTRODUCTION 1.1 Quick Reference 1.2 Copyright, License, and Warranty Disclaimer 2. USAGE 2.1 Targets 2.1.1 Wildcards 2.2 Options 2.2.1 Output redirection options 2.2.2 Output format options 2.2.3 Processing options 2.2.4 Diagnostic options 2.2.5 -# Version 2.2.6 -? Help 3. OUTPUT 3.1 File Header 3.2 Image 3.3 Extension Blocks 3.3.1 Generic extensions 3.3.2 Graphic Control Extension 3.3.3 Plain Text Extension 3.3.4 Comment Extension 3.3.5 Application Extension 3.4 Terminator 4. DIAGNOSTIC MESSAGES 4.1 Anomalies 4.2 Violations 4.3 Fascinatings 4.4 Nitpicks 4.5 Miscellaneous 5. THE END 1. INTRODUCTION --------------- GIFCHECK is a program of the Chiaro suite. It performs a complete analysis and verification of GIF-format images and displays the information it finds. It can also be used in batch files to determine whether GIF files need to be passed through the GIFSTRIP program to remove excess characters. 1.1 Quick Reference ------------------- This is a quick summary of the usage and options of GIFCHECK. Invoking GIFCHECK with the option "-?" will produce a similar summary. Complete explanations may be found in section 2. usage: gifcheck [options] target [target ...] target filename, filename with wildcards, or path. Wildcards are *, ?, and ranges or groups specified in []. A range or group may be complemented with ^ or !. -b BATCH: suppresses all console output. Should be first on the line. Returns ERRORLEVEL 0 if OK, 1 if a non-GIF file was found, 2 if unexpected EOF in a GIF file, 3 on other error, 4 if the GIF is OK but needs to be stripped, and 5 if the. GIF is OK but needs to be stripped with the -m option. -r Sends error messages to stderr instead of stdout. -p Turn off paging of output (paging always off when stdout is redirected or -b BATCH is in effect). -v VERBOSE: turns on verbose output. -h HEX DUMP: switches from ASCII to HEX dump for certain blocks. -c COLOR DUMP: dumps raw RGB values from all colormaps. -l Disables checking for leading junk characters (from a Mac, for example). -f FAST: disables decompression. Only block formatting is checked. -dn SET DISPLAY WARNING LEVEL: 0 displays anomalies and violations 1 (default) displays anomalies, violations, and fascinatings 2 displays anomalies, violations, fascinatings, and nitpicks -en SET EXIT WARNING LEVEL: 0 exits on anomalies only 1 (default) exits on anomalies and violations 2 exits on anomalies, violations, and fascinatings -# VERSION: Prints version information on internal modules, then exits. GIFCHECK automatically reads a list of targets from stdin if stdin is redirected. Targets must be separated by newlines, and are wildcard-expanded in the same way as command-line targets. Note that command-line targets are ignored if stdin is redirected; if stdin is not redirected, there must be at least one target on the command line. Options are case-insensitive, and may not be combined. 1.2 Copyright, License, and Warranty Disclaimer ----------------------------------------------- The Graphics Interchange Format(c) is the Copyright property of CompuServe Incorporated. GIF(sm) is a Service Mark property of CompuServe Incorporated. GIFCHECK is not in the public domain. All the files are copyright 1993 by James W. Birdsall, all rights reserved. The following license applies to the entire Chiaro suite, which is made up of all the files listed in the file MANIFEST. Permission is granted to do the following: You may freely redistribute this archive, so long as it contains all the files listed in the file MANIFEST, intact and unmodified. You may use the programs contained in this archive for a period of 30 days for evaluation purposes. Payment of the $10 shareware fee (which covers all the programs in the Chiaro suite) licenses you to use the Chiaro suite beyond the evaluation period. This license-to-use specifically includes use by bulletin board systems and other commercial or private information services. Registered users will also receive update notices and bug reports, and are entitled to use future versions without further payment. The contents of the distribution archive, and all other related files, information, and services are provided "as is" and without warranty. To the extent permitted by applicable law, the author disclaims all warranties, express or implied, including but not limited to, any implied warranty of merchantability or fitness for a particular purpose. While effort has been made to ensure that the files, information, and services are accurate and correct, the author shall not be liable for damages arising out of the use of or inability to use this product, including but not limited to, loss of profit, data, or use of this software, or special, incidental, or consequential damages or other similar claims, even if the author has been specifically advised of the possibility of such damages. Some states do not allow the exclusion of incidental or consequential damages, so the foregoing limitation may not apply to you. Information on contacting the author is provided at the end of this file. 2. USAGE -------- GIFCHECK may be called with zero or more options interspersed with zero or more targets. All options are scanned before any file searching is done, so order of options and targets is unimportant. If the standard input is not redirected, at least one target must be specified on the command line. If the standard input is redirected (is not the keyboard), GIFCHECK will detect this and will attempt to read a list of targets, separated with newlines, from the standard input. Targets on the command line will be ignored, but options on the command line will be used. This feature is for use with the "front end" option of CHILS. GIFCHECK requires 150,000 bytes of free conventional memory to run. 2.1 Targets ----------- A target can consist of a filename (optionally with path), a path, or a filename with wildcards (again, optionally with path). In the case of a filename, that file is checked. In the case of a path, all files in the specified directory are checked. In the case of a filename with wildcards, all files in the appropriate directory (the current directory if no path is given) matching the wildcards are checked. If a file that is checked is a GIF format file, it is processed. If it is not, it is not processed. 2.1.1 Wildcards --------------- Wildcards are much closer to the UNIX standard than MS-DOS. The characters * and ? retain their ordinary meanings but matching is much more intelligent. For example, *A.GIF will match LENNA.GIF but not CHESS.GIF, whereas under ordinary MS-DOS wildcards, *A.GIF would match both. Further, groups and ranges of characters may be specified in brackets []. For example, PIC0[1-5].GIF will match any filename starting with "PIC0", then containing any one of the characters "1", "2", "3", "4", or "5", and ending with ".GIF". PIC0[1-5A-F].GIF will match the names described before or those with the characters "A", "B", "C", "D", "E", or "F" between "PIC0" and ".GIF". A range or group may be complemented with the characters "^" or "!". For example, PIC0[^1-5].GIF will match any name with one character between "PIC0" and ".GIF" unless that character is "1", "2", "3", "4", or "5". Note that wildcards are only allowed in the filename portion, not the path portion. "\PICTURES\NEWGIF*\*.GIF" is illegal and an error message will be issued. 2.2 Options ----------- GIFCHECK has a variety of options to modify its function, including options to control the formatting of output, the level of processing done, etc. These options are described below. Options are case-insensitive ("-c" is the same as "-C") and may not be combined ("-cv" will not have the desired effect, since only the "-c" will be recognized; "-c -v" should be used instead). 2.2.1 Output redirection options -------------------------------- The following options specify where various types of output should be sent. 2.2.1.1 -b Batch ---------------- The -b option causes GIFCHECK to operate in batch mode. All console output is suppressed. This option should be first on the command line or other options may generate console output before this option is found. When in batch mode, GIFCHECK adjusts the ERRORLEVEL return to reflect the results of processing. ERRORLEVEL is set to: 0 if the file was a valid GIF format file and did not need stripping 1 if the file was not a GIF format file 2 if GIFCHECK encountered an unexpected end-of-file 3 if there was any other error 4 if the file is a valid GIF format file but needs to be stripped (excess characters on end of file) 5 if the file is a valid GIF format file but needs to be stripped with the -m option (excess characters at the beginning of the file and possibly the end as well) NOTE: the ERRORLEVEL reflects only the results of the last file processed. Therefore, in batch mode only one file should be specified on the command line. NOTE: this option automatically invokes the -p no-paging option. 2.2.1.2 -r Error Redirect ------------------------- Normally error messages are sent to the standard output stream, along with all other output from GIFCHECK. However, error messages can be redirected to the standard error output stream with the -r option. This means that error messages will appear on the screen even if output is redirected to a file with the DOS ">" or "|" output redirection operators. Note that the error messages will not appear in the file, however. 2.2.1.3 -p Disable paging ------------------------- Normally, output is paged (after a screenful of output has been printed, GIFCHECK pauses with the message "---more---" and waits for a keystroke before continuing). Also, an ASCII clockface is displayed when decompressing an image to indicate that GIFCHECK is still working. The -p option turns off both of these features. Note that the -p option is automatically invoked if the standard output is redirected, or if the -b option is in effect. 2.2.2 Output format options --------------------------- 2.2.2.1 -v Verbose ------------------ This option causes output to be in a more verbose format. No additional data is displayed. 2.2.2.2 -h Hex dump ------------------- This option causes output to be in a hex dump format instead of plain text format for Plain Text Extensions and Comment Extensions, extension blocks defined by the GIF89a standard. 2.2.2.3 -c Color dump --------------------- This option causes GIFCHECK to dump all colormaps found in an image, local and global, as decimal RGB triplets of the format RRR/GGG/BBB. 2.2.3 Processing options ------------------------ The following options determine what processing is performed on a file. 2.2.3.1 -l No leading junk check -------------------------------- The -l option disables checking for excess characters at the beginning of the file. This means that if a file does not begin with the GIF signature, the file is assumed not to be a GIF file. Excess characters are typically found at the beginnings of files which have been transferred incorrectly from a Macintosh computer. 2.2.3.2 -f Fast --------------- This option disables image decompression, in which the compressed image data is checked to ensure that the data has not been corrupted. This can be a slow process, especially for large files. However, an image cannot be completely verified if it is not decompressed, and some information can only be obtained by decompressing the image data. 2.2.4 Diagnostic options ------------------------ GIFCHECK produces a wide variety of diagnostics. They are described in more detail in section 4, but they all belong in one of four categories: ANOMALY a format error so major that GIFCHECK cannot decide how to continue processing the file VIOLATION a major violation of the GIF specification, which will probably cause most viewers to fail or display incorrectly FASCINATING (from Mr. Spock) a violation of the GIF specification, but most viewers should cope with the image OK NITPICK technically, a violation of the GIF specification, but so minor as to be meaningless -- a large fraction of all images contain NITPICK-type violations The diagnostic options determine which levels of diagnostics are displayed by GIFCHECK, and which levels cause GIFCHECK to skip immediately to the next image (or exit, if on the last image). Note that anomalies and violations are always displayed, anomalies always cause GIFCHECK to skip to the next image, and NITPICKS never cause GIFCHECK to skip. 2.2.3.1 -d Set display warning level ------------------------------------ The -d option takes an argument 0, 1, or 2, which must be immediately next to the "-d" (no space). -d0 display anomalies and violations -d1 (default) display anomalies, violations, and fascinatings -d2 display all 2.2.3.2 -e Set exit warning level --------------------------------- The -e option takes an argument 0, 1, or 2, which must be immediately next to the "-e" (no space). -e0 skip on anomalies only -e1 (default) skip on anomalies and violations -e2 skip on anomalies, violations, and fascinatings 2.2.4 -# Version ---------------- This option is intended more for bug reporting purposes than anything else. It causes GIFCHECK to print the version numbers and compilation dates of its component modules. No files are processed, and all other options are ignored. 2.2.5 -? Help ------------- This option, or any syntax error on the command line, causes a usage message to be displayed. No files are processed, and all other options are ignored. 3. OUTPUT --------- GIFCHECK recognizes eight different types of blocks within a GIF file: file headers, images, file terminators, and five types of extension block: generic extensions, Graphic Control Extensions, Plain Text Extensions, Comment Extensions, and Application Extensions (the last four are found only in GIF89a-format files). Data from each type of block is output in a different format, which may change according to the settings of the output format options. The various displays are explained below. Examples are surrounded by lines of equals signs for easier reading; the lines are not part of the display. 3.1 File Header --------------- The default file header display, with no options, looks like this: ===== Processing \JUNK2\DEVIL.GIF... FILE \JUNK2\DEVIL.GIF GIF87a 11209 bytes logical screen: 640 x 480 8 bits per color available on source GLOBAL COLOR TABLE: 6 bits (64 colors) bg index 0 (000/000/000) 59 unique colors. ===== The first line gives the filename, in case an error is encountered immediately. The second line repeats the filename and displays the format and filesize. The third line gives the size, in pixels, of the logical screen on which all images in the file will be displayed. The fourth line displays how many bits per color (R,G,B) were available on the machine on which the file was created. This value is frequently set incorrectly and may generally be ignored. The global color table, if present, has a subsection to itself. The sixth line indicates how large the global color table is. It also displays the background color index and the RGB value corresponding to that index. While the background color index is actually part of the file header proper, it is meaninless unless a global color table is present, so it is displayed in the global color table section. If the file is a GIF89a-format image, this line also indicates whether the global color table is sorted. Finally, the seventh line indicates how many unique colors are present in the global color table. If no global color table is present, a line stating this is displayed instead. If the -v VERBOSE option is in effect, the file header display looks like this: ===== Processing \JUNK2\DEVIL.GIF... FILE \JUNK2\DEVIL.GIF is GIF version GIF87a: file size (bytes): 11209 logical screen size (pixels, width x height): 640 x 480 8 bits per color available on source This file has a global color table. GLOBAL COLOR TABLE: 6 bits per index for a table size of 64 background index 0, RGB value 000/000/000 59 unique colors. ===== The data displayed is the same, but more explanatory text is included. If the -c color dump option is in effect, the file header display looks like this: ===== Processing \JUNK2\DEVIL.GIF... FILE \JUNK2\DEVIL.GIF GIF87a 11209 bytes logical screen: 640 x 480 8 bits per color available on source GLOBAL COLOR TABLE: 6 bits (64 colors) bg index 0 (000/000/000) 59 unique colors. 000/000/000 255/197/197 255/156/156 255/189/189 255/131/131 255/180/180 255/148/148 255/205/205 041/000/000 255/123/123 255/172/172 255/074/074 082/000/000 255/057/057 255/090/090 255/115/115 008/000/000 049/000/000 255/164/164 024/000/000 131/000/000 255/049/049 074/000/000 255/082/082 123/000/000 255/139/139 065/000/000 148/000/000 255/041/041 016/000/000 255/016/016 230/024/024 255/032/032 115/000/000 164/000/000 057/000/000 255/106/106 106/000/000 156/000/000 255/008/008 032/000/000 255/065/065 139/000/000 180/000/000 255/024/024 098/000/000 090/000/000 238/000/000 189/000/000 230/000/000 213/000/000 172/000/000 205/000/000 197/000/000 246/000/000 222/000/000 230/016/016 255/000/000 255/098/098 000/000/000 000/000/000 000/000/000 000/000/000 000/000/000 ===== The standard or verbose display is followed by a listing, in decimal, of the raw RGB values found in the global color map. 3.2 Image --------- The default image display, with no options, looks like this: ===== IMAGE 1: size 640 x 480 corner 0, 0 sequentially This image uses the global color table (no local color table). IMAGE DATA FOR IMAGE 1: code size 6 bits Reached end of codes in block 42 totals: 7906 codes packed into 10948 bytes in 43 blocks 307200 pixels extracted code table cleared 2 times compressed data is 4% of the size of uncompressed image 59 colors referenced (59 unique) ===== Each image in the file is numbered sequentially (by GIFCHECK) and has a section to itself. The first line gives the image number. The second line indicates the size (in pixels) of the image, the location (in pixels) of the upper left corner, and how the image is stored ("sequentially" or "interlaced"). The third line indicates whether there is a local color table, and whether the image uses the global or local color table. If there is a local color table, the third line is followed by a color table display similar to the one for global color tables, possibly including a dump of the raw RGB values if the -c color dump option is in effect. Very few images have local color tables. Image data is stored compressed, and the compressed data is further broken up into blocks (which are numbered by GIFCHECK starting at 0). The fifth line indicates the minimum code size for the image. The sixth line indicates the block in which the end-of-information (EOI) code was encountered. The seventh line indicates the number of codes extracted, the number of bytes which those codes occupied, and how many blocks those bytes were divided into. The eighth line indicates how many pixels were generated when the codes were decompressed, and the ninth line indicates how many times the table-clear code was encountered. The tenth line indicates the size of the compressed data as a percentage of the size of the uncompressed image. The size of the compressed data is the size of the data only, not including block headers, image headers, or any other non-image data, so the compression ratio is accurate. The size of the uncompressed image is calculated by multiplying the height and the width to get the total number of pixels in the image, then multiplying by the number of bits required to represent the global or local color table, depending on which one is used by the image. Finally, the eleventh line indicates how many colors were referenced by the image data, and how many of the colors referenced are unique. If the -v VERBOSE option is in effect, the image display looks like this: ===== IMAGE 1: image size (pixels, width x height): 640 x 480 image upper left corner (column, row): 0, 0 image is stored sequentially This image uses the global color table (no local color table). IMAGE DATA FOR IMAGE 1: code size 6 bits Reached end of codes in block 42 totals: 7906 codes packed into 10948 bytes in 43 blocks 307200 pixels extracted code table cleared 2 times compressed data is 4% of the size of uncompressed image 59 colors referenced (59 unique) ===== The data displayed is the same, but more explanatory text is included. If the -f FAST option is in effect, the image display looks like this: ===== IMAGE 1: size 640 x 480 corner 0, 0 sequentially This image uses the global color table (no local color table). IMAGE DATA FOR IMAGE 1: code size 6 bits totals: 10948 bytes in 43 blocks compressed data is 4% of the size of uncompressed image ===== Since the compressed image data is not decompressed under this option, less information is available. 3.3 Extension Blocks -------------------- Extension blocks have been part of the GIF format since the original GIF87a specification. Each extension block has a type identifier. In the GIF87a specification, all extension types were application-defined; there were no global standards. However, the GIF89a specification dedicated four type identifiers for specific functions, and defined the data formats used within those extension blocks. GIFCHECK recognizes these dedicated types and formats the data appropriately. 3.3.1 Generic extensions ------------------------ If the format is GIF87a, or the format is GIF89a but the type identifier is not one of the four dedicated types, the "generic extension" format is used. It looks like this: ===== GENERIC EXTENSION BLOCK 1: extension block type 255 BLOCK 0 (11 bytes): G 47 I 49 F 46 L 4C I 49 T 54 E 45 20 20 20 20 BLOCK 1 (6 bytes): 01 ) 29 p 70 0E 02 00 totals: 17 bytes of data in 2 blocks ===== Generic extension blocks are numbered sequentially (by GIFCHECK). The first line indicates that it is a generic extension block and gives the sequence number. The second line shows the type identifier value. The next lines display the data in hex dump format (generic extensions are always displayed in hex dump format, regardless of the setting of the -h option). Each data block is numbered sequentially (by GIFCHECK), starting at 0. A line giving the data block number and the number of bytes in the block is shown. Then the data bytes themselves are displayed, eight per line. If the byte is a printable character, the character is printed, followed by the hex value of the byte; otherwise a space is printed, followed by the hex value. Finally, a totals line is printed, indicating the total number of data bytes and the total number of data blocks. The -v VERBOSE option has no effect on generic extension displays. 3.3.2 Graphic Control Extension ------------------------------- Graphic Control Extensions (GCEs) are one of the GIF89a dedicated extension types. When encountered, they are displayed like this: ===== GRAPHIC CONTROL EXTENSION BLOCK 1: Disposal method: 0 (none) User input is not expected Transparency enabled (index 3) No delay time specified ===== GCEs are numbered sequentially (by GIFCHECK). The first line indicates that it is a GCE and gives the sequence number. The second line shows the disposal method (which applies to the next graphic -- i.e. image or Plain Text Extension -- in the file). This can be: "none", "leave in place", "restore to background", or "restore to previous". The third line indicates whether user input is expected before the next graphic will be displayed. The fourth line indicates whether a transparency color index (which indicates that pixels of the given color should be rendered "transparent" -- i.e., not drawn) has been specified, and if so, what the transparency index is. The last line indicates whether a delay time (delay before the next graphic is displayed) has been specified, and if so what the delay value is. The -v VERBOSE option has no effect on GCE displays. 3.3.3 Plain Text Extension -------------------------- Plain Text Extensions (PTEs) are one of the GIF89a dedicated extension types. When encountered, they are displayed like this: ===== PLAIN TEXT EXTENSION BLOCK 1: char cell 8 x 28, grid 96( 12) x 28( 1) at 0, 112 Foreground color index 0 (RGB 016/016/014) Background color index 3 (RGB 113/110/232) START TEXT (12 characters): Cass Berry END TEXT. ===== PTEs are numbered sequentially (by GIFCHECK). The first line indicates that it is a PTE and gives the sequence number. The second line shows: the size of the character cell, in pixels; the size of the character grid, in pixels and cells (cells in parentheses); and the location of the upper left corner of the grid. The third line shows the index and RGB values of the foreground text color, and the fourth line shows the same for the background color. The fifth line indicates the start of the text, and how many characters of text are present. It is followed by the text itself, which is in turn followed by an "END TEXT." marker on the next line. If the -v VERBOSE option is in effect, PTEs are displayed like this: ===== PLAIN TEXT EXTENSION BLOCK 1: Character cell grid upper left corner (column, row): 0, 112 Character cell size (pixels, width x height): 8 x 28 Character grid size (pixels, width x height): 96 x 28 Character grid size (chars, width x height): 12 x 1 Foreground color index 0 (RGB 016/016/014) Background color index 3 (RGB 113/110/232) START TEXT (12 characters): Cass Berry END TEXT. ===== The same information is displayed, but more explanatory text is included. If the -h hex dump option is in effect, the text is displayed in hex dump format, like this: ===== PLAIN TEXT EXTENSION BLOCK 1: char cell 8 x 28, grid 96( 12) x 28( 1) at 0, 112 Foreground color index 0 (RGB 016/016/014) Background color index 3 (RGB 113/110/232) START TEXT (12 characters): LINE 1: 20 C 43 a 61 s 73 s 73 20 B 42 e 65 r 72 r 72 y 79 20 END TEXT. ===== Instead of simply displaying the text, each character is displayed individually, followed by its hex value. 3.3.4 Comment Extension ----------------------- Comment Extensions are one of the GIF89a dedicated extension types. When encountered, they are displayed like this: ===== COMMENT EXTENSION BLOCK 1: START TEXT (161 characters): +-------------------------------------------------+ | Multi-image GIF89a created with CompuMake Tools | +-------------------------------------------------+ END TEXT. 0 unprintable characters. ===== Comment Extensions are numbered sequentially (by GIFCHECK). The first line indicates that it is a Comment Extension and gives the sequence number. The second line indicates the start of the text and how many characters of text are present. It is followed by the text itself, which is in turn followed by an "END TEXT." marker on the next line. The last line indicates how many unprintable characters were found in the text. The -v VERBOSE option has no effect on Comment Extension displays. If the -h hex dump option is in effect, the text is displayed in hex dump format, like this: ===== COMMENT EXTENSION BLOCK 1: START TEXT (161 characters): + 2B - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D + 2B 0D 0A | 7C 20 M 4D u 75 l 6C t 74 i 69 - 2D i 69 m 6D a 61 g 67 e 65 20 G 47 I 49 F 46 8 38 9 39 a 61 20 c 63 r 72 e 65 a 61 t 74 e 65 d 64 20 w 77 i 69 t 74 h 68 20 C 43 o 6F m 6D p 70 u 75 M 4D a 61 k 6B e 65 20 T 54 o 6F o 6F l 6C s 73 20 | 7C 0D 0A + 2B - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D - 2D + 2B 0D 0A 0D 0A END TEXT. 0 unprintable characters. ===== Instead of simply displaying the text, each character is displayed individually, followed by its hex value. 3.3.5 Application Extension --------------------------- Application Extensions are one of the GIF89a dedicated extension types. When encountered, they are displayed like this: ===== APPLICATION EXTENSION BLOCK 1: application identifier: f 66 r 72 a 61 c 63 t 74 i 69 n 6E t 74 application authentication code: 0 30 0 30 1 31 246 bytes of application data. F 46 r 72 a 61 c 63 t 74 a 61 l 6C 00 96 00 07 00 00 00 00 h 68 D8 FB 0C @ 40 00 00 00 20 0B FF 0C @ 40 00 00 00 00 00 E9 I 49 ? 3F 00 00 00 00 F0 = 3D 89 ? 3F 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 1B 00 80 02 E0 01 00 01 04 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 FF FF 01 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 E7 03 00 00 Z 5A 00 09 01 00 00 x 78 00 8C 00 ( 28 00 00 00 02 00 n 6E 00 00 00 00 00 01 00 FF FF 01 00 00 00 14 00 00 00 00 00 04 00 00 00 00 00 04 00 P 50 00 d 64 00 00 00 00 00 00 00 FF FF 00 00 00 h 68 D8 FB 0C @ 40 00 00 00 00 00 E9 I 49 ? 3F g 67 00 04 00 06 01 00 00 00 00 00 00 00 00 D8 A7 00 00 00 06 02 03 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01 00 00 00 00 00 ===== Application Extensions are numbered sequentially (by GIFCHECK). The first line indicates that it is an Application Extension and gives the sequence number. The second line introduces the eight-byte application identifier and is followed on the third line by the application identifier itself, in hex dump format. The fourth line introduces the three-byte application authentication code and is followed on the fifth line by the application authentication code itself, in hex dump format. The sixth line indicates how many bytes of application data are present, and is followed by the application data itself in hex dump format. All data in Application Extensions is always displayed in hex dump format, regardless of the setting of the -h option. The -v VERBOSE option has no effect on Application Extension displays. 3.4 Terminator -------------- When the GIF terminator block is encountered, GIFCHECK prints this line: ===== GIF TERMINATOR ===== 4. DIAGNOSTIC MESSAGES ---------------------- GIFCHECK can produce a variety of diagnostic messages. Some indicate problems within the GIF file. Others indicate malfunctions of GIFCHECK or the computer. Messages beginning with "ANOMALY", "VIOLATION", "FASCINATING", and "NITPICK" indicate problems within the GIF file. Messages beginning "FATAL ERROR", "ERROR", or "WARNING" indicate malfunctions. Messages beginning "STRIP" indicate that the file should be processed by the GIFSTRIP program to remove excess characters. 4.1 Anomalies ------------- Anomalies are radical violations of the GIF specification. Anomalies may be caused by massive corruption of the file, to the extent that GIFCHECK cannot determine how to proceed with processing of the file, or by individual values that make no sense. In either case, an image with anomalies will probably be rejected by most GIF viewers. Anomalies are always displayed and always cause GIFCHECK to skip immediately to the next file to be processed. The various anomalies that GIFCHECK can detect are explained below. ANOMALY: end of data reached before end of codes GIFCHECK reached the end of the packed image data without encountering an End-of-Information code, which signals the end of the image. ANOMALY: decompression error in block xxx at offset yyy GIFCHECK found a bad code in the packed image data. This indicates that the file has been corrupted, and most likely the rest of the file is trash. ANOMALY: foreground color index off end of global color table The color index specified for the foreground in a GIF89a Plain Text Extension is invalid (PTEs always use the global color table). ANOMALY: background color index off end of global color table The color index specified for the background in a GIF89a Plain Text Extension is invalid (PTEs always use the global color table). ANOMALY: an undefined disposal method (x) is given The value in the disposal method field of a Graphic Control Extension is undefined. ANOMALY: a ... cannot follow a ... Where "..." is a block type. The GIF89a specification includes a grammar which indicates which types of blocks can follow which other types of blocks (in the simpler GIF87a specification, there was no need). This grammar has been violated. 4.2 Violations -------------- Violations are major violations of the GIF specification. Violations may be caused by corruption of the file or by individual values that make no sense. In either case, an image with violations may be rejected by some viewers and displayed with difficulty by others. Violations are always displayed, and by default cause GIFCHECK to skip immediately to the next file to be processed. The various violations that GIFCHECK can detect are explained below. VIOLATION: image does not fit on logical screen Either the size of the image given in the image header is greater in one or both dimensions than the size of the logical screen given in the file header, or the position of the image given in the image header causes parts of the image to extend beyond the boundaries of the logical screen. VIOLATION: bad transparency index (x) If processing a Plain Text Extension, indicates that the transparency index specified by a preceeding Graphic Control Extension is off the end of the global color table (PTEs always use the global color table). If processing an image, indicates that the transparency index is off the end of either the global or local color table, depending on which one the image uses. VIOLATION: character grid does not fit on logical screen Either the size of the character grid given in the Plain Text Extension is greater in one or both dimensions than the size of the logical screen given in the file header, or the position of the character grid causes parts of it to extend beyond the boundaries of the logical screen. VIOLATION: text is too long to fit into character grid The text given in the Plain Text Extension contains more characters than there are character cells in the character grid. VIOLATION: xxx garbage characters found between blocks There should be no garbage characters between blocks. While this can theoretically be solved by using the GIFSTRIP program, it probably indicates that the file has been corrupted. 4.3 Fascinatings ---------------- Fascinatings are not always violations of the GIF specification. Fascinatings are unusual conditions which most viewers can cope with, but are still worthy of note. Fascinatings are displayed by default, and by default do NOT cause GIFCHECK to skip. The various fascinatings that GIFCHECK can detect are explained below. FASCINATING: this file does not contain a global color table The GIF specification allows for files without global color tables; they are to be displayed using the global color table from a previous file, or a default table chosen by the viewer if no previous global color table is available. Files without global color tables are extremely rare. This message is displayed when an image which uses the global color table is encountered in a file which has none. FASCINATING: xxx extra blocks on end of image The packed image data continued for one or more blocks after the block in which the End-of-Information (EOI) code was found. While this condition is suspicious, the data before the EOI decompressed correctly (otherwise an ANOMALY would have occurred), so there's no concrete evidence that the image is bad. FASCINATING: too few pixels extracted (xxx lines found, yyy pixels missing) When the End-of-Information code was encountered, too few pixels had been extracted to fill the image (e.g. for a 320 by 200 image, which should have 64,000 pixels, only 63,999 were extracted). Typically only one or two are missing. Viewers seem to cope with this condition OK. FASCINATING: too many pixels extracted (xxx extra) When the End-of-Information code was encountered, more pixels than necessary had been extracted (e.g. for a 320 by 200 image, which should have 64,000 pixels, 64,001 were extracted). Typically only one or two extras are present. Viewers seem to cope with this condition OK. FASCINATING: code table never cleared While this is technically OK, it is recommended practice to clear the decompression code table (via a clear code in the compressed image data) immediately at the beginning of the image. If the code table was never cleared, the initial clear never occurred. FASCINATING: this file has no Global Color Table (GCT is used by PTE) The GIF specification allows for files without global color tables; they are to be displayed using the global color table from a previous file, or a default table chosen by the viewer if no previous global color table is available. Files without global color tables are extremely rare. This message is displayed when a Plain Text Extension (PTEs always use the global color table) is encountered in a file which has no global color table. FASCINATING: transparency index does not match either foreground or background This message is displayed when processing a Plain Text Extension and the transparency index specified by a preceeding Graphic Control Extension does not match either the foreground or the background color indices of the PTE. This renders the transparency index meaningless, since it only applies to the next graphic (image or PTE) following the GCE. Nothing is technically wrong, but it's weird. 4.4 Nitpicks ------------ Nitpicks are minor technical violations of the GIF specification. Some are very common, and they should not cause problems when viewing the file. Nitpicks are by default NOT displayed, and never cause GIFCHECK to skip. The various nitpicks that GIFCHECK can detect are explained below. Most of these messages are self-explanatory, so no detailed explanation is given. NITPICK: byte 6 of the logical screen descriptor should be 0 for GIF87A NITPICK: bit 3, byte 4 of logical screen descriptor should be 0 for GIF87A NITPICK: bits 3 and 4, byte 9 of image descriptor should be 0 NITPICK: bit 5, byte 9 of image descriptor should be 0 for GIF87A NITPICK: bits 5-7, byte 1 of graphic control extension should be 0 NITPICK: local color table size is nonzero The image does not have a local color table, but the local color table size is nonzero. NITPICK: character grid not an integral number of character cells wide The horizontal size of the character grid in a Plain Text Extension is not evenly divisible by the horizontal size of a character cell. NITPICK: character grid not an integral number of character cells high The vertical size of the character grid in a Plain Text Extension is not evenly divisible by the vertical size of a character cell. NITPICK: xxx unprintable characters in text Plain Text Extensions are supposed to contain only "7-bit printable ASCII characters". 4.5 Miscellaneous ----------------- GIFCHECK can also produce messages beginning with "FATAL ERROR", "ERROR", "WARNING", and "STRIP". The first three indicate malfunctions of GIFCHECK or the computer, and typically cause GIFCHECK to exit immediately. Messages beginning with "STRIP" indicate that the file should be processed by the GIFSTRIP program to remove excess characters, and are simply for the user's information. Processing of the file is not affected. 5. THE END ---------- Technical support via email is available from the following addresses: INTERNET (the following are alternate addresses for the same place): support@picarefy.com picarefy!support@amc.com picarefy!support@netcom.com uunet!uw-coco!amc-gw!picarefy!support COMPUSERVE: 71261,1731 GENIE: J.BIRDSALL2 Registrations should be sent to: James W. Birdsall 11112 NE 124 LN #D204 Kirkland, WA 98034 If you have an email address on any of the networks listed above, please include it when registering. It is much easier to send updates by email. Also, please specify what sort of archive (ZIP, ZOO, ARC, LZH, ARJ, UNIX shar) you can handle most easily. NOTE: IF YOU DO NOT PROVIDE AN EMAIL ADDRESS, YOU WILL ONLY RECEIVE MAJOR VERSION UPDATES. YOU WILL NOT RECEIVE MINOR VERSIONS. PLEASE PROVIDE AN EMAIL ADDRESS IF YOU HAVE ANY WAY OF DOING SO.