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avoid an unneeded check in the FT1 has_chars() method implementation
[imager.git] / fileformatdocs / spec-gif89a.txt
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5 Cover Sheet for the GIF89a Specification
6
7
8 DEFERRED CLEAR CODE IN LZW COMPRESSION
9
10 There has been confusion about where clear codes can be found in the
11 data stream. As the specification says, they may appear at anytime. There
12 is not a requirement to send a clear code when the string table is full.
13
14 It is the encoder's decision as to when the table should be cleared. When
15 the table is full, the encoder can chose to use the table as is, making no
16 changes to it until the encoder chooses to clear it. The encoder during
17 this time sends out codes that are of the maximum Code Size.
18
19 As we can see from the above, when the decoder's table is full, it must
20 not change the table until a clear code is received. The Code Size is that
21 of the maximum Code Size. Processing other than this is done normally.
22
23 Because of a large base of decoders that do not handle the decompression in
24 this manner, we ask developers of GIF encoding software to NOT implement
25 this feature until at least January 1991 and later if they see that their
26 particular market is not ready for it. This will give developers of GIF
27 decoding software time to implement this feature and to get it into the
28 hands of their clients before the decoders start "breaking" on the new
29 GIF's. It is not required that encoders change their software to take
30 advantage of the deferred clear code, but it is for decoders.
31
32 APPLICATION EXTENSION BLOCK - APPLICATION IDENTIFIER
33
34 There will be a Courtesy Directory file located on CompuServe in the PICS
35 forum. This directory will contain Application Identifiers for Application
36 Extension Blocks that have been used by developers of GIF applications.
37 This file is intended to help keep developers that wish to create
38 Application Extension Blocks from using the same Application Identifiers.
39 This is not an official directory; it is for voluntary participation only
40 and does not guarantee that someone will not use the same identifier.
41
42 E-Mail can be sent to Larry Wood (forum manager of PICS) indicating the
43 request for inclusion in this file with an identifier.
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85 GRAPHICS INTERCHANGE FORMAT(sm)
86
87 Version 89a
88
89 (c)1987,1988,1989,1990
90
91 Copyright
92 CompuServe Incorporated
93 Columbus, Ohio
94
95
96
97
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124CompuServe Incorporated Graphics Interchange Format
125Document Date : 31 July 1990 Programming Reference
126
127
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133
134
135
136 Table of Contents
137
138Disclaimer................................................................. 1
139
140Foreword................................................................... 1
141
142Licensing.................................................................. 1
143
144About the Document......................................................... 2
145
146General Description........................................................ 2
147
148Version Numbers............................................................ 2
149
150The Encoder................................................................ 3
151
152The Decoder................................................................ 3
153
154Compliance................................................................. 3
155
156About Recommendations...................................................... 4
157
158About Color Tables......................................................... 4
159
160Blocks, Extensions and Scope............................................... 4
161
162Block Sizes................................................................ 5
163
164Using GIF as an embedded protocol.......................................... 5
165
166Data Sub-blocks............................................................ 5
167
168Block Terminator........................................................... 6
169
170Header..................................................................... 7
171
172Logical Screen Descriptor.................................................. 8
173
174Global Color Table......................................................... 10
175
176Image Descriptor........................................................... 11
177
178Local Color Table.......................................................... 13
179
180Table Based Image Data..................................................... 14
181
182Graphic Control Extension.................................................. 15
183
184Comment Extension.......................................................... 17
185
186Plain Text Extension....................................................... 18
187
188Application Extension...................................................... 21
189
190Trailer.................................................................... 23
191
192
193
194
195
196
197
198
199
200
201
202Quick Reference Table...................................................... 24
203
204GIF Grammar................................................................ 25
205
206Glossary................................................................... 27
207
208Conventions................................................................ 28
209
210Interlaced Images.......................................................... 29
211
212Variable-Length-Code LZW Compression....................................... 30
213
214On-line Capabilities Dialogue.............................................. 33
215
216
217
218
219
220
221
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223
224
225
226
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265 1
266
267
2681. Disclaimer.
269
270The information provided herein is subject to change without notice. In no
271event will CompuServe Incorporated be liable for damages, including any loss of
272revenue, loss of profits or other incidental or consequential damages arising
273out of the use or inability to use the information; CompuServe Incorporated
274makes no claim as to the suitability of the information.
275
276
2772. Foreword.
278
279This document defines the Graphics Interchange Format(sm). The specification
280given here defines version 89a, which is an extension of version 87a.
281
282The Graphics Interchange Format(sm) as specified here should be considered
283complete; any deviation from it should be considered invalid, including but not
284limited to, the use of reserved or undefined fields within control or data
285blocks, the inclusion of extraneous data within or between blocks, the use of
286methods or algorithms not specifically listed as part of the format, etc. In
287general, any and all deviations, extensions or modifications not specified in
288this document should be considered to be in violation of the format and should
289be avoided.
290
291
2923. Licensing.
293
294The Graphics Interchange Format(c) is the copyright property of CompuServe
295Incorporated. Only CompuServe Incorporated is authorized to define, redefine,
296enhance, alter, modify or change in any way the definition of the format.
297
298CompuServe Incorporated hereby grants a limited, non-exclusive, royalty-free
299license for the use of the Graphics Interchange Format(sm) in computer
300software; computer software utilizing GIF(sm) must acknowledge ownership of the
301Graphics Interchange Format and its Service Mark by CompuServe Incorporated, in
302User and Technical Documentation. Computer software utilizing GIF, which is
303distributed or may be distributed without User or Technical Documentation must
304display to the screen or printer a message acknowledging ownership of the
305Graphics Interchange Format and the Service Mark by CompuServe Incorporated; in
306this case, the acknowledgement may be displayed in an opening screen or leading
307banner, or a closing screen or trailing banner. A message such as the following
308may be used:
309
310 "The Graphics Interchange Format(c) is the Copyright property of
311 CompuServe Incorporated. GIF(sm) is a Service Mark property of
312 CompuServe Incorporated."
313
314For further information, please contact :
315
316 CompuServe Incorporated
317 Graphics Technology Department
318 5000 Arlington Center Boulevard
319 Columbus, Ohio 43220
320 U. S. A.
321
322CompuServe Incorporated maintains a mailing list with all those individuals and
323organizations who wish to receive copies of this document when it is corrected
324
325
326
327
328
329
330
331 2
332
333
334or revised. This service is offered free of charge; please provide us with your
335mailing address.
336
337
3384. About the Document.
339
340This document describes in detail the definition of the Graphics Interchange
341Format. This document is intended as a programming reference; it is
342recommended that the entire document be read carefully before programming,
343because of the interdependence of the various parts. There is an individual
344section for each of the Format blocks. Within each section, the sub-section
345labeled Required Version refers to the version number that an encoder will have
346to use if the corresponding block is used in the Data Stream. Within each
347section, a diagram describes the individual fields in the block; the diagrams
348are drawn vertically; top bytes in the diagram appear first in the Data Stream.
349Bits within a byte are drawn most significant on the left end. Multi-byte
350numeric fields are ordered Least Significant Byte first. Numeric constants are
351represented as Hexadecimal numbers, preceded by "0x". Bit fields within a byte
352are described in order from most significant bits to least significant bits.
353
354
3555. General Description.
356
357The Graphics Interchange Format(sm) defines a protocol intended for the on-line
358transmission and interchange of raster graphic data in a way that is
359independent of the hardware used in their creation or display.
360
361The Graphics Interchange Format is defined in terms of blocks and sub-blocks
362which contain relevant parameters and data used in the reproduction of a
363graphic. A GIF Data Stream is a sequence of protocol blocks and sub-blocks
364representing a collection of graphics. In general, the graphics in a Data
365Stream are assumed to be related to some degree, and to share some control
366information; it is recommended that encoders attempt to group together related
367graphics in order to minimize hardware changes during processing and to
368minimize control information overhead. For the same reason, unrelated graphics
369or graphics which require resetting hardware parameters should be encoded
370separately to the extent possible.
371
372A Data Stream may originate locally, as when read from a file, or it may
373originate remotely, as when transmitted over a data communications line. The
374Format is defined with the assumption that an error-free Transport Level
375Protocol is used for communications; the Format makes no provisions for
376error-detection and error-correction.
377
378The GIF Data Stream must be interpreted in context, that is, the application
379program must rely on information external to the Data Stream to invoke the
380decoder process.
381
382
3836. Version Numbers.
384
385The version number in the Header of a Data Stream is intended to identify the
386minimum set of capabilities required of a decoder in order to fully process the
387Data Stream. An encoder should use the earliest possible version number that
388includes all the blocks used in the Data Stream. Within each block section in
389this document, there is an entry labeled Required Version which specifies the
390
391
392
393
394
395
396
397 3
398
399
400earliest version number that includes the corresponding block. The encoder
401should make every attempt to use the earliest version number covering all the
402blocks in the Data Stream; the unnecessary use of later version numbers will
403hinder processing by some decoders.
404
405
4067. The Encoder.
407
408The Encoder is the program used to create a GIF Data Stream. From raster data
409and other information, the encoder produces the necessary control and data
410blocks needed for reproducing the original graphics.
411
412The encoder has the following primary responsibilities.
413
414 - Include in the Data Stream all the necessary information to
415 reproduce the graphics.
416
417 - Insure that a Data Stream is labeled with the earliest possible
418 Version Number that will cover the definition of all the blocks in
419 it; this is to ensure that the largest number of decoders can
420 process the Data Stream.
421
422 - Ensure encoding of the graphics in such a way that the decoding
423 process is optimized. Avoid redundant information as much as
424 possible.
425
426 - To the extent possible, avoid grouping graphics which might
427 require resetting hardware parameters during the decoding process.
428
429 - Set to zero (off) each of the bits of each and every field
430 designated as reserved. Note that some fields in the Logical Screen
431 Descriptor and the Image Descriptor were reserved under Version
432 87a, but are used under version 89a.
433
434
4358. The Decoder.
436
437The Decoder is the program used to process a GIF Data Stream. It processes the
438Data Stream sequentially, parsing the various blocks and sub-blocks, using the
439control information to set hardware and process parameters and interpreting the
440data to render the graphics.
441
442The decoder has the following primary responsibilities.
443
444 - Process each graphic in the Data Stream in sequence, without
445 delays other than those specified in the control information.
446
447 - Set its hardware parameters to fit, as closely as possible, the
448 control information contained in the Data Stream.
449
450
4519. Compliance.
452
453An encoder or a decoder is said to comply with a given version of the Graphics
454Interchange Format if and only if it fully conforms with and correctly
455implements the definition of the standard associated with that version. An
456
457
458
459
460
461
462
463 4
464
465
466encoder or a decoder may be compliant with a given version number and not
467compliant with some subsequent version.
468
469
47010. About Recommendations.
471
472Each block section in this document contains an entry labeled Recommendation;
473this section lists a set of recommendations intended to guide and organize the
474use of the particular blocks. Such recommendations are geared towards making
475the functions of encoders and decoders more efficient, as well as making
476optimal use of the communications bandwidth. It is advised that these
477recommendations be followed.
478
479
48011. About Color Tables.
481
482The GIF format utilizes color tables to render raster-based graphics. A color
483table can have one of two different scopes: global or local. A Global Color
484Table is used by all those graphics in the Data Stream which do not have a
485Local Color Table associated with them. The scope of the Global Color Table is
486the entire Data Stream. A Local Color Table is always associated with the
487graphic that immediately follows it; the scope of a Local Color Table is
488limited to that single graphic. A Local Color Table supersedes a Global Color
489Table, that is, if a Data Stream contains a Global Color Table, and an image
490has a Local Color Table associated with it, the decoder must save the Global
491Color Table, use the Local Color Table to render the image, and then restore
492the Global Color Table. Both types of color tables are optional, making it
493possible for a Data Stream to contain numerous graphics without a color table
494at all. For this reason, it is recommended that the decoder save the last
495Global Color Table used until another Global Color Table is encountered. In
496this way, a Data Stream which does not contain either a Global Color Table or
497a Local Color Table may be processed using the last Global Color Table saved.
498If a Global Color Table from a previous Stream is used, that table becomes the
499Global Color Table of the present Stream. This is intended to reduce the
500overhead incurred by color tables. In particular, it is recommended that an
501encoder use only one Global Color Table if all the images in related Data
502Streams can be rendered with the same table. If no color table is available at
503all, the decoder is free to use a system color table or a table of its own. In
504that case, the decoder may use a color table with as many colors as its
505hardware is able to support; it is recommended that such a table have black and
506white as its first two entries, so that monochrome images can be rendered
507adequately.
508
509The Definition of the GIF Format allows for a Data Stream to contain only the
510Header, the Logical Screen Descriptor, a Global Color Table and the GIF
511Trailer. Such a Data Stream would be used to load a decoder with a Global Color
512Table, in preparation for subsequent Data Streams without a color table at all.
513
514
51512. Blocks, Extensions and Scope.
516
517Blocks can be classified into three groups : Control, Graphic-Rendering and
518Special Purpose. Control blocks, such as the Header, the Logical Screen
519Descriptor, the Graphic Control Extension and the Trailer, contain information
520used to control the process of the Data Stream or information used in setting
521hardware parameters. Graphic-Rendering blocks such as the Image Descriptor and
522
523
524
525
526
527
528
529 5
530
531
532the Plain Text Extension contain information and data used to render a graphic
533on the display device. Special Purpose blocks such as the Comment Extension and
534the Application Extension are neither used to control the process of the Data
535Stream nor do they contain information or data used to render a graphic on the
536display device. With the exception of the Logical Screen Descriptor and the
537Global Color Table, whose scope is the entire Data Stream, all other Control
538blocks have a limited scope, restricted to the Graphic-Rendering block that
539follows them. Special Purpose blocks do not delimit the scope of any Control
540blocks; Special Purpose blocks are transparent to the decoding process.
541Graphic-Rendering blocks and extensions are used as scope delimiters for
542Control blocks and extensions. The labels used to identify labeled blocks fall
543into three ranges : 0x00-0x7F (0-127) are the Graphic Rendering blocks,
544excluding the Trailer (0x3B); 0x80-0xF9 (128-249) are the Control blocks;
5450xFA-0xFF (250-255) are the Special Purpose blocks. These ranges are defined so
546that decoders can handle block scope by appropriately identifying block labels,
547even when the block itself cannot be processed.
548
549
55013. Block Sizes.
551
552The Block Size field in a block, counts the number of bytes remaining in the
553block, not counting the Block Size field itself, and not counting the Block
554Terminator, if one is to follow. Blocks other than Data Blocks are intended to
555be of fixed length; the Block Size field is provided in order to facilitate
556skipping them, not to allow their size to change in the future. Data blocks
557and sub-blocks are of variable length to accommodate the amount of data.
558
559
56014. Using GIF as an embedded protocol.
561
562As an embedded protocol, GIF may be part of larger application protocols,
563within which GIF is used to render graphics. In such a case, the application
564protocol could define a block within which the GIF Data Stream would be
565contained. The application program would then invoke a GIF decoder upon
566encountering a block of type GIF. This approach is recommended in favor of
567using Application Extensions, which become overhead for all other applications
568that do not process them. Because a GIF Data Stream must be processed in
569context, the application must rely on some means of identifying the GIF Data
570Stream outside of the Stream itself.
571
572
57315. Data Sub-blocks.
574
575 a. Description. Data Sub-blocks are units containing data. They do not
576 have a label, these blocks are processed in the context of control
577 blocks, wherever data blocks are specified in the format. The first byte
578 of the Data sub-block indicates the number of data bytes to follow. A
579 data sub-block may contain from 0 to 255 data bytes. The size of the
580 block does not account for the size byte itself, therefore, the empty
581 sub-block is one whose size field contains 0x00.
582
583 b. Required Version. 87a.
584
585
586
587
588
589
590
591
592
593
594
595 6
596
597
598 c. Syntax.
599
600 7 6 5 4 3 2 1 0 Field Name Type
601 +---------------+
602 0 | | Block Size Byte
603 +---------------+
604 1 | |
605 +- -+
606 2 | |
607 +- -+
608 3 | |
609 +- -+
610 | | Data Values Byte
611 +- -+
612 up | |
613 +- . . . . -+
614 to | |
615 +- -+
616 | |
617 +- -+
618255 | |
619 +---------------+
620
621 i) Block Size - Number of bytes in the Data Sub-block; the size
622 must be within 0 and 255 bytes, inclusive.
623
624 ii) Data Values - Any 8-bit value. There must be exactly as many
625 Data Values as specified by the Block Size field.
626
627 d. Extensions and Scope. This type of block always occurs as part of a
628 larger unit. It does not have a scope of itself.
629
630 e. Recommendation. None.
631
632
63316. Block Terminator.
634
635 a. Description. This zero-length Data Sub-block is used to terminate a
636 sequence of Data Sub-blocks. It contains a single byte in the position of
637 the Block Size field and does not contain data.
638
639 b. Required Version. 87a.
640
641 c. Syntax.
642
643 7 6 5 4 3 2 1 0 Field Name Type
644 +---------------+
645 0 | | Block Size Byte
646 +---------------+
647
648 i) Block Size - Number of bytes in the Data Sub-block; this field
649 contains the fixed value 0x00.
650
651 ii) Data Values - This block does not contain any data.
652
653
654
655
656
657
658
659
660
661 7
662
663
664 d. Extensions and Scope. This block terminates the immediately preceding
665 sequence of Data Sub-blocks. This block cannot be modified by any
666 extension.
667
668 e. Recommendation. None.
669
670
67117. Header.
672
673 a. Description. The Header identifies the GIF Data Stream in context. The
674 Signature field marks the beginning of the Data Stream, and the Version
675 field identifies the set of capabilities required of a decoder to fully
676 process the Data Stream. This block is REQUIRED; exactly one Header must
677 be present per Data Stream.
678
679 b. Required Version. Not applicable. This block is not subject to a
680 version number. This block must appear at the beginning of every Data
681 Stream.
682
683 c. Syntax.
684
685
686 7 6 5 4 3 2 1 0 Field Name Type
687 +---------------+
688 0 | | Signature 3 Bytes
689 +- -+
690 1 | |
691 +- -+
692 2 | |
693 +---------------+
694 3 | | Version 3 Bytes
695 +- -+
696 4 | |
697 +- -+
698 5 | |
699 +---------------+
700
701 i) Signature - Identifies the GIF Data Stream. This field contains
702 the fixed value 'GIF'.
703
704 ii) Version - Version number used to format the data stream.
705 Identifies the minimum set of capabilities necessary to a decoder
706 to fully process the contents of the Data Stream.
707
708 Version Numbers as of 10 July 1990 : "87a" - May 1987
709 "89a" - July 1989
710
711 Version numbers are ordered numerically increasing on the first two
712 digits starting with 87 (87,88,...,99,00,...,85,86) and
713 alphabetically increasing on the third character (a,...,z).
714
715 iii) Extensions and Scope. The scope of this block is the entire
716 Data Stream. This block cannot be modified by any extension.
717
718
719
720
721
722
723
724
725
726
727 8
728
729
730 d. Recommendations.
731
732 i) Signature - This field identifies the beginning of the GIF Data
733 Stream; it is not intended to provide a unique signature for the
734 identification of the data. It is recommended that the GIF Data
735 Stream be identified externally by the application. (Refer to
736 Appendix G for on-line identification of the GIF Data Stream.)
737
738 ii) Version - ENCODER : An encoder should use the earliest possible
739 version number that defines all the blocks used in the Data Stream.
740 When two or more Data Streams are combined, the latest of the
741 individual version numbers should be used for the resulting Data
742 Stream. DECODER : A decoder should attempt to process the data
743 stream to the best of its ability; if it encounters a version
744 number which it is not capable of processing fully, it should
745 nevertheless, attempt to process the data stream to the best of its
746 ability, perhaps after warning the user that the data may be
747 incomplete.
748
749
75018. Logical Screen Descriptor.
751
752 a. Description. The Logical Screen Descriptor contains the parameters
753 necessary to define the area of the display device within which the
754 images will be rendered. The coordinates in this block are given with
755 respect to the top-left corner of the virtual screen; they do not
756 necessarily refer to absolute coordinates on the display device. This
757 implies that they could refer to window coordinates in a window-based
758 environment or printer coordinates when a printer is used.
759
760 This block is REQUIRED; exactly one Logical Screen Descriptor must be
761 present per Data Stream.
762
763 b. Required Version. Not applicable. This block is not subject to a
764 version number. This block must appear immediately after the Header.
765
766 c. Syntax.
767
768 7 6 5 4 3 2 1 0 Field Name Type
769 +---------------+
770 0 | | Logical Screen Width Unsigned
771 +- -+
772 1 | |
773 +---------------+
774 2 | | Logical Screen Height Unsigned
775 +- -+
776 3 | |
777 +---------------+
778 4 | | | | | <Packed Fields> See below
779 +---------------+
780 5 | | Background Color Index Byte
781 +---------------+
782 6 | | Pixel Aspect Ratio Byte
783 +---------------+
784
785
786
787
788
789
790
791
792
793 9
794
795
796 <Packed Fields> = Global Color Table Flag 1 Bit
797 Color Resolution 3 Bits
798 Sort Flag 1 Bit
799 Size of Global Color Table 3 Bits
800
801 i) Logical Screen Width - Width, in pixels, of the Logical Screen
802 where the images will be rendered in the displaying device.
803
804 ii) Logical Screen Height - Height, in pixels, of the Logical
805 Screen where the images will be rendered in the displaying device.
806
807 iii) Global Color Table Flag - Flag indicating the presence of a
808 Global Color Table; if the flag is set, the Global Color Table will
809 immediately follow the Logical Screen Descriptor. This flag also
810 selects the interpretation of the Background Color Index; if the
811 flag is set, the value of the Background Color Index field should
812 be used as the table index of the background color. (This field is
813 the most significant bit of the byte.)
814
815 Values : 0 - No Global Color Table follows, the Background
816 Color Index field is meaningless.
817 1 - A Global Color Table will immediately follow, the
818 Background Color Index field is meaningful.
819
820 iv) Color Resolution - Number of bits per primary color available
821 to the original image, minus 1. This value represents the size of
822 the entire palette from which the colors in the graphic were
823 selected, not the number of colors actually used in the graphic.
824 For example, if the value in this field is 3, then the palette of
825 the original image had 4 bits per primary color available to create
826 the image. This value should be set to indicate the richness of
827 the original palette, even if not every color from the whole
828 palette is available on the source machine.
829
830 v) Sort Flag - Indicates whether the Global Color Table is sorted.
831 If the flag is set, the Global Color Table is sorted, in order of
832 decreasing importance. Typically, the order would be decreasing
833 frequency, with most frequent color first. This assists a decoder,
834 with fewer available colors, in choosing the best subset of colors;
835 the decoder may use an initial segment of the table to render the
836 graphic.
837
838 Values : 0 - Not ordered.
839 1 - Ordered by decreasing importance, most
840 important color first.
841
842 vi) Size of Global Color Table - If the Global Color Table Flag is
843 set to 1, the value in this field is used to calculate the number
844 of bytes contained in the Global Color Table. To determine that
845 actual size of the color table, raise 2 to [the value of the field
846 + 1]. Even if there is no Global Color Table specified, set this
847 field according to the above formula so that decoders can choose
848 the best graphics mode to display the stream in. (This field is
849 made up of the 3 least significant bits of the byte.)
850
851 vii) Background Color Index - Index into the Global Color Table for
852
853
854
855
856
857
858
859 10
860
861
862 the Background Color. The Background Color is the color used for
863 those pixels on the screen that are not covered by an image. If the
864 Global Color Table Flag is set to (zero), this field should be zero
865 and should be ignored.
866
867 viii) Pixel Aspect Ratio - Factor used to compute an approximation
868 of the aspect ratio of the pixel in the original image. If the
869 value of the field is not 0, this approximation of the aspect ratio
870 is computed based on the formula:
871
872 Aspect Ratio = (Pixel Aspect Ratio + 15) / 64
873
874 The Pixel Aspect Ratio is defined to be the quotient of the pixel's
875 width over its height. The value range in this field allows
876 specification of the widest pixel of 4:1 to the tallest pixel of
877 1:4 in increments of 1/64th.
878
879 Values : 0 - No aspect ratio information is given.
880 1..255 - Value used in the computation.
881
882 d. Extensions and Scope. The scope of this block is the entire Data
883 Stream. This block cannot be modified by any extension.
884
885 e. Recommendations. None.
886
887
88819. Global Color Table.
889
890 a. Description. This block contains a color table, which is a sequence of
891 bytes representing red-green-blue color triplets. The Global Color Table
892 is used by images without a Local Color Table and by Plain Text
893 Extensions. Its presence is marked by the Global Color Table Flag being
894 set to 1 in the Logical Screen Descriptor; if present, it immediately
895 follows the Logical Screen Descriptor and contains a number of bytes
896 equal to
897 3 x 2^(Size of Global Color Table+1).
898
899 This block is OPTIONAL; at most one Global Color Table may be present
900 per Data Stream.
901
902 b. Required Version. 87a
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925 11
926
927
928 c. Syntax.
929
930 7 6 5 4 3 2 1 0 Field Name Type
931 +===============+
932 0 | | Red 0 Byte
933 +- -+
934 1 | | Green 0 Byte
935 +- -+
936 2 | | Blue 0 Byte
937 +- -+
938 3 | | Red 1 Byte
939 +- -+
940 | | Green 1 Byte
941 +- -+
942 up | |
943 +- . . . . -+ ...
944 to | |
945 +- -+
946 | | Green 255 Byte
947 +- -+
948767 | | Blue 255 Byte
949 +===============+
950
951
952 d. Extensions and Scope. The scope of this block is the entire Data
953 Stream. This block cannot be modified by any extension.
954
955 e. Recommendation. None.
956
957
95820. Image Descriptor.
959
960 a. Description. Each image in the Data Stream is composed of an Image
961 Descriptor, an optional Local Color Table, and the image data. Each
962 image must fit within the boundaries of the Logical Screen, as defined
963 in the Logical Screen Descriptor.
964
965 The Image Descriptor contains the parameters necessary to process a table
966 based image. The coordinates given in this block refer to coordinates
967 within the Logical Screen, and are given in pixels. This block is a
968 Graphic-Rendering Block, optionally preceded by one or more Control
969 blocks such as the Graphic Control Extension, and may be optionally
970 followed by a Local Color Table; the Image Descriptor is always followed
971 by the image data.
972
973 This block is REQUIRED for an image. Exactly one Image Descriptor must
974 be present per image in the Data Stream. An unlimited number of images
975 may be present per Data Stream.
976
977 b. Required Version. 87a.
978
979
980
981
982
983
984
985
986
987
988
989
990
991 12
992
993
994 c. Syntax.
995
996 7 6 5 4 3 2 1 0 Field Name Type
997 +---------------+
998 0 | | Image Separator Byte
999 +---------------+
1000 1 | | Image Left Position Unsigned
1001 +- -+
1002 2 | |
1003 +---------------+
1004 3 | | Image Top Position Unsigned
1005 +- -+
1006 4 | |
1007 +---------------+
1008 5 | | Image Width Unsigned
1009 +- -+
1010 6 | |
1011 +---------------+
1012 7 | | Image Height Unsigned
1013 +- -+
1014 8 | |
1015 +---------------+
1016 9 | | | | | | <Packed Fields> See below
1017 +---------------+
1018
1019 <Packed Fields> = Local Color Table Flag 1 Bit
1020 Interlace Flag 1 Bit
1021 Sort Flag 1 Bit
1022 Reserved 2 Bits
1023 Size of Local Color Table 3 Bits
1024
1025 i) Image Separator - Identifies the beginning of an Image
1026 Descriptor. This field contains the fixed value 0x2C.
1027
1028 ii) Image Left Position - Column number, in pixels, of the left edge
1029 of the image, with respect to the left edge of the Logical Screen.
1030 Leftmost column of the Logical Screen is 0.
1031
1032 iii) Image Top Position - Row number, in pixels, of the top edge of
1033 the image with respect to the top edge of the Logical Screen. Top
1034 row of the Logical Screen is 0.
1035
1036 iv) Image Width - Width of the image in pixels.
1037
1038 v) Image Height - Height of the image in pixels.
1039
1040 vi) Local Color Table Flag - Indicates the presence of a Local Color
1041 Table immediately following this Image Descriptor. (This field is
1042 the most significant bit of the byte.)
1043
1044
1045 Values : 0 - Local Color Table is not present. Use
1046 Global Color Table if available.
1047 1 - Local Color Table present, and to follow
1048 immediately after this Image Descriptor.
1049
1050
1051
1052
1053
1054
1055
1056
1057 13
1058
1059
1060 vii) Interlace Flag - Indicates if the image is interlaced. An image
1061 is interlaced in a four-pass interlace pattern; see Appendix E for
1062 details.
1063
1064 Values : 0 - Image is not interlaced.
1065 1 - Image is interlaced.
1066
1067 viii) Sort Flag - Indicates whether the Local Color Table is
1068 sorted. If the flag is set, the Local Color Table is sorted, in
1069 order of decreasing importance. Typically, the order would be
1070 decreasing frequency, with most frequent color first. This assists
1071 a decoder, with fewer available colors, in choosing the best subset
1072 of colors; the decoder may use an initial segment of the table to
1073 render the graphic.
1074
1075 Values : 0 - Not ordered.
1076 1 - Ordered by decreasing importance, most
1077 important color first.
1078
1079 ix) Size of Local Color Table - If the Local Color Table Flag is
1080 set to 1, the value in this field is used to calculate the number
1081 of bytes contained in the Local Color Table. To determine that
1082 actual size of the color table, raise 2 to the value of the field
1083 + 1. This value should be 0 if there is no Local Color Table
1084 specified. (This field is made up of the 3 least significant bits
1085 of the byte.)
1086
1087 d. Extensions and Scope. The scope of this block is the Table-based Image
1088 Data Block that follows it. This block may be modified by the Graphic
1089 Control Extension.
1090
1091 e. Recommendation. None.
1092
1093
109421. Local Color Table.
1095
1096 a. Description. This block contains a color table, which is a sequence of
1097 bytes representing red-green-blue color triplets. The Local Color Table
1098 is used by the image that immediately follows. Its presence is marked by
1099 the Local Color Table Flag being set to 1 in the Image Descriptor; if
1100 present, the Local Color Table immediately follows the Image Descriptor
1101 and contains a number of bytes equal to
1102 3x2^(Size of Local Color Table+1).
1103 If present, this color table temporarily becomes the active color table
1104 and the following image should be processed using it. This block is
1105 OPTIONAL; at most one Local Color Table may be present per Image
1106 Descriptor and its scope is the single image associated with the Image
1107 Descriptor that precedes it.
1108
1109 b. Required Version. 87a.
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123 14
1124
1125
1126 c. Syntax.
1127
1128 7 6 5 4 3 2 1 0 Field Name Type
1129 +===============+
1130 0 | | Red 0 Byte
1131 +- -+
1132 1 | | Green 0 Byte
1133 +- -+
1134 2 | | Blue 0 Byte
1135 +- -+
1136 3 | | Red 1 Byte
1137 +- -+
1138 | | Green 1 Byte
1139 +- -+
1140 up | |
1141 +- . . . . -+ ...
1142 to | |
1143 +- -+
1144 | | Green 255 Byte
1145 +- -+
1146767 | | Blue 255 Byte
1147 +===============+
1148
1149
1150 d. Extensions and Scope. The scope of this block is the Table-based Image
1151 Data Block that immediately follows it. This block cannot be modified by
1152 any extension.
1153
1154 e. Recommendations. None.
1155
1156
115722. Table Based Image Data.
1158
1159 a. Description. The image data for a table based image consists of a
1160 sequence of sub-blocks, of size at most 255 bytes each, containing an
1161 index into the active color table, for each pixel in the image. Pixel
1162 indices are in order of left to right and from top to bottom. Each index
1163 must be within the range of the size of the active color table, starting
1164 at 0. The sequence of indices is encoded using the LZW Algorithm with
1165 variable-length code, as described in Appendix F
1166
1167 b. Required Version. 87a.
1168
1169 c. Syntax. The image data format is as follows:
1170
1171 7 6 5 4 3 2 1 0 Field Name Type
1172 +---------------+
1173 | | LZW Minimum Code Size Byte
1174 +---------------+
1175
1176 +===============+
1177 | |
1178 / / Image Data Data Sub-blocks
1179 | |
1180 +===============+
1181
1182
1183
1184
1185
1186
1187
1188
1189 15
1190
1191
1192 i) LZW Minimum Code Size. This byte determines the initial number
1193 of bits used for LZW codes in the image data, as described in
1194 Appendix F.
1195
1196 d. Extensions and Scope. This block has no scope, it contains raster
1197 data. Extensions intended to modify a Table-based image must appear
1198 before the corresponding Image Descriptor.
1199
1200 e. Recommendations. None.
1201
1202
120323. Graphic Control Extension.
1204
1205 a. Description. The Graphic Control Extension contains parameters used
1206 when processing a graphic rendering block. The scope of this extension is
1207 the first graphic rendering block to follow. The extension contains only
1208 one data sub-block.
1209
1210 This block is OPTIONAL; at most one Graphic Control Extension may precede
1211 a graphic rendering block. This is the only limit to the number of
1212 Graphic Control Extensions that may be contained in a Data Stream.
1213
1214 b. Required Version. 89a.
1215
1216 c. Syntax.
1217
1218 7 6 5 4 3 2 1 0 Field Name Type
1219 +---------------+
1220 0 | | Extension Introducer Byte
1221 +---------------+
1222 1 | | Graphic Control Label Byte
1223 +---------------+
1224
1225 +---------------+
1226 0 | | Block Size Byte
1227 +---------------+
1228 1 | | | | | <Packed Fields> See below
1229 +---------------+
1230 2 | | Delay Time Unsigned
1231 +- -+
1232 3 | |
1233 +---------------+
1234 4 | | Transparent Color Index Byte
1235 +---------------+
1236
1237 +---------------+
1238 0 | | Block Terminator Byte
1239 +---------------+
1240
1241
1242 <Packed Fields> = Reserved 3 Bits
1243 Disposal Method 3 Bits
1244 User Input Flag 1 Bit
1245 Transparent Color Flag 1 Bit
1246
1247 i) Extension Introducer - Identifies the beginning of an extension
1248
1249
1250
1251
1252
1253
1254
1255 16
1256
1257
1258 block. This field contains the fixed value 0x21.
1259
1260 ii) Graphic Control Label - Identifies the current block as a
1261 Graphic Control Extension. This field contains the fixed value
1262 0xF9.
1263
1264 iii) Block Size - Number of bytes in the block, after the Block
1265 Size field and up to but not including the Block Terminator. This
1266 field contains the fixed value 4.
1267
1268 iv) Disposal Method - Indicates the way in which the graphic is to
1269 be treated after being displayed.
1270
1271 Values : 0 - No disposal specified. The decoder is
1272 not required to take any action.
1273 1 - Do not dispose. The graphic is to be left
1274 in place.
1275 2 - Restore to background color. The area used by the
1276 graphic must be restored to the background color.
1277 3 - Restore to previous. The decoder is required to
1278 restore the area overwritten by the graphic with
1279 what was there prior to rendering the graphic.
1280 4-7 - To be defined.
1281
1282 v) User Input Flag - Indicates whether or not user input is
1283 expected before continuing. If the flag is set, processing will
1284 continue when user input is entered. The nature of the User input
1285 is determined by the application (Carriage Return, Mouse Button
1286 Click, etc.).
1287
1288 Values : 0 - User input is not expected.
1289 1 - User input is expected.
1290
1291 When a Delay Time is used and the User Input Flag is set,
1292 processing will continue when user input is received or when the
1293 delay time expires, whichever occurs first.
1294
1295 vi) Transparency Flag - Indicates whether a transparency index is
1296 given in the Transparent Index field. (This field is the least
1297 significant bit of the byte.)
1298
1299 Values : 0 - Transparent Index is not given.
1300 1 - Transparent Index is given.
1301
1302 vii) Delay Time - If not 0, this field specifies the number of
1303 hundredths (1/100) of a second to wait before continuing with the
1304 processing of the Data Stream. The clock starts ticking immediately
1305 after the graphic is rendered. This field may be used in
1306 conjunction with the User Input Flag field.
1307
1308 viii) Transparency Index - The Transparency Index is such that when
1309 encountered, the corresponding pixel of the display device is not
1310 modified and processing goes on to the next pixel. The index is
1311 present if and only if the Transparency Flag is set to 1.
1312
1313 ix) Block Terminator - This zero-length data block marks the end of
1314
1315
1316
1317
1318
1319
1320
1321 17
1322
1323 the Graphic Control Extension.
1324
1325 d. Extensions and Scope. The scope of this Extension is the graphic
1326 rendering block that follows it; it is possible for other extensions to
1327 be present between this block and its target. This block can modify the
1328 Image Descriptor Block and the Plain Text Extension.
1329
1330 e. Recommendations.
1331
1332 i) Disposal Method - The mode Restore To Previous is intended to be
1333 used in small sections of the graphic; the use of this mode imposes
1334 severe demands on the decoder to store the section of the graphic
1335 that needs to be saved. For this reason, this mode should be used
1336 sparingly. This mode is not intended to save an entire graphic or
1337 large areas of a graphic; when this is the case, the encoder should
1338 make every attempt to make the sections of the graphic to be
1339 restored be separate graphics in the data stream. In the case where
1340 a decoder is not capable of saving an area of a graphic marked as
1341 Restore To Previous, it is recommended that a decoder restore to
1342 the background color.
1343
1344 ii) User Input Flag - When the flag is set, indicating that user
1345 input is expected, the decoder may sound the bell (0x07) to alert
1346 the user that input is being expected. In the absence of a
1347 specified Delay Time, the decoder should wait for user input
1348 indefinitely. It is recommended that the encoder not set the User
1349 Input Flag without a Delay Time specified.
1350
1351
135224. Comment Extension.
1353
1354 a. Description. The Comment Extension contains textual information which
1355 is not part of the actual graphics in the GIF Data Stream. It is suitable
1356 for including comments about the graphics, credits, descriptions or any
1357 other type of non-control and non-graphic data. The Comment Extension
1358 may be ignored by the decoder, or it may be saved for later processing;
1359 under no circumstances should a Comment Extension disrupt or interfere
1360 with the processing of the Data Stream.
1361
1362 This block is OPTIONAL; any number of them may appear in the Data Stream.
1363
1364 b. Required Version. 89a.
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387 18
1388
1389
1390 c. Syntax.
1391
1392 7 6 5 4 3 2 1 0 Field Name Type
1393 +---------------+
1394 0 | | Extension Introducer Byte
1395 +---------------+
1396 1 | | Comment Label Byte
1397 +---------------+
1398
1399 +===============+
1400 | |
1401 N | | Comment Data Data Sub-blocks
1402 | |
1403 +===============+
1404
1405 +---------------+
1406 0 | | Block Terminator Byte
1407 +---------------+
1408
1409 i) Extension Introducer - Identifies the beginning of an extension
1410 block. This field contains the fixed value 0x21.
1411
1412 ii) Comment Label - Identifies the block as a Comment Extension.
1413 This field contains the fixed value 0xFE.
1414
1415 iii) Comment Data - Sequence of sub-blocks, each of size at most
1416 255 bytes and at least 1 byte, with the size in a byte preceding
1417 the data. The end of the sequence is marked by the Block
1418 Terminator.
1419
1420 iv) Block Terminator - This zero-length data block marks the end of
1421 the Comment Extension.
1422
1423 d. Extensions and Scope. This block does not have scope. This block
1424 cannot be modified by any extension.
1425
1426 e. Recommendations.
1427
1428 i) Data - This block is intended for humans. It should contain
1429 text using the 7-bit ASCII character set. This block should
1430 not be used to store control information for custom processing.
1431
1432 ii) Position - This block may appear at any point in the Data
1433 Stream at which a block can begin; however, it is recommended that
1434 Comment Extensions do not interfere with Control or Data blocks;
1435 they should be located at the beginning or at the end of the Data
1436 Stream to the extent possible.
1437
1438
143925. Plain Text Extension.
1440
1441 a. Description. The Plain Text Extension contains textual data and the
1442 parameters necessary to render that data as a graphic, in a simple form.
1443 The textual data will be encoded with the 7-bit printable ASCII
1444 characters. Text data are rendered using a grid of character cells
1445
1446
1447
1448
1449
1450
1451
1452
1453 19
1454
1455
1456 defined by the parameters in the block fields. Each character is rendered
1457 in an individual cell. The textual data in this block is to be rendered
1458 as mono-spaced characters, one character per cell, with a best fitting
1459 font and size. For further information, see the section on
1460 Recommendations below. The data characters are taken sequentially from
1461 the data portion of the block and rendered within a cell, starting with
1462 the upper left cell in the grid and proceeding from left to right and
1463 from top to bottom. Text data is rendered until the end of data is
1464 reached or the character grid is filled. The Character Grid contains an
1465 integral number of cells; in the case that the cell dimensions do not
1466 allow for an integral number, fractional cells must be discarded; an
1467 encoder must be careful to specify the grid dimensions accurately so that
1468 this does not happen. This block requires a Global Color Table to be
1469 available; the colors used by this block reference the Global Color Table
1470 in the Stream if there is one, or the Global Color Table from a previous
1471 Stream, if one was saved. This block is a graphic rendering block,
1472 therefore it may be modified by a Graphic Control Extension. This block
1473 is OPTIONAL; any number of them may appear in the Data Stream.
1474
1475 b. Required Version. 89a.
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519 20
1520
1521
1522 c. Syntax.
1523
1524 7 6 5 4 3 2 1 0 Field Name Type
1525 +---------------+
1526 0 | | Extension Introducer Byte
1527 +---------------+
1528 1 | | Plain Text Label Byte
1529 +---------------+
1530
1531 +---------------+
1532 0 | | Block Size Byte
1533 +---------------+
1534 1 | | Text Grid Left Position Unsigned
1535 +- -+
1536 2 | |
1537 +---------------+
1538 3 | | Text Grid Top Position Unsigned
1539 +- -+
1540 4 | |
1541 +---------------+
1542 5 | | Text Grid Width Unsigned
1543 +- -+
1544 6 | |
1545 +---------------+
1546 7 | | Text Grid Height Unsigned
1547 +- -+
1548 8 | |
1549 +---------------+
1550 9 | | Character Cell Width Byte
1551 +---------------+
1552 10 | | Character Cell Height Byte
1553 +---------------+
1554 11 | | Text Foreground Color Index Byte
1555 +---------------+
1556 12 | | Text Background Color Index Byte
1557 +---------------+
1558
1559 +===============+
1560 | |
1561 N | | Plain Text Data Data Sub-blocks
1562 | |
1563 +===============+
1564
1565 +---------------+
1566 0 | | Block Terminator Byte
1567 +---------------+
1568
1569 i) Extension Introducer - Identifies the beginning of an extension
1570 block. This field contains the fixed value 0x21.
1571
1572 ii) Plain Text Label - Identifies the current block as a Plain Text
1573 Extension. This field contains the fixed value 0x01.
1574
1575 iii) Block Size - Number of bytes in the extension, after the Block
1576 Size field and up to but not including the beginning of the data
1577 portion. This field contains the fixed value 12.
1578
1579
1580
1581
1582
1583
1584
1585 21
1586
1587
1588 iv) Text Grid Left Position - Column number, in pixels, of the left
1589 edge of the text grid, with respect to the left edge of the Logical
1590 Screen.
1591
1592 v) Text Grid Top Position - Row number, in pixels, of the top edge
1593 of the text grid, with respect to the top edge of the Logical
1594 Screen.
1595
1596 vi) Image Grid Width - Width of the text grid in pixels.
1597
1598 vii) Image Grid Height - Height of the text grid in pixels.
1599
1600 viii) Character Cell Width - Width, in pixels, of each cell in the
1601 grid.
1602
1603 ix) Character Cell Height - Height, in pixels, of each cell in the
1604 grid.
1605
1606 x) Text Foreground Color Index - Index into the Global Color Table
1607 to be used to render the text foreground.
1608
1609 xi) Text Background Color Index - Index into the Global Color Table
1610 to be used to render the text background.
1611
1612 xii) Plain Text Data - Sequence of sub-blocks, each of size at most
1613 255 bytes and at least 1 byte, with the size in a byte preceding
1614 the data. The end of the sequence is marked by the Block
1615 Terminator.
1616
1617 xiii) Block Terminator - This zero-length data block marks the end
1618 of the Plain Text Data Blocks.
1619
1620 d. Extensions and Scope. The scope of this block is the Plain Text Data
1621 Block contained in it. This block may be modified by the Graphic Control
1622 Extension.
1623
1624 e. Recommendations. The data in the Plain Text Extension is assumed to be
1625 preformatted. The selection of font and size is left to the discretion of
1626 the decoder. If characters less than 0x20 or greater than 0xf7 are
1627 encountered, it is recommended that the decoder display a Space character
1628 (0x20). The encoder should use grid and cell dimensions such that an
1629 integral number of cells fit in the grid both horizontally as well as
1630 vertically. For broadest compatibility, character cell dimensions should
1631 be around 8x8 or 8x16 (width x height); consider an image for unusual
1632 sized text.
1633
1634
163526. Application Extension.
1636
1637 a. Description. The Application Extension contains application-specific
1638 information; it conforms with the extension block syntax, as described
1639 below, and its block label is 0xFF.
1640
1641 b. Required Version. 89a.
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651 22
1652
1653
1654 c. Syntax.
1655
1656 7 6 5 4 3 2 1 0 Field Name Type
1657 +---------------+
1658 0 | | Extension Introducer Byte
1659 +---------------+
1660 1 | | Extension Label Byte
1661 +---------------+
1662
1663 +---------------+
1664 0 | | Block Size Byte
1665 +---------------+
1666 1 | |
1667 +- -+
1668 2 | |
1669 +- -+
1670 3 | | Application Identifier 8 Bytes
1671 +- -+
1672 4 | |
1673 +- -+
1674 5 | |
1675 +- -+
1676 6 | |
1677 +- -+
1678 7 | |
1679 +- -+
1680 8 | |
1681 +---------------+
1682 9 | |
1683 +- -+
1684 10 | | Appl. Authentication Code 3 Bytes
1685 +- -+
1686 11 | |
1687 +---------------+
1688
1689 +===============+
1690 | |
1691 | | Application Data Data Sub-blocks
1692 | |
1693 | |
1694 +===============+
1695
1696 +---------------+
1697 0 | | Block Terminator Byte
1698 +---------------+
1699
1700 i) Extension Introducer - Defines this block as an extension. This
1701 field contains the fixed value 0x21.
1702
1703 ii) Application Extension Label - Identifies the block as an
1704 Application Extension. This field contains the fixed value 0xFF.
1705
1706 iii) Block Size - Number of bytes in this extension block,
1707 following the Block Size field, up to but not including the
1708 beginning of the Application Data. This field contains the fixed
1709 value 11.
1710
1711
1712
1713
1714
1715
1716
1717 23
1718
1719
1720 iv) Application Identifier - Sequence of eight printable ASCII
1721 characters used to identify the application owning the Application
1722 Extension.
1723
1724 v) Application Authentication Code - Sequence of three bytes used
1725 to authenticate the Application Identifier. An Application program
1726 may use an algorithm to compute a binary code that uniquely
1727 identifies it as the application owning the Application Extension.
1728
1729
1730 d. Extensions and Scope. This block does not have scope. This block
1731 cannot be modified by any extension.
1732
1733 e. Recommendation. None.
1734
1735
173627. Trailer.
1737
1738 a. Description. This block is a single-field block indicating the end of
1739 the GIF Data Stream. It contains the fixed value 0x3B.
1740
1741 b. Required Version. 87a.
1742
1743 c. Syntax.
1744
1745 7 6 5 4 3 2 1 0 Field Name Type
1746 +---------------+
1747 0 | | GIF Trailer Byte
1748 +---------------+
1749
1750 d. Extensions and Scope. This block does not have scope, it terminates
1751 the GIF Data Stream. This block may not be modified by any extension.
1752
1753 e. Recommendations. None.
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783 24
1784
1785
1786Appendix
1787A. Quick Reference Table.
1788
1789Block Name Required Label Ext. Vers.
1790Application Extension Opt. (*) 0xFF (255) yes 89a
1791Comment Extension Opt. (*) 0xFE (254) yes 89a
1792Global Color Table Opt. (1) none no 87a
1793Graphic Control Extension Opt. (*) 0xF9 (249) yes 89a
1794Header Req. (1) none no N/A
1795Image Descriptor Opt. (*) 0x2C (044) no 87a (89a)
1796Local Color Table Opt. (*) none no 87a
1797Logical Screen Descriptor Req. (1) none no 87a (89a)
1798Plain Text Extension Opt. (*) 0x01 (001) yes 89a
1799Trailer Req. (1) 0x3B (059) no 87a
1800
1801Unlabeled Blocks
1802Header Req. (1) none no N/A
1803Logical Screen Descriptor Req. (1) none no 87a (89a)
1804Global Color Table Opt. (1) none no 87a
1805Local Color Table Opt. (*) none no 87a
1806
1807Graphic-Rendering Blocks
1808Plain Text Extension Opt. (*) 0x01 (001) yes 89a
1809Image Descriptor Opt. (*) 0x2C (044) no 87a (89a)
1810
1811Control Blocks
1812Graphic Control Extension Opt. (*) 0xF9 (249) yes 89a
1813
1814Special Purpose Blocks
1815Trailer Req. (1) 0x3B (059) no 87a
1816Comment Extension Opt. (*) 0xFE (254) yes 89a
1817Application Extension Opt. (*) 0xFF (255) yes 89a
1818
1819legend: (1) if present, at most one occurrence
1820 (*) zero or more occurrences
1821 (+) one or more occurrences
1822
1823Notes : The Header is not subject to Version Numbers.
1824(89a) The Logical Screen Descriptor and the Image Descriptor retained their
1825syntax from version 87a to version 89a, but some fields reserved under version
182687a are used under version 89a.
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849 25
1850
1851
1852Appendix
1853B. GIF Grammar.
1854
1855A Grammar is a form of notation to represent the sequence in which certain
1856objects form larger objects. A grammar is also used to represent the number of
1857objects that can occur at a given position. The grammar given here represents
1858the sequence of blocks that form the GIF Data Stream. A grammar is given by
1859listing its rules. Each rule consists of the left-hand side, followed by some
1860form of equals sign, followed by the right-hand side. In a rule, the
1861right-hand side describes how the left-hand side is defined. The right-hand
1862side consists of a sequence of entities, with the possible presence of special
1863symbols. The following legend defines the symbols used in this grammar for GIF.
1864
1865Legend: <> grammar word
1866 ::= defines symbol
1867 * zero or more occurrences
1868 + one or more occurrences
1869 | alternate element
1870 [] optional element
1871
1872Example:
1873
1874<GIF Data Stream> ::= Header <Logical Screen> <Data>* Trailer
1875
1876This rule defines the entity <GIF Data Stream> as follows. It must begin with a
1877Header. The Header is followed by an entity called Logical Screen, which is
1878defined below by another rule. The Logical Screen is followed by the entity
1879Data, which is also defined below by another rule. Finally, the entity Data is
1880followed by the Trailer. Since there is no rule defining the Header or the
1881Trailer, this means that these blocks are defined in the document. The entity
1882Data has a special symbol (*) following it which means that, at this position,
1883the entity Data may be repeated any number of times, including 0 times. For
1884further reading on this subject, refer to a standard text on Programming
1885Languages.
1886
1887
1888The Grammar.
1889
1890<GIF Data Stream> ::= Header <Logical Screen> <Data>* Trailer
1891
1892<Logical Screen> ::= Logical Screen Descriptor [Global Color Table]
1893
1894<Data> ::= <Graphic Block> |
1895 <Special-Purpose Block>
1896
1897<Graphic Block> ::= [Graphic Control Extension] <Graphic-Rendering Block>
1898
1899<Graphic-Rendering Block> ::= <Table-Based Image> |
1900 Plain Text Extension
1901
1902<Table-Based Image> ::= Image Descriptor [Local Color Table] Image Data
1903
1904<Special-Purpose Block> ::= Application Extension |
1905 Comment Extension
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915 26
1916
1917
1918NOTE : The grammar indicates that it is possible for a GIF Data Stream to
1919contain the Header, the Logical Screen Descriptor, a Global Color Table and the
1920GIF Trailer. This special case is used to load a GIF decoder with a Global
1921Color Table, in preparation for subsequent Data Streams without color tables at
1922all.
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981 27
1982
1983
1984Appendix
1985C. Glossary.
1986
1987Active Color Table - Color table used to render the next graphic. If the next
1988graphic is an image which has a Local Color Table associated with it, the
1989active color table becomes the Local Color Table associated with that image.
1990If the next graphic is an image without a Local Color Table, or a Plain Text
1991Extension, the active color table is the Global Color Table associated with the
1992Data Stream, if there is one; if there is no Global Color Table in the Data
1993Stream, the active color table is a color table saved from a previous Data
1994Stream, or one supplied by the decoder.
1995
1996Block - Collection of bytes forming a protocol unit. In general, the term
1997includes labeled and unlabeled blocks, as well as Extensions.
1998
1999Data Stream - The GIF Data Stream is composed of blocks and sub-blocks
2000representing images and graphics, together with control information to render
2001them on a display device. All control and data blocks in the Data Stream must
2002follow the Header and must precede the Trailer.
2003
2004Decoder - A program capable of processing a GIF Data Stream to render the
2005images and graphics contained in it.
2006
2007Encoder - A program capable of capturing and formatting image and graphic
2008raster data, following the definitions of the Graphics Interchange Format.
2009
2010Extension - A protocol block labeled by the Extension Introducer 0x21.
2011
2012Extension Introducer - Label (0x21) defining an Extension.
2013
2014Graphic - Data which can be rendered on the screen by virtue of some algorithm.
2015The term graphic is more general than the term image; in addition to images,
2016the term graphic also includes data such as text, which is rendered using
2017character bit-maps.
2018
2019Image - Data representing a picture or a drawing; an image is represented by an
2020array of pixels called the raster of the image.
2021
2022Raster - Array of pixel values representing an image.
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047 28
2048
2049
2050Appendix
2051D. Conventions.
2052
2053Animation - The Graphics Interchange Format is not intended as a platform for
2054animation, even though it can be done in a limited way.
2055
2056Byte Ordering - Unless otherwise stated, multi-byte numeric fields are ordered
2057with the Least Significant Byte first.
2058
2059Color Indices - Color indices always refer to the active color table, either
2060the Global Color Table or the Local Color Table.
2061
2062Color Order - Unless otherwise stated, all triple-component RGB color values
2063are specified in Red-Green-Blue order.
2064
2065Color Tables - Both color tables, the Global and the Local, are optional; if
2066present, the Global Color Table is to be used with every image in the Data
2067Stream for which a Local Color Table is not given; if present, a Local Color
2068Table overrides the Global Color Table. However, if neither color table is
2069present, the application program is free to use an arbitrary color table. If
2070the graphics in several Data Streams are related and all use the same color
2071table, an encoder could place the color table as the Global Color Table in the
2072first Data Stream and leave subsequent Data Streams without a Global Color
2073Table or any Local Color Tables; in this way, the overhead for the table is
2074eliminated. It is recommended that the decoder save the previous Global Color
2075Table to be used with the Data Stream that follows, in case it does not contain
2076either a Global Color Table or any Local Color Tables. In general, this allows
2077the application program to use past color tables, significantly reducing
2078transmission overhead.
2079
2080Extension Blocks - Extensions are defined using the Extension Introducer code
2081to mark the beginning of the block, followed by a block label, identifying the
2082type of extension. Extension Codes are numbers in the range from 0x00 to 0xFF,
2083inclusive. Special purpose extensions are transparent to the decoder and may be
2084omitted when transmitting the Data Stream on-line. The GIF capabilities
2085dialogue makes the provision for the receiver to request the transmission of
2086all blocks; the default state in this regard is no transmission of Special
2087purpose blocks.
2088
2089Reserved Fields - All Reserved Fields are expected to have each bit set to zero
2090(off).
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113 29
2114
2115
2116Appendix
2117E. Interlaced Images.
2118
2119The rows of an Interlaced images are arranged in the following order:
2120
2121 Group 1 : Every 8th. row, starting with row 0. (Pass 1)
2122 Group 2 : Every 8th. row, starting with row 4. (Pass 2)
2123 Group 3 : Every 4th. row, starting with row 2. (Pass 3)
2124 Group 4 : Every 2nd. row, starting with row 1. (Pass 4)
2125
2126The Following example illustrates how the rows of an interlaced image are
2127ordered.
2128
2129 Row Number Interlace Pass
2130
2131 0 ----------------------------------------- 1
2132 1 ----------------------------------------- 4
2133 2 ----------------------------------------- 3
2134 3 ----------------------------------------- 4
2135 4 ----------------------------------------- 2
2136 5 ----------------------------------------- 4
2137 6 ----------------------------------------- 3
2138 7 ----------------------------------------- 4
2139 8 ----------------------------------------- 1
2140 9 ----------------------------------------- 4
2141 10 ----------------------------------------- 3
2142 11 ----------------------------------------- 4
2143 12 ----------------------------------------- 2
2144 13 ----------------------------------------- 4
2145 14 ----------------------------------------- 3
2146 15 ----------------------------------------- 4
2147 16 ----------------------------------------- 1
2148 17 ----------------------------------------- 4
2149 18 ----------------------------------------- 3
2150 19 ----------------------------------------- 4
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179 30
2180
2181
2182Appendix
2183F. Variable-Length-Code LZW Compression.
2184
2185The Variable-Length-Code LZW Compression is a variation of the Lempel-Ziv
2186Compression algorithm in which variable-length codes are used to replace
2187patterns detected in the original data. The algorithm uses a code or
2188translation table constructed from the patterns encountered in the original
2189data; each new pattern is entered into the table and its index is used to
2190replace it in the compressed stream.
2191
2192The compressor takes the data from the input stream and builds a code or
2193translation table with the patterns as it encounters them; each new pattern is
2194entered into the code table and its index is added to the output stream; when a
2195pattern is encountered which had been detected since the last code table
2196refresh, its index from the code table is put on the output stream, thus
2197achieving the data compression. The expander takes input from the compressed
2198data stream and builds the code or translation table from it; as the compressed
2199data stream is processed, codes are used to index into the code table and the
2200corresponding data is put on the decompressed output stream, thus achieving
2201data decompression. The details of the algorithm are explained below. The
2202Variable-Length-Code aspect of the algorithm is based on an initial code size
2203(LZW-initial code size), which specifies the initial number of bits used for
2204the compression codes. When the number of patterns detected by the compressor
2205in the input stream exceeds the number of patterns encodable with the current
2206number of bits, the number of bits per LZW code is increased by one.
2207
2208The Raster Data stream that represents the actual output image can be
2209represented as:
2210
2211 7 6 5 4 3 2 1 0
2212 +---------------+
2213 | LZW code size |
2214 +---------------+
2215
2216 +---------------+ ----+
2217 | block size | |
2218 +---------------+ |
2219 | | +-- Repeated as many
2220 | data bytes | | times as necessary.
2221 | | |
2222 +---------------+ ----+
2223
2224 . . . . . . ------- The code that terminates the LZW
2225 compressed data must appear before
2226 Block Terminator.
2227 +---------------+
2228 |0 0 0 0 0 0 0 0| Block Terminator
2229 +---------------+
2230
2231The conversion of the image from a series of pixel values to a transmitted or
2232stored character stream involves several steps. In brief these steps are:
2233
22341. Establish the Code Size - Define the number of bits needed to represent the
2235actual data.
2236
22372. Compress the Data - Compress the series of image pixels to a series of
2238
2239
2240
2241
2242
2243
2244
2245 31
2246
2247
2248compression codes.
2249
22503. Build a Series of Bytes - Take the set of compression codes and convert to a
2251string of 8-bit bytes.
2252
22534. Package the Bytes - Package sets of bytes into blocks preceded by character
2254counts and output.
2255
2256ESTABLISH CODE SIZE
2257
2258The first byte of the Compressed Data stream is a value indicating the minimum
2259number of bits required to represent the set of actual pixel values. Normally
2260this will be the same as the number of color bits. Because of some algorithmic
2261constraints however, black & white images which have one color bit must be
2262indicated as having a code size of 2.
2263This code size value also implies that the compression codes must start out one
2264bit longer.
2265
2266COMPRESSION
2267
2268The LZW algorithm converts a series of data values into a series of codes which
2269may be raw values or a code designating a series of values. Using text
2270characters as an analogy, the output code consists of a character or a code
2271representing a string of characters.
2272
2273The LZW algorithm used in GIF matches algorithmically with the standard LZW
2274algorithm with the following differences:
2275
22761. A special Clear code is defined which resets all compression/decompression
2277parameters and tables to a start-up state. The value of this code is 2**<code
2278size>. For example if the code size indicated was 4 (image was 4 bits/pixel)
2279the Clear code value would be 16 (10000 binary). The Clear code can appear at
2280any point in the image data stream and therefore requires the LZW algorithm to
2281process succeeding codes as if a new data stream was starting. Encoders should
2282output a Clear code as the first code of each image data stream.
2283
22842. An End of Information code is defined that explicitly indicates the end of
2285the image data stream. LZW processing terminates when this code is encountered.
2286It must be the last code output by the encoder for an image. The value of this
2287code is <Clear code>+1.
2288
22893. The first available compression code value is <Clear code>+2.
2290
22914. The output codes are of variable length, starting at <code size>+1 bits per
2292code, up to 12 bits per code. This defines a maximum code value of 4095
2293(0xFFF). Whenever the LZW code value would exceed the current code length, the
2294code length is increased by one. The packing/unpacking of these codes must then
2295be altered to reflect the new code length.
2296
2297BUILD 8-BIT BYTES
2298
2299Because the LZW compression used for GIF creates a series of variable length
2300codes, of between 3 and 12 bits each, these codes must be reformed into a
2301series of 8-bit bytes that will be the characters actually stored or
2302transmitted. This provides additional compression of the image. The codes are
2303formed into a stream of bits as if they were packed right to left and then
2304
2305
2306
2307
2308
2309
2310
2311 32
2312
2313
2314picked off 8 bits at a time to be output.
2315
2316Assuming a character array of 8 bits per character and using 5 bit codes to be
2317packed, an example layout would be similar to:
2318
2319
2320 +---------------+
2321 0 | | bbbaaaaa
2322 +---------------+
2323 1 | | dcccccbb
2324 +---------------+
2325 2 | | eeeedddd
2326 +---------------+
2327 3 | | ggfffffe
2328 +---------------+
2329 4 | | hhhhhggg
2330 +---------------+
2331 . . .
2332 +---------------+
2333 N | |
2334 +---------------+
2335
2336
2337Note that the physical packing arrangement will change as the number of bits
2338per compression code change but the concept remains the same.
2339
2340PACKAGE THE BYTES
2341
2342Once the bytes have been created, they are grouped into blocks for output by
2343preceding each block of 0 to 255 bytes with a character count byte. A block
2344with a zero byte count terminates the Raster Data stream for a given image.
2345These blocks are what are actually output for the GIF image. This block format
2346has the side effect of allowing a decoding program the ability to read past the
2347actual image data if necessary by reading block counts and then skipping over
2348the data.
2349
2350
2351
2352FURTHER READING
2353
2354[1] Ziv, J. and Lempel, A. : "A Universal Algorithm for Sequential Data
2355Compression", IEEE Transactions on Information Theory, May 1977.
2356[2] Welch, T. : "A Technique for High-Performance Data Compression", Computer,
2357June 1984.
2358[3] Nelson, M.R. : "LZW Data Compression", Dr. Dobb's Journal, October 1989.
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377 33
2378
2379
2380Appendix
2381G. On-line Capabilities Dialogue.
2382
2383NOTE : This section is currently (10 July 1990) under revision; the information
2384provided here should be used as general guidelines. Code written based on this
2385information should be designed in a flexible way to accommodate any changes
2386resulting from the revisions.
2387
2388The following sequences are defined for use in mediating control between a GIF
2389sender and GIF receiver over an interactive communications line. These
2390sequences do not apply to applications that involve downloading of static GIF
2391files and are not considered part of a GIF file.
2392
2393GIF CAPABILITIES ENQUIRY
2394
2395The GIF Capabilities Enquiry sequence is issued from a host and requests an
2396interactive GIF decoder to return a response message that defines the graphics
2397parameters for the decoder. This involves returning information about available
2398screen sizes, number of bits/color supported and the amount of color detail
2399supported. The escape sequence for the GIF Capabilities Enquiry is defined as:
2400
2401ESC[>0g 0x1B 0x5B 0x3E 0x30 0x67
2402
2403GIF CAPABILITIES RESPONSE
2404
2405The GIF Capabilities Response message is returned by an interactive GIF decoder
2406and defines the decoder's display capabilities for all graphics modes that are
2407supported by the software. Note that this can also include graphics printers as
2408well as a monitor screen. The general format of this message is:
2409
2410#version;protocol{;dev, width, height, color-bits, color-res}...<CR>
2411
2412
2413'#' GIF Capabilities Response identifier character.
2414version GIF format version number; initially '87a'.
2415protocol='0' No end-to-end protocol supported by decoder Transfer as direct
2416 8-bit data stream.
2417protocol='1' Can use CIS B+ error correction protocol to transfer GIF data
2418 interactively from the host directly to the display.
2419dev = '0' Screen parameter set follows.
2420dev = '1' Printer parameter set follows.
2421width Maximum supported display width in pixels.
2422height Maximum supported display height in pixels.
2423color-bits Number of bits per pixel supported. The number of supported
2424 colors is therefore 2**color-bits.
2425color-res Number of bits per color component supported in the hardware
2426 color palette. If color-res is '0' then no hardware palette
2427 table is available.
2428
2429Note that all values in the GIF Capabilities Response are returned as ASCII
2430decimal numbers and the message is terminated by a Carriage Return character.
2431
2432The following GIF Capabilities Response message describes three standard IBM PC
2433Enhanced Graphics Adapter configurations with no printer; the GIF data stream
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443 34
2444
2445
2446can be processed within an error correcting protocol:
2447
2448#87a;1;0,320,200,4,0;0,640,200,2,2;0,640,350,4,2<CR>
2449
2450ENTER GIF GRAPHICS MODE
2451
2452Two sequences are currently defined to invoke an interactive GIF decoder into
2453action. The only difference between them is that different output media are
2454selected. These sequences are:
2455
2456ESC[>1g Display GIF image on screen
2457
2458 0x1B 0x5B 0x3E 0x31 0x67
2459
2460ESC[>2g Display image directly to an attached graphics printer. The image may
2461optionally be displayed on the screen as well.
2462
2463 0x1B 0x5B 0x3E 0x32 0x67
2464
2465Note that the 'g' character terminating each sequence is in lowercase.
2466
2467INTERACTIVE ENVIRONMENT
2468
2469The assumed environment for the transmission of GIF image data from an
2470interactive application is a full 8-bit data stream from host to micro. All
2471256 character codes must be transferrable. The establishing of an 8-bit data
2472path for communications will normally be taken care of by the host application
2473programs. It is however up to the receiving communications programs supporting
2474GIF to be able to receive and pass on all 256 8-bit codes to the GIF decoder
2475software.
2476.
Image manipulation for perl