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Monster Media 1993 #2
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Cover Sheet for the GIF89a Specification
DEFERRED CLEAR CODE IN LZW COMPRESSION
There has been confusion about where clear codes can be found in the
data stream. As the specification says, they may appear at anytime. There
is not a requirement to send a clear code when the string table is full.
It is the encoder's decision as to when the table should be cleared. When
the table is full, the encoder can chose to use the table as is, making no
changes to it until the encoder chooses to clear it. The encoder during
this time sends out codes that are of the maximum Code Size.
As we can see from the above, when the decoder's table is full, it must
not change the table until a clear code is received. The Code Size is that
of the maximum Code Size. Processing other than this is done normally.
Because of a large base of decoders that do not handle the decompression in
this manner, we ask developers of GIF encoding software to NOT implement
this feature until at least January 1991 and later if they see that their
particular market is not ready for it. This will give developers of GIF
decoding software time to implement this feature and to get it into the
hands of their clients before the decoders start "breaking" on the new
GIF's. It is not required that encoders change their software to take
advantage of the deferred clear code, but it is for decoders.
APPLICATION EXTENSION BLOCK - APPLICATION IDENTIFIER
There will be a Courtesy Directory file located on CompuServe in the PICS
forum. This directory will contain Application Identifiers for Application
Extension Blocks that have been used by developers of GIF applications.
This file is intended to help keep developers that wish to create
Application Extension Blocks from using the same Application Identifiers.
This is not an official directory; it is for voluntary participation only
and does not guarantee that someone will not use the same identifier.
E-Mail can be sent to Larry Wood (forum manager of PICS) indicating the
request for inclusion in this file with an identifier.
GRAPHICS INTERCHANGE FORMAT(sm)
Version 89a
(c)1987,1988,1989,1990
Copyright
CompuServe Incorporated
Columbus, Ohio
CompuServe Incorporated Graphics Interchange Format
Document Date : 31 July 1990 Programming Reference
Table of Contents
Disclaimer................................................................. 1
Foreword................................................................... 1
Licensing.................................................................. 1
About the Document......................................................... 2
General Description........................................................ 2
Version Numbers............................................................ 2
The Encoder................................................................ 3
The Decoder................................................................ 3
Compliance................................................................. 3
About Recommendations...................................................... 4
About Color Tables......................................................... 4
Blocks, Extensions and Scope............................................... 4
Block Sizes................................................................ 5
Using GIF as an embedded protocol.......................................... 5
Data Sub-blocks............................................................ 5
Block Terminator........................................................... 6
Header..................................................................... 7
Logical Screen Descriptor.................................................. 8
Global Color Table......................................................... 10
Image Descriptor........................................................... 11
Local Color Table.......................................................... 13
Table Based Image Data..................................................... 14
Graphic Control Extension.................................................. 15
Comment Extension.......................................................... 17
Plain Text Extension....................................................... 18
Application Extension...................................................... 21
Trailer.................................................................... 23
Quick Reference Table...................................................... 24
GIF Grammar................................................................ 25
Glossary................................................................... 27
Conventions................................................................ 28
Interlaced Images.......................................................... 29
Variable-Length-Code LZW Compression....................................... 30
On-line Capabilities Dialogue.............................................. 33
1
1. Disclaimer.
The information provided herein is subject to change without notice. In no
event will CompuServe Incorporated be liable for damages, including any loss of
revenue, loss of profits or other incidental or consequential damages arising
out of the use or inability to use the information; CompuServe Incorporated
makes no claim as to the suitability of the information.
2. Foreword.
This document defines the Graphics Interchange Format(sm). The specification
given here defines version 89a, which is an extension of version 87a.
The Graphics Interchange Format(sm) as specified here should be considered
complete; any deviation from it should be considered invalid, including but not
limited to, the use of reserved or undefined fields within control or data
blocks, the inclusion of extraneous data within or between blocks, the use of
methods or algorithms not specifically listed as part of the format, etc. In
general, any and all deviations, extensions or modifications not specified in
this document should be considered to be in violation of the format and should
be avoided.
3. Licensing.
The Graphics Interchange Format(c) is the copyright property of CompuServe
Incorporated. Only CompuServe Incorporated is authorized to define, redefine,
enhance, alter, modify or change in any way the definition of the format.
CompuServe Incorporated hereby grants a limited, non-exclusive, royalty-free
license for the use of the Graphics Interchange Format(sm) in computer
software; computer software utilizing GIF(sm) must acknowledge ownership of the
Graphics Interchange Format and its Service Mark by CompuServe Incorporated, in
User and Technical Documentation. Computer software utilizing GIF, which is
distributed or may be distributed without User or Technical Documentation must
display to the screen or printer a message acknowledging ownership of the
Graphics Interchange Format and the Service Mark by CompuServe Incorporated; in
this case, the acknowledgement may be displayed in an opening screen or leading
banner, or a closing screen or trailing banner. A message such as the following
may be used:
"The Graphics Interchange Format(c) is the Copyright property of
CompuServe Incorporated. GIF(sm) is a Service Mark property of
CompuServe Incorporated."
For further information, please contact :
CompuServe Incorporated
Graphics Technology Department
5000 Arlington Center Boulevard
Columbus, Ohio 43220
U. S. A.
CompuServe Incorporated maintains a mailing list with all those individuals and
organizations who wish to receive copies of this document when it is corrected
2
or revised. This service is offered free of charge; please provide us with your
mailing address.
4. About the Document.
This document describes in detail the definition of the Graphics Interchange
Format. This document is intended as a programming reference; it is
recommended that the entire document be read carefully before programming,
because of the interdependence of the various parts. There is an individual
section for each of the Format blocks. Within each section, the sub-section
labeled Required Version refers to the version number that an encoder will have
to use if the corresponding block is used in the Data Stream. Within each
section, a diagram describes the individual fields in the block; the diagrams
are drawn vertically; top bytes in the diagram appear first in the Data Stream.
Bits within a byte are drawn most significant on the left end. Multi-byte
numeric fields are ordered Least Significant Byte first. Numeric constants are
represented as Hexadecimal numbers, preceded by "0x". Bit fields within a byte
are described in order from most significant bits to least significant bits.
5. General Description.
The Graphics Interchange Format(sm) defines a protocol intended for the on-line
transmission and interchange of raster graphic data in a way that is
independent of the hardware used in their creation or display.
The Graphics Interchange Format is defined in terms of blocks and sub-blocks
which contain relevant parameters and data used in the reproduction of a
graphic. A GIF Data Stream is a sequence of protocol blocks and sub-blocks
representing a collection of graphics. In general, the graphics in a Data
Stream are assumed to be related to some degree, and to share some control
information; it is recommended that encoders attempt to group together related
graphics in order to minimize hardware changes during processing and to
minimize control information overhead. For the same reason, unrelated graphics
or graphics which require resetting hardware parameters should be encoded
separately to the extent possible.
A Data Stream may originate locally, as when read from a file, or it may
originate remotely, as when transmitted over a data communications line. The
Format is defined with the assumption that an error-free Transport Level
Protocol is used for communications; the Format makes no provisions for
error-detection and error-correction.
The GIF Data Stream must be interpreted in context, that is, the application
program must rely on information external to the Data Stream to invoke the
decoder process.
6. Version Numbers.
The version number in the Header of a Data Stream is intended to identify the
minimum set of capabilities required of a decoder in order to fully process the
Data Stream. An encoder should use the earliest possible version number that
includes all the blocks used in the Data Stream. Within each block section in
this document, there is an entry labeled Required Version which specifies the
3
earliest version number that includes the corresponding block. The encoder
should make every attempt to use the earliest version number covering all the
blocks in the Data Stream; the unnecessary use of later version numbers will
hinder processing by some decoders.
7. The Encoder.
The Encoder is the program used to create a GIF Data Stream. From raster data
and other information, the encoder produces the necessary control and data
blocks needed for reproducing the original graphics.
The encoder has the following primary responsibilities.
- Include in the Data Stream all the necessary information to
reproduce the graphics.
- Insure that a Data Stream is labeled with the earliest possible
Version Number that will cover the definition of all the blocks in
it; this is to ensure that the largest number of decoders can
process the Data Stream.
- Ensure encoding of the graphics in such a way that the decoding
process is optimized. Avoid redundant information as much as
possible.
- To the extent possible, avoid grouping graphics which might
require resetting hardware parameters during the decoding process.
- Set to zero (off) each of the bits of each and every field
designated as reserved. Note that some fields in the Logical Screen
Descriptor and the Image Descriptor were reserved under Version
87a, but are used under version 89a.
8. The Decoder.
The Decoder is the program used to process a GIF Data Stream. It processes the
Data Stream sequentially, parsing the various blocks and sub-blocks, using the
control information to set hardware and process parameters and interpreting the
data to render the graphics.
The decoder has the following primary responsibilities.
- Process each graphic in the Data Stream in sequence, without
delays other than those specified in the control information.
- Set its hardware parameters to fit, as closely as possible, the
control information contained in the Data Stream.
9. Compliance.
An encoder or a decoder is said to comply with a given version of the Graphics
Interchange Format if and only if it fully conforms with and correctly
implements the definition of the standard associated with that version. An
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encoder or a decoder may be compliant with a given version number and not
compliant with some subsequent version.
10. About Recommendations.
Each block section in this document contains an entry labeled Recommendation;
this section lists a set of recommendations intended to guide and organize the
use of the particular blocks. Such recommendations are geared towards making
the functions of encoders and decoders more efficient, as well as making
optimal use of the communications bandwidth. It is advised that these
recommendations be followed.
11. About Color Tables.
The GIF format utilizes color tables to render raster-based graphics. A color
table can have one of two different scopes: global or local. A Global Color
Table is used by all those graphics in the Data Stream which do not have a
Local Color Table associated with them. The scope of the Global Color Table is
the entire Data Stream. A Local Color Table is always associated with the
graphic that immediately follows it; the scope of a Local Color Table is
limited to that single graphic. A Local Color Table supersedes a Global Color
Table, that is, if a Data Stream contains a Global Color Table, and an image
has a Local Color Table associated with it, the decoder must save the Global
Color Table, use the Local Color Table to render the image, and then restore
the Global Color Table. Both types of color tables are optional, making it
possible for a Data Stream to contain numerous graphics without a color table
at all. For this reason, it is recommended that the decoder save the last
Global Color Table used until another Global Color Table is encountered. In
this way, a Data Stream which does not contain either a Global Color Table or
a Local Color Table may be processed using the last Global Color Table saved.
If a Global Color Table from a previous Stream is used, that table becomes the
Global Color Table of the present Stream. This is intended to reduce the
overhead incurred by color tables. In particular, it is recommended that an
encoder use only one Global Color Table if all the images in related Data
Streams can be rendered with the same table. If no color table is available at
all, the decoder is free to use a system color table or a table of its own. In
that case, the decoder may use a color table with as many colors as its
hardware is able to support; it is recommended that such a table have black and
white as its first two entries, so that monochrome images can be rendered
adequately.
The Definition of the GIF Format allows for a Data Stream to contain only the
Header, the Logical Screen Descriptor, a Global Color Table and the GIF
Trailer. Such a Data Stream would be used to load a decoder with a Global Color
Table, in preparation for subsequent Data Streams without a color table at all.
12. Blocks, Extensions and Scope.
Blocks can be classified into three groups : Control, Graphic-Rendering and
Special Purpose. Control blocks, such as the Header, the Logical Screen
Descriptor, the Graphic Control Extension and the Trailer, contain information
used to control the process of the Data Stream or information used in setting
hardware parameters. Graphic-Rendering blocks such as the Image Descriptor and
5
the Plain Text Extension contain information and data used to render a graphic
on the display device. Special Purpose blocks such as the Comment Extension and
the Application Extension are neither used to control the process of the Data
Stream nor do they contain information or data used to render a graphic on the
display device. With the exception of the Logical Screen Descriptor and the
Global Color Table, whose scope is the entire Data Stream, all other Control
blocks have a limited scope, restricted to the Graphic-Rendering block that
follows them. Special Purpose blocks do not delimit the scope of any Control
blocks; Special Purpose blocks are transparent to the decoding process.
Graphic-Rendering blocks and extensions are used as scope delimiters for
Control blocks and extensions. The labels used to identify labeled blocks fall
into three ranges : 0x00-0x7F (0-127) are the Graphic Rendering blocks,
excluding the Trailer (0x3B); 0x80-0xF9 (128-249) are the Control blocks;
0xFA-0xFF (250-255) are the Special Purpose blocks. These ranges are defined so
that decoders can handle block scope by appropriately identifying block labels,
even when the block itself cannot be processed.
13. Block Sizes.
The Block Size field in a block, counts the number of bytes remaining in the
block, not counting the Block Size field itself, and not counting the Block
Terminator, if one is to follow. Blocks other than Data Blocks are intended to
be of fixed length; the Block Size field is provided in order to facilitate
skipping them, not to allow their size to change in the future. Data blocks
and sub-blocks are of variable length to accommodate the amount of data.
14. Using GIF as an embedded protocol.
As an embedded protocol, GIF may be part of larger application protocols,
within which GIF is used to render graphics. In such a case, the application
protocol could define a block within which the GIF Data Stream would be
contained. The application program would then invoke a GIF decoder upon
encountering a block of type GIF. This approach is recommended in favor of
using Application Extensions, which become overhead for all other applications
that do not process them. Because a GIF Data Stream must be processed in
context, the application must rely on some means of identifying the GIF Data
Stream outside of the Stream itself.
15. Data Sub-blocks.
a. Description. Data Sub-blocks are units containing data. They do not
have a label, these blocks are processed in the context of control
blocks, wherever data blocks are specified in the format. The first byte
of the Data sub-block indicates the number of data bytes to follow. A
data sub-block may contain from 0 to 255 data bytes. The size of the
block does not account for the size byte itself, therefore, the empty
sub-block is one whose size field contains 0x00.
b. Required Version. 87a.
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c. Syntax.
7 6 5 4 3 2 1 0 Field Name Type
+---------------+
0 | | Block Size Byte
+---------------+
1 | |
+- -+
2 | |
+- -+
3 | |
+- -+
| | Data Values Byte
+- -+
up | |
+- . . . . -+
to | |
+- -+
| |
+- -+
255 | |
+---------------+
i) Block Size - Number of bytes in the Data Sub-block; the size
must be within 0 and 255 bytes, inclusive.
ii) Data Values - Any 8-bit value. There must be exactly as many
Data Values as specified by the Block Size field.
d. Extensions and Scope. This type of block always occurs as part of a
larger unit. It does not have a scope of itself.
e. Recommendation. None.
16. Block Terminator.
a. Description. This zero-length Data Sub-block is used to terminate a
sequence of Data Sub-blocks. It contains a single byte in the position of
the Block Size field and does not contain data.
b. Required Version. 87a.
c. Syntax.
7 6 5 4 3 2 1 0 Field Name Type
+---------------+
0 | | Block Size Byte
+---------------+
i) Block Size - Number of bytes in the Data Sub-block; this field
contains the fixed value 0x00.
ii) Data Values - This block does not contain any data.
7
d. Extensions and Scope. This block terminates the immediately preceding
sequence of Data Sub-blocks. This block cannot be modified by any
extension.
e. Recommendation. None.
17. Header.
a. Description. The Header identifies the GIF Data Stream in context. The
Signature field marks the beginning of the Data Stream, and the Version
field identifies the set of capabilities required of a decoder to fully
process the Data Stream. This block is REQUIRED; exactly one Header must
be present per Data Stream.
b. Required Version. Not applicable. This block is not subject to a
version number. This block must appear at the beginning of every Data
Stream.
c. Syntax.
7 6 5 4 3 2 1 0 Field Name Type
+---------------+
0 | | Signature 3 Bytes
+- -+
1 | |
+- -+
2 | |
+---------------+
3 | | Version 3 Bytes
+- -+
4 | |
+- -+
5 | |
+---------------+
i) Signature - Identifies the GIF Data Stream. This field contains
the fixed value 'GIF'.
ii) Version - Version number used to format the data stream.
Identifies the minimum set of capabilities necessary to a decoder
to fully process the contents of the Data Stream.
Version Numbers as of 10 July 1990 : "87a" - May 1987
"89a" - July 1989
Version numbers are ordered numerically increasing on the first two
digits starting with 87 (87,88,...,99,00,...,85,86) and
alphabetically increasing on the third character (a,...,z).
iii) Extensions and Scope. The scope of this block is the entire
Data Stream. This block cannot be modified by any extension.
8
d. Recommendations.
i) Signature - This field identifies the beginning of the GIF Data
Stream; it is not intended to provide a unique signature for the
identification of the data. It is recommended that the GIF Data
Stream be identified externally by the application. (Refer to
Appendix G for on-line identification of the GIF Data Stream.)
ii) Version - ENCODER : An encoder should use the earliest possible
version number that defines all the blocks used in the Data Stream.
When two or more Data Streams are combined, the latest of the
individual version numbers should be used for the resulting Data
Stream. DECODER : A decoder should attempt to process the data
stream to the best of its ability; if it encounters a version
number which it is not capable of processing fully, it should
nevertheless, attempt to process the data stream to the best of its
ability, perhaps after warning the user that the data may be
incomplete.
18. Logical Screen Descriptor.
a. Description. The Logical Screen Descriptor contains the parameters
necessary to define the area of the display device within which the
images will be rendered. The coordinates in this block are given with
respect to the top-left corner of the virtual screen; they do not
necessarily refer to absolute coordinates on the display device. This
implies that they could refer to window coordinates in a window-based
environment or printer coordinates when a printer is used.
This block is REQUIRED; exactly one Logical Screen Descriptor must be
present per Data Stream.
b. Required Version. Not applicable. This block is not subject to a
version number. This block must appear immediately after the Header.
c. Syntax.
7 6 5 4 3 2 1 0 Field Name Type
+---------------+
0 | | Logical Screen Width Unsigned
+- -+
1 | |
+---------------+
2 | | Logical Screen Height Unsigned
+- -+
3 | |
+---------------+
4 | | | | | <Packed Fields> See below
+---------------+
5 | | Background Color Index Byte
+---------------+
6 | | Pixel Aspect Ratio Byte
+---------------+
9
<Packed Fields> = Global Color Table Flag 1 Bit
Color Resolution 3 Bits
Sort Flag 1 Bit
Size of Global Color Table 3 Bits
i) Logical Screen Width - Width, in pixels, of the Logical Screen
where the images will be rendered in the displaying device.
ii) Logical Screen Height - Height, in pixels, of the Logical
Screen where the images will be rendered in the displaying device.
iii) Global Color Table Flag - Flag indicating the presence of a
Global Color Table; if the flag is set, the Global Color Table will
immediately follow the Logical Screen Descriptor. This flag also
selects the interpretation of the Background Color Index; if the
flag is set, the value of the Background Color Index field should
be used as the table index of the background color. (This field is
the most significant bit of the byte.)
Values : 0 - No Global Color Table follows, the Background
Color Index field is meaningless.
1 - A Global Color Table will immediately follow, the
Background Color Index field is meaningful.
iv) Color Resolution - Number of bits per primary color available
to the original image, minus 1. This value represents the size of
the entire palette from which the colors in the graphic were
selected, not the number of colors actually used in the graphic.
For example, if the value in this field is 3, then the palette of
the original image had 4 bits per primary color available to create
the image. This value should be set to indicate the richness of
the original palette, even if not every color from the whole
palette is available on the source machine.
v) Sort Flag - Indicates whether the Global Color Table is sorted.
If the flag is set, the Global Color Table is sorted, in order of
decreasing importance. Typically, the order would be decreasing
frequency, with most frequent color first. This assists a decoder,
with fewer available colors, in choosing the best subset of colors;
the decoder may use an initial segment of the table to render the
graphic.
Values : 0 - Not ordered.
1 - Ordered by decreasing importance, most
important color first.
vi) Size of Global Color Table - If the Global Color Table Flag is
set to 1, the value in this field is used to calculate the number
of bytes contained in the Global Color Table. To determine that
actual size of the color table, raise 2 to [the value of the field
+ 1]. Even if there is no Global Color Table specified, set this
field according to the above formula so that decoders can choose
the best graphics mode to display the stream in. (This field is
made up of the 3 least significant bits of the byte.)
vii) Background Color Index - Index into the Global Color Table for
10
the Background Color. The Background Color is the color used for
those pixels on the screen that are not covered by an image. If the
Global Color Table Flag is set to (zero), this field should be zero
and should be ignored.
viii) Pixel Aspect Ratio - Factor used to compute an approximation
of the aspect ratio of the pixel in the original image. If the
value of the field is not 0, this approximation of the aspect ratio
is computed based on the formula:
Aspect Ratio = (Pixel Aspect Ratio + 15) / 64
The Pixel Aspect Ratio is defined to be the quotient of the pixel's
width over its height. The value range in this field allows
specification of the widest pixel of 4:1 to the tallest pixel of
1:4 in increments of 1/64th.
Values : 0 - No aspect ratio information is given.
1..255 - Value used in the computation.
d. Extensions and Scope. The scope of this block is the entire Data
Stream. This block cannot be modified by any extension.
e. Recommendations. None.
19. Global Color Table.
a. Description. This block contains a color table, which is a sequence of
bytes representing red-green-blue color triplets. The Global Color Table
is used by images without a Local Color Table and by Plain Text
Extensions. Its presence is marked by the Global Color Table Flag being
set to 1 in the Logical Screen Descriptor; if present, it immediately
follows the Logical Screen Descriptor and contains a number of bytes
equal to
3 x 2^(Size of Global Color Table+1).
This block is OPTIONAL; at most one Global Color Table may be present
per Data Stream.
b. Required Version. 87a
11
c. Syntax.
7 6 5 4 3 2 1 0 Field Name Type
+===============+
0 | | Red 0 Byte
+- -+
1 | | Green 0 Byte
+- -+
2 | | Blue 0 Byte
+- -+
3 | | Red 1 Byte
+- -+
| | Green 1 Byte
+- -+
up | |
+- . . . . -+ ...
to | |
+- -+
| | Green 255 Byte
+- -+
767 | | Blue 255 Byte
+===============+
d. Extensions and Scope. The scope of this block is the entire Data
Stream. This block cannot be modified by any extension.
e. Recommendation. None.
20. Image Descriptor.
a. Description. Each image in the Data Stream is composed of an Image
Descriptor, an optional Local Color Table, and the image data. Each
image must fit within the boundaries of the Logical Screen, as defined
in the Logical Screen Descriptor.
The Image Descriptor contains the parameters necessary to process a table
based image. The coordinates given in this block refer to coordinates
within the Logical Screen, and are given in pixels. This block is a
Graphic-Rendering Block, optionally preceded by one or more Control
blocks such as the Graphic Control Extension, and may be optionally
followed by a Local Color Table; the Image Descriptor is always followed
by the image data.
This block is REQUIRED for an image. Exactly one Image Descriptor must
be present per image in the Data Stream. An unlimited number of images
may be present per Data Stream.
b. Required Version. 87a.
12
c. Syntax.
7 6 5 4 3 2 1 0 Field Name Type
+---------------+
0 | | Image Separator Byte
+---------------+
1 | | Image Left Position Unsigned
+- -+
2 | |
+---------------+
3 | | Image Top Position Unsigned
+- -+
4 | |
+---------------+
5 | | Image Width Unsigned
+- -+
6 | |
+---------------+
7 | | Image Height Unsigned
+- -+
8 | |
+---------------+
9 | | | | | | <Packed Fields> See below
+---------------+
<Packed Fields> = Local Color Table Flag 1 Bit
Interlace Flag 1 Bit
Sort Flag 1 Bit
Reserved 2 Bits
Size of Local Color Table 3 Bits
i) Image Separator - Identifies the beginning of an Image
Descriptor. This field contains the fixed value 0x2C.
ii) Image Left Position - Column number, in pixels, of the left edge
of the image, with respect to the left edge of the Logical Screen.
Leftmost column of the Logical Screen is 0.
iii) Image Top Position - Row number, in pixels, of the top edge of
the image with respect to the top edge of the Logical Screen. Top
row of the Logical Screen is 0.
iv) Image Width - Width of the image in pixels.
v) Image Height - Height of the image in pixels.
vi) Local Color Table Flag - Indicates the presence of a Local Color
Table immediately following this Image Descriptor. (This field is
the most significant bit of the byte.)
Values : 0 - Local Color Table is not present. Use
Global Color Table if available.
1 - Local Color Table present, and to follow
immediately after this Image Descriptor.
13
vii) Interlace Flag - Indicates if the image is interlaced. An image
is interlaced in a four-pass interlace pattern; see Appendix E for
details.
Values : 0 - Image is not interlaced.
1 - Image is interlaced.
viii) Sort Flag - Indicates whether the Local Color Table is
sorted. If the flag is set, the Local Color Table is sorted, in
order of decreasing importance. Typically, the order would be
decreasing frequency, with most frequent color first. This assists
a decoder, with fewer available colors, in choosing the best subset
of colors; the decoder may use an initial segment of the table to
render the graphic.
Values : 0 - Not ordered.
1 - Ordered by decreasing importance, most
important color first.
ix) Size of Local Color Table - If the Local Color Table Flag is
set to 1, the value in this field is used to calculate the number
of bytes contained in the Local Color Table. To determine that
actual size of the color table, raise 2 to the value of the field
+ 1. This value should be 0 if there is no Local Color Table
specified. (This field is made up of the 3 least significant bits
of the byte.)
d. Extensions and Scope. The scope of this block is the Table-based Image
Data Block that follows it. This block may be modified by the Graphic
Control Extension.
e. Recommendation. None.
21. Local Color Table.
a. Description. This block contains a color table, which is a sequence of
bytes representing red-green-blue color triplets. The Local Color Table
is used by the image that immediately follows. Its presence is marked by
the Local Color Table Flag being set to 1 in the Image Descriptor; if
present, the Local Color Table immediately follows the Image Descriptor
and contains a number of bytes equal to
3x2^(Size of Local Color Table+1).
If present, this color table temporarily becomes the active color table
and the following image should be processed using it. This block is
OPTIONAL; at most one Local Color Table may be present per Image
Descriptor and its scope is the single image associated with the Image
Descriptor that precedes it.
b. Required Version. 87a.
14
c. Syntax.
7 6 5 4 3 2 1 0 Field Name Type
+===============+
0 | | Red 0 Byte
+- -+
1 | | Green 0 Byte
+- -+
2 | | Blue 0 Byte
+- -+
3 | | Red 1 Byte
+- -+
| | Green 1 Byte
+- -+
up | |
+- . . . . -+ ...
to | |
+- -+
| | Green 255 Byte
+- -+
767 | | Blue 255 Byte
+===============+
d. Extensions and Scope. The scope of this block is the Table-based Image
Data Block that immediately follows it. This block cannot be modified by
any extension.
e. Recommendations. None.
22. Table Based Image Data.
a. Description. The image data for a table based image consists of a
sequence of sub-blocks, of size at most 255 bytes each, containing an
index into the active color table, for each pixel in the image. Pixel
indices are in order of left to right and from top to bottom. Each index
must be within the range of the size of the active color table, starting
at 0. The sequence of indices is encoded using the LZW Algorithm with
variable-length code, as described in Appendix F
b. Required Version. 87a.
c. Syntax. The image data format is as follows:
7 6 5 4 3 2 1 0 Field Name Type
+---------------+
| | LZW Minimum Code Size Byte
+---------------+
+===============+
| |
/ / Image Data Data Sub-blocks
| |
+===============+
15
i) LZW Minimum Code Size. This byte determines the initial number
of bits used for LZW codes in the image data, as described in
Appendix F.
d. Extensions and Scope. This block has no scope, it contains raster
data. Extensions intended to modify a Table-based image must appear
before the corresponding Image Descriptor.
e. Recommendations. None.
23. Graphic Control Extension.
a. Description. The Graphic Control Extension contains parameters used
when processing a graphic rendering block. The scope of this extension is
the first graphic rendering block to follow. The extension contains only
one data sub-block.
This block is OPTIONAL; at most one Graphic Control Extension may precede
a graphic rendering block. This is the only limit to the number of
Graphic Control Extensions that may be contained in a Data Stream.
b. Required Version. 89a.
c. Syntax.
7 6 5 4 3 2 1 0 Field Name Type
+---------------+
0 | | Extension Introducer Byte
+---------------+
1 | | Graphic Control Label Byte
+---------------+
+---------------+
0 | | Block Size Byte
+---------------+
1 | | | | | <Packed Fields> See below
+---------------+
2 | | Delay Time Unsigned
+- -+
3 | |
+---------------+
4 | | Transparent Color Index Byte
+---------------+
+---------------+
0 | | Block Terminator Byte
+---------------+
<Packed Fields> = Reserved 3 Bits
Disposal Method 3 Bits
User Input Flag 1 Bit
Transparent Color Flag 1 Bit
i) Extension Introducer - Identifies the beginning of an extension
16
block. This field contains the fixed value 0x21.
ii) Graphic Control Label - Identifies the current block as a
Graphic Control Extension. This field contains the fixed value
0xF9.
iii) Block Size - Number of bytes in the block, after the Block
Size field and up to but not including the Block Terminator. This
field contains the fixed value 4.
iv) Disposal Method - Indicates the way in which the graphic is to
be treated after being displayed.
Values : 0 - No disposal specified. The decoder is
not required to take any action.
1 - Do not dispose. The graphic is to be left
in place.
2 - Restore to background color. The area used by the
graphic must be restored to the background color.
3 - Restore to previous. The decoder is required to
restore the area overwritten by the graphic with
what was there prior to rendering the graphic.
4-7 - To be defined.
v) User Input Flag - Indicates whether or not user input is
expected before continuing. If the flag is set, processing will
continue when user input is entered. The nature of the User input
is determined by the application (Carriage Return, Mouse Button
Click, etc.).
Values : 0 - User input is not expected.
1 - User input is expected.
When a Delay Time is used and the User Input Flag is set,
processing will continue when user input is received or when the
delay time expires, whichever occurs first.
vi) Transparency Flag - Indicates whether a transparency index is
given in the Transparent Index field. (This field is the least
significant bit of the byte.)
Values : 0 - Transparent Index is not given.
1 - Transparent Index is given.
vii) Delay Time - If not 0, this field specifies the number of
hundredths (1/100) of a second to wait before continuing with the
processing of the Data Stream. The clock starts ticking immediately
after the graphic is rendered. This field may be used in
conjunction with the User Input Flag field.
viii) Transparency Index - The Transparency Index is such that when
encountered, the corresponding pixel of the display device is not
modified and processing goes on to the next pixel. The index is
present if and only if the Transparency Flag is set to 1.
ix) Block Terminator - This zero-length data block marks the end of
17
the Graphic Control Extension.
d. Extensions and Scope. The scope of this Extension is the graphic
rendering block that follows it; it is possible for other extensions to
be present between this block and its target. This block can modify the
Image Descriptor Block and the Plain Text Extension.
e. Recommendations.
i) Disposal Method - The mode Restore To Previous is intended to be
used in small sections of the graphic; the use of this mode imposes
severe demands on the decoder to store the section of the graphic
that needs to be saved. For this reason, this mode should be used
sparingly. This mode is not intended to save an entire graphic or
large areas of a graphic; when this is the case, the encoder should
make every attempt to make the sections of the graphic to be
restored be separate graphics in the data stream. In the case where
a decoder is not capable of saving an area of a graphic marked as
Restore To Previous, it is recommended that a decoder restore to
the background color.
ii) User Input Flag - When the flag is set, indicating that user
input is expected, the decoder may sound the bell (0x07) to alert
the user that input is being expected. In the absence of a
specified Delay Time, the decoder should wait for user input
indefinitely. It is recommended that the encoder not set the User
Input Flag without a Delay Time specified.
24. Comment Extension.
a. Description. The Comment Extension contains textual information which
is not part of the actual graphics in the GIF Data Stream. It is suitable
for including comments about the graphics, credits, descriptions or any
other type of non-control and non-graphic data. The Comment Extension
may be ignored by the decoder, or it may be saved for later processing;
under no circumstances should a Comment Extension disrupt or interfere
with the processing of the Data Stream.
This block is OPTIONAL; any number of them may appear in the Data Stream.
b. Required Version. 89a.
18
c. Syntax.
7 6 5 4 3 2 1 0 Field Name Type
+---------------+
0 | | Extension Introducer Byte
+---------------+
1 | | Comment Label Byte
+---------------+
+===============+
| |
N | | Comment Data Data Sub-blocks
| |
+===============+
+---------------+
0 | | Block Terminator Byte
+---------------+
i) Extension Introducer - Identifies the beginning of an extension
block. This field contains the fixed value 0x21.
ii) Comment Label - Identifies the block as a Comment Extension.
This field contains the fixed value 0xFE.
iii) Comment Data - Sequence of sub-blocks, each of size at most
255 bytes and at least 1 byte, with the size in a byte preceding
the data. The end of the sequence is marked by the Block
Terminator.
iv) Block Terminator - This zero-length data block marks the end of
the Comment Extension.
d. Extensions and Scope. This block does not have scope. This block
cannot be modified by any extension.
e. Recommendations.
i) Data - This block is intended for humans. It should contain
text using the 7-bit ASCII character set. This block should
not be used to store control information for custom processing.
ii) Position - This block may appear at any point in the Data
Stream at which a block can begin; however, it is recommended that
Comment Extensions do not interfere with Control or Data blocks;
they should be located at the beginning or at the end of the Data
Stream to the extent possible.
25. Plain Text Extension.
a. Description. The Plain Text Extension contains textual data and the
parameters necessary to render that data as a graphic, in a simple form.
The textual data will be encoded with the 7-bit printable ASCII
characters. Text data are rendered using a grid of character cells
19
defined by the parameters in the block fields. Each character is rendered
in an individual cell. The textual data in this block is to be rendered
as mono-spaced characters, one character per cell, with a best fitting
font and size. For further information, see the section on
Recommendations below. The data characters are taken sequentially from
the data portion of the block and rendered within a cell, starting with
the upper left cell in the grid and proceeding from left to right and
from top to bottom. Text data is rendered until the end of data is
reached or the character grid is filled. The Character Grid contains an
integral number of cells; in the case that the cell dimensions do not
allow for an integral number, fractional cells must be discarded; an
encoder must be careful to specify the grid dimensions accurately so that
this does not happen. This block requires a Global Color Table to be
available; the colors used by this block reference the Global Color Table
in the Stream if there is one, or the Global Color Table from a previous
Stream, if one was saved. This block is a graphic rendering block,
therefore it may be modified by a Graphic Control Extension. This block
is OPTIONAL; any number of them may appear in the Data Stream.
b. Required Version. 89a.
20
c. Syntax.
7 6 5 4 3 2 1 0 Field Name Type
+---------------+
0 | | Extension Introducer Byte
+---------------+
1 | | Plain Text Label Byte
+---------------+
+---------------+
0 | | Block Size Byte
+---------------+
1 | | Text Grid Left Position Unsigned
+- -+
2 | |
+---------------+
3 | | Text Grid Top Position Unsigned
+- -+
4 | |
+---------------+
5 | | Text Grid Width Unsigned
+- -+
6 | |
+---------------+
7 | | Text Grid Height Unsigned
+- -+
8 | |
+---------------+
9 | | Character Cell Width Byte
+---------------+
10 | | Character Cell Height Byte
+---------------+
11 | | Text Foreground Color Index Byte
+---------------+
12 | | Text Background Color Index Byte
+---------------+
+===============+
| |
N | | Plain Text Data Data Sub-blocks
| |
+===============+
+---------------+
0 | | Block Terminator Byte
+---------------+
i) Extension Introducer - Identifies the beginning of an extension
block. This field contains the fixed value 0x21.
ii) Plain Text Label - Identifies the current block as a Plain Text
Extension. This field contains the fixed value 0x01.
iii) Block Size - Number of bytes in the extension, after the Block
Size field and up to but not including the beginning of the data
portion. This field contains the fixed value 12.
21
iv) Text Grid Left Position - Column number, in pixels, of the left
edge of the text grid, with respect to the left edge of the Logical
Screen.
v) Text Grid Top Position - Row number, in pixels, of the top edge
of the text grid, with respect to the top edge of the Logical
Screen.
vi) Image Grid Width - Width of the text grid in pixels.
vii) Image Grid Height - Height of the text grid in pixels.
viii) Character Cell Width - Width, in pixels, of each cell in the
grid.
ix) Character Cell Height - Height, in pixels, of each cell in the
grid.
x) Text Foreground Color Index - Index into the Global Color Table
to be used to render the text foreground.
xi) Text Background Color Index - Index into the Global Color Table
to be used to render the text background.
xii) Plain Text Data - Sequence of sub-blocks, each of size at most
255 bytes and at least 1 byte, with the size in a byte preceding
the data. The end of the sequence is marked by the Block
Terminator.
xiii) Block Terminator - This zero-length data block marks the end
of the Plain Text Data Blocks.
d. Extensions and Scope. The scope of this block is the Plain Text Data
Block contained in it. This block may be modified by the Graphic Control
Extension.
e. Recommendations. The data in the Plain Text Extension is assumed to be
preformatted. The selection of font and size is left to the discretion of
the decoder. If characters less than 0x20 or greater than 0xf7 are
encountered, it is recommended that the decoder display a Space character
(0x20). The encoder should use grid and cell dimensions such that an
integral number of cells fit in the grid both horizontally as well as
vertically. For broadest compatibility, character cell dimensions should
be around 8x8 or 8x16 (width x height); consider an image for unusual
sized text.
26. Application Extension.
a. Description. The Application Extension contains application-specific
information; it conforms with the extension block syntax, as described
below, and its block label is 0xFF.
b. Required Version. 89a.
22
c. Syntax.
7 6 5 4 3 2 1 0 Field Name Type
+---------------+
0 | | Extension Introducer Byte
+---------------+
1 | | Extension Label Byte
+---------------+
+---------------+
0 | | Block Size Byte
+---------------+
1 | |
+- -+
2 | |
+- -+
3 | | Application Identifier 8 Bytes
+- -+
4 | |
+- -+
5 | |
+- -+
6 | |
+- -+
7 | |
+- -+
8 | |
+---------------+
9 | |
+- -+
10 | | Appl. Authentication Code 3 Bytes
+- -+
11 | |
+---------------+
+===============+
| |
| | Application Data Data Sub-blocks
| |
| |
+===============+
+---------------+
0 | | Block Terminator Byte
+---------------+
i) Extension Introducer - Defines this block as an extension. This
field contains the fixed value 0x21.
ii) Application Extension Label - Identifies the block as an
Application Extension. This field contains the fixed value 0xFF.
iii) Block Size - Number of bytes in this extension block,
following the Block Size field, up to but not including the
beginning of the Application Data. This field contains the fixed
value 11.
23
iv) Application Identifier - Sequence of eight printable ASCII
characters used to identify the application owning the Application
Extension.
v) Application Authentication Code - Sequence of three bytes used
to authenticate the Application Identifier. An Application program
may use an algorithm to compute a binary code that uniquely
identifies it as the application owning the Application Extension.
d. Extensions and Scope. This block does not have scope. This block
cannot be modified by any extension.
e. Recommendation. None.
27. Trailer.
a. Description. This block is a single-field block indicating the end of
the GIF Data Stream. It contains the fixed value 0x3B.
b. Required Version. 87a.
c. Syntax.
7 6 5 4 3 2 1 0 Field Name Type
+---------------+
0 | | GIF Trailer Byte
+---------------+
d. Extensions and Scope. This block does not have scope, it terminates
the GIF Data Stream. This block may not be modified by any extension.
e. Recommendations. None.
24
Appendix
A. Quick Reference Table.
Block Name Required Label Ext. Vers.
Application Extension Opt. (*) 0xFF (255) yes 89a
Comment Extension Opt. (*) 0xFE (254) yes 89a
Global Color Table Opt. (1) none no 87a
Graphic Control Extension Opt. (*) 0xF9 (249) yes 89a
Header Req. (1) none no N/A
Image Descriptor Opt. (*) 0x2C (044) no 87a (89a)
Local Color Table Opt. (*) none no 87a
Logical Screen Descriptor Req. (1) none no 87a (89a)
Plain Text Extension Opt. (*) 0x01 (001) yes 89a
Trailer Req. (1) 0x3B (059) no 87a
Unlabeled Blocks
Header Req. (1) none no N/A
Logical Screen Descriptor Req. (1) none no 87a (89a)
Global Color Table Opt. (1) none no 87a
Local Color Table Opt. (*) none no 87a
Graphic-Rendering Blocks
Plain Text Extension Opt. (*) 0x01 (001) yes 89a
Image Descriptor Opt. (*) 0x2C (044) no 87a (89a)
Control Blocks
Graphic Control Extension Opt. (*) 0xF9 (249) yes 89a
Special Purpose Blocks
Trailer Req. (1) 0x3B (059) no 87a
Comment Extension Opt. (*) 0xFE (254) yes 89a
Application Extension Opt. (*) 0xFF (255) yes 89a
legend: (1) if present, at most one occurrence
(*) zero or more occurrences
(+) one or more occurrences
Notes : The Header is not subject to Version Numbers.
(89a) The Logical Screen Descriptor and the Image Descriptor retained their
syntax from version 87a to version 89a, but some fields reserved under version
87a are used under version 89a.
25
Appendix
B. GIF Grammar.
A Grammar is a form of notation to represent the sequence in which certain
objects form larger objects. A grammar is also used to represent the number of
objects that can occur at a given position. The grammar given here represents
the sequence of blocks that form the GIF Data Stream. A grammar is given by
listing its rules. Each rule consists of the left-hand side, followed by some
form of equals sign, followed by the right-hand side. In a rule, the
right-hand side describes how the left-hand side is defined. The right-hand
side consists of a sequence of entities, with the possible presence of special
symbols. The following legend defines the symbols used in this grammar for GIF.
Legend: <> grammar word
::= defines symbol
* zero or more occurrences
+ one or more occurrences
| alternate element
[] optional element
Example:
<GIF Data Stream> ::= Header <Logical Screen> <Data>* Trailer
This rule defines the entity <GIF Data Stream> as follows. It must begin with a
Header. The Header is followed by an entity called Logical Screen, which is
defined below by another rule. The Logical Screen is followed by the entity
Data, which is also defined below by another rule. Finally, the entity Data is
followed by the Trailer. Since there is no rule defining the Header or the
Trailer, this means that these blocks are defined in the document. The entity
Data has a special symbol (*) following it which means that, at this position,
the entity Data may be repeated any number of times, including 0 times. For
further reading on this subject, refer to a standard text on Programming
Languages.
The Grammar.
<GIF Data Stream> ::= Header <Logical Screen> <Data>* Trailer
<Logical Screen> ::= Logical Screen Descriptor [Global Color Table]
<Data> ::= <Graphic Block> |
<Special-Purpose Block>
<Graphic Block> ::= [Graphic Control Extension] <Graphic-Rendering Block>
<Graphic-Rendering Block> ::= <Table-Based Image> |
Plain Text Extension
<Table-Based Image> ::= Image Descriptor [Local Color Table] Image Data
<Special-Purpose Block> ::= Application Extension |
Comment Extension
26
NOTE : The grammar indicates that it is possible for a GIF Data Stream to
contain the Header, the Logical Screen Descriptor, a Global Color Table and the
GIF Trailer. This special case is used to load a GIF decoder with a Global
Color Table, in preparation for subsequent Data Streams without color tables at
all.
27
Appendix
C. Glossary.
Active Color Table - Color table used to render the next graphic. If the next
graphic is an image which has a Local Color Table associated with it, the
active color table becomes the Local Color Table associated with that image.
If the next graphic is an image without a Local Color Table, or a Plain Text
Extension, the active color table is the Global Color Table associated with the
Data Stream, if there is one; if there is no Global Color Table in the Data
Stream, the active color table is a color table saved from a previous Data
Stream, or one supplied by the decoder.
Block - Collection of bytes forming a protocol unit. In general, the term
includes labeled and unlabeled blocks, as well as Extensions.
Data Stream - The GIF Data Stream is composed of blocks and sub-blocks
representing images and graphics, together with control information to render
them on a display device. All control and data blocks in the Data Stream must
follow the Header and must precede the Trailer.
Decoder - A program capable of processing a GIF Data Stream to render the
images and graphics contained in it.
Encoder - A program capable of capturing and formatting image and graphic
raster data, following the definitions of the Graphics Interchange Format.
Extension - A protocol block labeled by the Extension Introducer 0x21.
Extension Introducer - Label (0x21) defining an Extension.
Graphic - Data which can be rendered on the screen by virtue of some algorithm.
The term graphic is more general than the term image; in addition to images,
the term graphic also includes data such as text, which is rendered using
character bit-maps.
Image - Data representing a picture or a drawing; an image is represented by an
array of pixels called the raster of the image.
Raster - Array of pixel values representing an image.
28
Appendix
D. Conventions.
Animation - The Graphics Interchange Format is not intended as a platform for
animation, even though it can be done in a limited way.
Byte Ordering - Unless otherwise stated, multi-byte numeric fields are ordered
with the Least Significant Byte first.
Color Indices - Color indices always refer to the active color table, either
the Global Color Table or the Local Color Table.
Color Order - Unless otherwise stated, all triple-component RGB color values
are specified in Red-Green-Blue order.
Color Tables - Both color tables, the Global and the Local, are optional; if
present, the Global Color Table is to be used with every image in the Data
Stream for which a Local Color Table is not given; if present, a Local Color
Table overrides the Global Color Table. However, if neither color table is
present, the application program is free to use an arbitrary color table. If
the graphics in several Data Streams are related and all use the same color
table, an encoder could place the color table as the Global Color Table in the
first Data Stream and leave subsequent Data Streams without a Global Color
Table or any Local Color Tables; in this way, the overhead for the table is
eliminated. It is recommended that the decoder save the previous Global Color
Table to be used with the Data Stream that follows, in case it does not contain
either a Global Color Table or any Local Color Tables. In general, this allows
the application program to use past color tables, significantly reducing
transmission overhead.
Extension Blocks - Extensions are defined using the Extension Introducer code
to mark the beginning of the block, followed by a block label, identifying the
type of extension. Extension Codes are numbers in the range from 0x00 to 0xFF,
inclusive. Special purpose extensions are transparent to the decoder and may be
omitted when transmitting the Data Stream on-line. The GIF capabilities
dialogue makes the provision for the receiver to request the transmission of
all blocks; the default state in this regard is no transmission of Special
purpose blocks.
Reserved Fields - All Reserved Fields are expected to have each bit set to zero
(off).
29
Appendix
E. Interlaced Images.
The rows of an Interlaced images are arranged in the following order:
Group 1 : Every 8th. row, starting with row 0. (Pass 1)
Group 2 : Every 8th. row, starting with row 4. (Pass 2)
Group 3 : Every 4th. row, starting with row 2. (Pass 3)
Group 4 : Every 2nd. row, starting with row 1. (Pass 4)
The Following example illustrates how the rows of an interlaced image are
ordered.
Row Number Interlace Pass
0 ----------------------------------------- 1
1 ----------------------------------------- 4
2 ----------------------------------------- 3
3 ----------------------------------------- 4
4 ----------------------------------------- 2
5 ----------------------------------------- 4
6 ----------------------------------------- 3
7 ----------------------------------------- 4
8 ----------------------------------------- 1
9 ----------------------------------------- 4
10 ----------------------------------------- 3
11 ----------------------------------------- 4
12 ----------------------------------------- 2
13 ----------------------------------------- 4
14 ----------------------------------------- 3
15 ----------------------------------------- 4
16 ----------------------------------------- 1
17 ----------------------------------------- 4
18 ----------------------------------------- 3
19 ----------------------------------------- 4
30
Appendix
F. Variable-Length-Code LZW Compression.
The Variable-Length-Code LZW Compression is a variation of the Lempel-Ziv
Compression algorithm in which variable-length codes are used to replace
patterns detected in the original data. The algorithm uses a code or
translation table constructed from the patterns encountered in the original
data; each new pattern is entered into the table and its index is used to
replace it in the compressed stream.
The compressor takes the data from the input stream and builds a code or
translation table with the patterns as it encounters them; each new pattern is
entered into the code table and its index is added to the output stream; when a
pattern is encountered which had been detected since the last code table
refresh, its index from the code table is put on the output stream, thus
achieving the data compression. The expander takes input from the compressed
data stream and builds the code or translation table from it; as the compressed
data stream is processed, codes are used to index into the code table and the
corresponding data is put on the decompressed output stream, thus achieving
data decompression. The details of the algorithm are explained below. The
Variable-Length-Code aspect of the algorithm is based on an initial code size
(LZW-initial code size), which specifies the initial number of bits used for
the compression codes. When the number of patterns detected by the compressor
in the input stream exceeds the number of patterns encodable with the current
number of bits, the number of bits per LZW code is increased by one.
The Raster Data stream that represents the actual output image can be
represented as:
7 6 5 4 3 2 1 0
+---------------+
| LZW code size |
+---------------+
+---------------+ ----+
| block size | |
+---------------+ |
| | +-- Repeated as many
| data bytes | | times as necessary.
| | |
+---------------+ ----+
. . . . . . ------- The code that terminates the LZW
compressed data must appear before
Block Terminator.
+---------------+
|0 0 0 0 0 0 0 0| Block Terminator
+---------------+
The conversion of the image from a series of pixel values to a transmitted or
stored character stream involves several steps. In brief these steps are:
1. Establish the Code Size - Define the number of bits needed to represent the
actual data.
2. Compress the Data - Compress the series of image pixels to a series of
31
compression codes.
3. Build a Series of Bytes - Take the set of compression codes and convert to a
string of 8-bit bytes.
4. Package the Bytes - Package sets of bytes into blocks preceded by character
counts and output.
ESTABLISH CODE SIZE
The first byte of the Compressed Data stream is a value indicating the minimum
number of bits required to represent the set of actual pixel values. Normally
this will be the same as the number of color bits. Because of some algorithmic
constraints however, black & white images which have one color bit must be
indicated as having a code size of 2.
This code size value also implies that the compression codes must start out one
bit longer.
COMPRESSION
The LZW algorithm converts a series of data values into a series of codes which
may be raw values or a code designating a series of values. Using text
characters as an analogy, the output code consists of a character or a code
representing a string of characters.
The LZW algorithm used in GIF matches algorithmically with the standard LZW
algorithm with the following differences:
1. A special Clear code is defined which resets all compression/decompression
parameters and tables to a start-up state. The value of this code is 2**<code
size>. For example if the code size indicated was 4 (image was 4 bits/pixel)
the Clear code value would be 16 (10000 binary). The Clear code can appear at
any point in the image data stream and therefore requires the LZW algorithm to
process succeeding codes as if a new data stream was starting. Encoders should
output a Clear code as the first code of each image data stream.
2. An End of Information code is defined that explicitly indicates the end of
the image data stream. LZW processing terminates when this code is encountered.
It must be the last code output by the encoder for an image. The value of this
code is <Clear code>+1.
3. The first available compression code value is <Clear code>+2.
4. The output codes are of variable length, starting at <code size>+1 bits per
code, up to 12 bits per code. This defines a maximum code value of 4095
(0xFFF). Whenever the LZW code value would exceed the current code length, the
code length is increased by one. The packing/unpacking of these codes must then
be altered to reflect the new code length.
BUILD 8-BIT BYTES
Because the LZW compression used for GIF creates a series of variable length
codes, of between 3 and 12 bits each, these codes must be reformed into a
series of 8-bit bytes that will be the characters actually stored or
transmitted. This provides additional compression of the image. The codes are
formed into a stream of bits as if they were packed right to left and then
32
picked off 8 bits at a time to be output.
Assuming a character array of 8 bits per character and using 5 bit codes to be
packed, an example layout would be similar to:
+---------------+
0 | | bbbaaaaa
+---------------+
1 | | dcccccbb
+---------------+
2 | | eeeedddd
+---------------+
3 | | ggfffffe
+---------------+
4 | | hhhhhggg
+---------------+
. . .
+---------------+
N | |
+---------------+
Note that the physical packing arrangement will change as the number of bits
per compression code change but the concept remains the same.
PACKAGE THE BYTES
Once the bytes have been created, they are grouped into blocks for output by
preceding each block of 0 to 255 bytes with a character count byte. A block
with a zero byte count terminates the Raster Data stream for a given image.
These blocks are what are actually output for the GIF image. This block format
has the side effect of allowing a decoding program the ability to read past the
actual image data if necessary by reading block counts and then skipping over
the data.
FURTHER READING
[1] Ziv, J. and Lempel, A. : "A Universal Algorithm for Sequential Data
Compression", IEEE Transactions on Information Theory, May 1977.
[2] Welch, T. : "A Technique for High-Performance Data Compression", Computer,
June 1984.
[3] Nelson, M.R. : "LZW Data Compression", Dr. Dobb's Journal, October 1989.
33
Appendix
G. On-line Capabilities Dialogue.
NOTE : This section is currently (10 July 1990) under revision; the information
provided here should be used as general guidelines. Code written based on this
information should be designed in a flexible way to accommodate any changes
resulting from the revisions.
The following sequences are defined for use in mediating control between a GIF
sender and GIF receiver over an interactive communications line. These
sequences do not apply to applications that involve downloading of static GIF
files and are not considered part of a GIF file.
GIF CAPABILITIES ENQUIRY
The GIF Capabilities Enquiry sequence is issued from a host and requests an
interactive GIF decoder to return a response message that defines the graphics
parameters for the decoder. This involves returning information about available
screen sizes, number of bits/color supported and the amount of color detail
supported. The escape sequence for the GIF Capabilities Enquiry is defined as:
ESC[>0g 0x1B 0x5B 0x3E 0x30 0x67
GIF CAPABILITIES RESPONSE
The GIF Capabilities Response message is returned by an interactive GIF decoder
and defines the decoder's display capabilities for all graphics modes that are
supported by the software. Note that this can also include graphics printers as
well as a monitor screen. The general format of this message is:
#version;protocol{;dev, width, height, color-bits, color-res}...<CR>
'#' GIF Capabilities Response identifier character.
version GIF format version number; initially '87a'.
protocol='0' No end-to-end protocol supported by decoder Transfer as direct
8-bit data stream.
protocol='1' Can use CIS B+ error correction protocol to transfer GIF data
interactively from the host directly to the display.
dev = '0' Screen parameter set follows.
dev = '1' Printer parameter set follows.
width Maximum supported display width in pixels.
height Maximum supported display height in pixels.
color-bits Number of bits per pixel supported. The number of supported
colors is therefore 2**color-bits.
color-res Number of bits per color component supported in the hardware
color palette. If color-res is '0' then no hardware palette
table is available.
Note that all values in the GIF Capabilities Response are returned as ASCII
decimal numbers and the message is terminated by a Carriage Return character.
The following GIF Capabilities Response message describes three standard IBM PC
Enhanced Graphics Adapter configurations with no printer; the GIF data stream
34
can be processed within an error correcting protocol:
#87a;1;0,320,200,4,0;0,640,200,2,2;0,640,350,4,2<CR>
ENTER GIF GRAPHICS MODE
Two sequences are currently defined to invoke an interactive GIF decoder into
action. The only difference between them is that different output media are
selected. These sequences are:
ESC[>1g Display GIF image on screen
0x1B 0x5B 0x3E 0x31 0x67
ESC[>2g Display image directly to an attached graphics printer. The image may
optionally be displayed on the screen as well.
0x1B 0x5B 0x3E 0x32 0x67
Note that the 'g' character terminating each sequence is in lowercase.
INTERACTIVE ENVIRONMENT
The assumed environment for the transmission of GIF image data from an
interactive application is a full 8-bit data stream from host to micro. All
256 character codes must be transferrable. The establishing of an 8-bit data
path for communications will normally be taken care of by the host application
programs. It is however up to the receiving communications programs supporting
GIF to be able to receive and pass on all 256 8-bit codes to the GIF decoder
software.
