GIF and LZW References

lzw-refs.md

LZW References

LZW data compression algorithm (US PAT.)

• https://www.google.com/patents/US7696906 (Apr 13, 2010)
• https://www.google.com/patents/US20100039300 (Feb 18, 2010)

Compression of Individual Sequences via Variable Rate Coding (IEEE)

• Jacob Ziv and Abraham Lempel
• IEEE Transactions on Information Theory, Sep. 1978, pp. 530-536, vol. 24, No. 5.
• http://ieeexplore.ieee.org/document/1055934/

A Technique For High-Performance Data Compression (IEEE)

• Terry Welch
• Computer Volume: 17, Issue: 6, June 1984
• http://ieeexplore.ieee.org/document/1659158/

LZW Data Compression

• Nelson, M.R.
• Dr. Dobb's Journal, October 1989.
• http://marknelson.us/1989/10/01/lzw-data-compression/
• http://marknelson.us/2011/11/08/lzw-revisited/ (in November 8th, 2011)

The LZW algorithm (Data Compression Reference Center)

• https://web.archive.org/web/20150428121506/http://oldwww.rasip.fer.hr:80/research/compress/algorithms/fund/lz/lzw.html
• https://web.archive.org/web/20150428140733/http://oldwww.rasip.fer.hr:80/research/compress/algorithms/fund/lz/lz78.html
• https://web.archive.org/web/20150428140733/http://oldwww.rasip.fer.hr:80/research/compress/algorithms/fund/lz/lz77.html

GIF Specification (W3C)

• https://www.w3.org/Graphics/GIF/spec-gif89a.txt
• https://www.w3.org/Graphics/GIF/spec-gif87.txt

GIF 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.

      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
     +===============+

Animated-GIF Looping extension (unofficial)

http://www.vurdalakov.net/misc/gif/netscape-looping-application-extension

Syntax

    +---------------+
 0  |     0x21      |  Extension Label
    +---------------+
 1  |     0xFF      |  Application Extension Label
    +---------------+
 2  |     0x0B      |  Block Size
    +---------------+
 3  |               | 
    +-             -+
 4  |               | 
    +-             -+
 5  |               | 
    +-             -+
 6  |               | 
    +-  NETSCAPE   -+  Application Identifier (8 bytes)
 7  |               | 
    +-             -+
 8  |               | 
    +-             -+
 9  |               | 
    +-             -+
10  |               | 
    +---------------+
11  |               | 
    +-             -+
12  |      2.0      |  Application Authentication Code (3 bytes)
    +-             -+
13  |               | 
    +===============+                      --+
14  |     0x03      |  Sub-block Data Size   |
    +---------------+                        |
15  |     0x01      |  Sub-block ID          |
    +---------------+                        | Application Data Sub-block
16  |               |                        |
    +-             -+  Loop Count (2 bytes)  |
17  |               |                        |
    +===============+                      --+
18  |     0x00      |  Block Terminator
    +---------------+

• Extension Label - Defines this block as an extension. This field contains the fixed value 0x21 (33).
• Application Extension Label - Identifies the block as an Application Extension. This field contains the fixed value 0xFF (255).
• 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 0x0B (11).
• Application Identifier - Sequence of eight printable ASCII characters used to identify the application owning the Application Extension. This field contains the fixed value "NETSCAPE".
• 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. This field contains the fixed value "2.0". Sometimes Application Identifier and Application Authentication Code fields are referred as one "NETSCAPE2.0" field.
• Sub-block Data Size - Indicates the number of data bytes to follow. The size of the block does not account for the size byte itself. This field contains the fixed value 0x03 (3).
• Sub-block ID - Identifies the Netscape Looping Extension. This field contains the fixed value 0x01 (1).
• Loop Count - Indicates the number of iterations the animated GIF should be executed. This field is an unsigned 2-byte integer in little-endian (least significant byte first) byte order. 0x00 (0) means infinite loop.
• Block Terminator - This zero-length data block marks the end of the Application Extension. This field contains the fixed value 0x00 (0).

Netscape Buffering Application Extension (GIF Unofficial Specification)

    +---------------+
 0  |     0x21      |  Extension Label
    +---------------+
 1  |     0xFF      |  Application Extension Label
    +---------------+
 2  |     0x0B      |  Block Size
    +---------------+
 3  |               | 
    +-             -+
 4  |               | 
    +-             -+
 5  |               | 
    +-             -+
 6  |               | 
    +-  NETSCAPE   -+  Application Identifier (8 bytes)
 7  |               | 
    +-             -+
 8  |               | 
    +-             -+
 9  |               | 
    +-             -+
10  |               | 
    +---------------+
11  |               | 
    +-             -+
12  |      2.0      |  Application Authentication Code (3 bytes)
    +-             -+
13  |               | 
    +===============+                      --+
14  |     0x05      |  Sub-block Data Size   |
    +---------------+                        |
15  |     0x02      |  Sub-block ID          |
    +---------------+                        |
16  |               |                        |
    +-             -+                        | Application Data Sub-block
17  |               |                        |
    +-             -+  Buffer Size (4 bytes) |
18  |               |                        |
    +-             -+                        |
19  |               |                        |
    +===============+                      --+
20  |     0x00      |  Block Terminator
    +---------------+

http://www.vurdalakov.net/misc/gif/netscape-buffering-application-extension