gif89m.txt

CompuServe公司GIF格式标准

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      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.







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            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.









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      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.







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            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.





























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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.






















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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 g