LECTURE D'UNE IMAGE GIF

Commentaire de BombStrike le 27/06/2004 19:08:19

voici ce que tu cherche :)




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.