Method Image.GIF._decode()

Method _decode

array _decode(string gifdata)
array _decode(array __decoded)

Description

Decodes a GIF image structure down to chunks, and also decode the images in the render chunks.

({int xsize,int ysize,    // 0: size of image drawing area
  void|object colortable, // 2: opt. global colortable
  ({ int aspx, int aspy,  // 3 0: aspect ratio or 0, 0 if not set
     int background }),   //   2: index of background color

followed by any number these blocks in any order (gce chunks are decoded and incorporated in the render chunks):

  ({ GIF.RENDER,          //   0: block identifier
    int x, int y,         //   1: position of render
    object image,         //   3: render image
    void|object alpha,    //   4: 0 or render alpha channel
    object colortable,    //   5: colortable (may be same as global)
 
    int interlace,        //   6: interlace flag
    int trans_index,      //   7: 0 or transparent color index
    int delay,            //   8: 0 or delay in centiseconds
    int user_input,       //   9: user input flag
    int disposal})        //  10: disposal method number (0..7)
 
  ({ GIF.EXTENSION,       //   0: block identifier
     int extension,       //   1: extension number
     string data })       //   2: extension data
 

and possibly ended with one of these:

  ({ GIF.ERROR_PREMATURE_EOD })   // premature end-of-data
 
  ({ GIF.ERROR_TOO_MUCH_DATA,     // data following end marker
     string data })               // (rest of file)
 
  ({ GIF.ERROR_UNKNOWN_DATA,      // unknown data
     string data })               // (rest of file)

The decode method uses this data in a way similar to this program:

import Image;
 
object my_decode_gif(string data)
{
   array a=GIF._decode(data);
   object img=image(a[0],a[1]);
   foreach (a[4..],array b)
      if (b[0]==GIF.RENDER)
         if (b[4]) img->paste_alpha(b[3],b[4],b[1],b[2]);
         else img->paste(b[3],b[1],b[2]);
   return img;
}

Parameter gifdata

GIF data (with header and all)

Parameter __decoded

GIF data as from __decode

Returns

the above array

Note

May throw errors if the GIF header is incomplete or illegal.

This is in the very advanced sector of the GIF support; please read about how GIF files works.