From MultimediaWiki
Jump to navigation Jump to search
  • Company: Nordic Company

This is a rather simple quadtree-based paletted video codec with its name probably coming from its developer initials.

The image is split into squares of 4nx4n pixels large for the closest match with frame dimensions (e.g. 256x256 squares for 320x240 video) that get divided into 16 parts with a flag telling if the part is fill/skip or should be further subdivided (or coded as raw for 4x4 squares). Flags and pixel data are grouped into separate data chunks inside the frame.

Frame consists of the following data: 32-bit frame flags, 32-bit frame size (not counting frame flags), tile flags data (starting with 32-bit size), and pixel data. There are two known frame flags: bit 0 signals inter frame coding and bit 1 signals that pixel data is LZW-compressed (with 9- to 12-bit index, code 0x100 used as EOS and no restart code).

Simplified decoding process:

 tile_size = 0x4000;
 while (tile_size >= max(width, height))
   tile_size >>= 2;
 cur_tile_size = tile_size;
 for each tile in image
   decode_tile(tile_pos, cur_tile_size);
 decode_tile(pos, cur_tile_size) {
   flags = get_16bit_le(flags_data);
   for each one of 16 subparts in tile {
     if ((flag & 1) == 0) {
        pix = get_byte(pixel_data);
        if (intra || pix != 0) {
            fill subpart with pix value
     } else if (cur_tile_size > 4) {
        decode_tile(subpart, cur_tile_size >> 2);
     } else {
        fill 4x4 block with 16 values read from pixel data
     flags >>= 1;