Escape 122

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The ESCAPE 122 codec is a relatively simple 8-bit palettized video compression engine based on multipass BTC encoding and operates with 8x8 "superblocks" combined from 2x2 macroblocks.

The packed video stream is usually stored within an ARMovie container. Each frame chunk begins with an 8-byte header (here and below all numbers are little-endian):

dword video_size - size of the video portion
dword audio_size - size of the audio portion

Video data starts immediately after this header and is followed by audio data (if any).

Note: The existing original decoder implementation only supports 320x200 video. Only one sample file encoded in this resolution is known so far.

Video portion

The first 2 bytes of the chunk indicate the length of the following palette block. Palette is stored as standard VGA DAC B-G-R triplets and includes a variable number of color entries, which can be determined by dividing palette length by 3 and rounding result toward zero (thus, block length may not necessarily be a multiple of 3). Right after the palette block the actual compressed frame data begins.

The Video decoding algorithm operates as follows:

repeat
  skip_value = RICE_Decode()
  skip skip_value number of 8x8 superblocks (keep them unchanged from the last frame)

  if all blocks drawn/skipped
    finish

  while next bit of the stream is zero
    new_block = Decode_MacroBlock()
    mask = read 4*4 (number of macroblocks in superblock) bits
    for each macroblock of superblock (left to right, top to bottom)
      if next bit of mask (from low to high) is set
        macroblock = new_block

  if next bit of the stream is zero
    mask = read 4*4 (number of macroblocks in superblock) bits
    for each macroblock of superblock (left to right, top to bottom)
      if next bit of mask (from low to high) is set
        macroblock = Decode_MacroBlock()

  advance to the next superblock


The RICE_Decode performs RICE-like value unpacking and operates as follows:

 value = 0
 more_bits = read 1 bit from the stream
 if 1 bit of more_bits is set
   value = value + more_bits
   more_bits = read 3 bits from the stream
   value = value + more_bits
   if 3 bits of more_bits is set
     more_bits = read 7 bits from the stream
     value = value + more_bits
     if 7 bits of more_bits is set
       more_bits = read 12 bits from the stream
       value = value + more_bits


Decode_Macroblock performs 2x2 macroblock decoding:

 read 2*2 (number of pixels in macroblock) mask bits from the stream
 if all mask bits are set or clear
   read 7 top bits of color from the stream
   the lowest bit of color is the any bit of mask
   fill whole macroblock with color
 else
   read 8 bits of color0 from the stream
   read 8 bits of color1 from the stream
   for each pixel of macroblock (left to right, top to bottom)
     if next bit of mask (from low to high) is set 
       pixel = color1
     else
       pixel = color0


Audio portion

TODO: Insert IMA ADPCM description & link here