On2 VP4

• FourCC: VP40
• Company: On2
• Samples: http://samples.mplayerhq.hu/V-codecs/VP4/

This codec has source code base common with its predecessor but sets version to 2.

Frame header

The same as VP3 with some extensions.

• Frame type - 1 bit
• Unused - 1 bit
• DCT Q mask - 6 bits

For frame type 0 (BASE_FRAME):

• version byte 0 - 8 bits
• version - 5 bits (should be 2)
• key frame type - 1 bit
• spare bits - 2 bits
• ??? - 8 bits
• ??? - 8 bits
• ??? - 5 bits
• ??? - 3 bits
• ??? - 5 bits
• ??? - 3 bits
• ??? - 2 bits

Display fragments

This is present for non-base frames only.

First there's MBFullyFlags[] coded as flag array. Then for all macroblocks that are not fully coded (i.e. MBFullyFlags[mb] is zero) there are flag values in MBCoded[] coded the same way. And finally for coded blocks there's some Huffman-coded value.

Flag arrays coding

Flag arrays are run-coded using some strange variation of VLC and having initial flag value signalled at the start.

 flag = getbit();
 len  = get_mplayer();
 for (i = 0; i < size; i++) {
   if (!len) {
     flag = !flag;
     len  = get_mplayer();
   }
   array[i] = flag;
   len--;
 }

Variable-length code

 int get_mplayer() {
   val = 1;
   tail = 0;
   while (getbit()) {
     val++;
     for (i = 0; i < 8; i++) {
       if (!getbit())
         break;
       val += 1 << i;
     }
     if (i < 8) {
       tail = i;
       break;
     }
   }
   return val + getbits(tail);
 }