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 Huffman-coded CBP:

 cbptab = 0;
 for (all MBs) {
   if (MBFullyFlags[mb]) {
     cbp = 0xF;
   } else if (MBCoded[mb]) {
     cbp = get_huff(cbp_vlc[cbptab]);
     cbptab = (cbp == 0x3 || cbp == 0x7 || cbp == 0xD || cbp == 0xE);
   } else {
     cbp = 0x0;
   }
 }

Huffman codes for table 0:

 000   -> 0x1
 001   -> 0x8
 010   -> 0x4
 0110  -> 0xB
 01110 -> 0x6
 01111 -> 0x9
 1000  -> 0xA
 1001  -> 0x3
 1010  -> 0xE
 1011  -> 0x7
 1100  -> 0xD
 1101  -> 0x5
 1110  -> 0xC
 1111  -> 0x2

Huffman codes for table 1:

 000   -> 0x7
 001   -> 0xB
 010   -> 0xE
 01100 -> 0x9
 01101 -> 0x6
 0111  -> 0x1
 1000  -> 0x5
 1001  -> 0x3
 1010  -> 0x2
 1011  -> 0xC
 1100  -> 0x4
 1101  -> 0xA
 1110  -> 0x8
 1111  -> 0xD

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);
 }

Intraframe data

• Luma DC Huffman table - 4 bits
• Luma AC Huffman table - 4 bits
• Chroma DC Huffman table - 4 bits
• Chroma AC Huffman table - 4 bits

Then macroblock data follows — first full data for luma (4 blocks per MB) and then for chroma (also 4 blocks per MB). Macroblock data coding seems to be exactly the same as in VP3.