Sorenson Video 3

• FOURCCs: SVQ3
• Company: Sorenson Media
• Samples: http://samples.mplayerhq.hu/V-codecs/SVQ3/

Video codec apparently based on an early H.264 draft.

The decoder in FFmpeg produces heavy artifacting for some uncommon samples.

Decoding Process

This codec extensively uses Golomb coding.

Slice Header

TODO

Macroblock layer

Each macroblock starts with Golomb code signalling MB type:

• 0 - skip block
• 1-7 - inter 16x16 blocks, type specifies prediction direction
• 8 and 33 - intra 4x4 block (type 33 means that there are no coefficients for this block)
• 9-32 - intra 16x16 blocks, type is used to set prediction direction and coded block pattern

In any case coefficients are stored in 4x4 (sub)blocks.

Dezigzag pattern (from H.264):

 o-->o-->o   o
         |  /|
 o   o   o / o
 | / |   |/  |
 o   o   o   o
   /
 o-->o-->o-->o

Coefficient decoding

Each coefficient is stored as Golomb codeword, last bit is coefficient sign, code = 0 means end of nonzero coefficients.

Codes for block type 3:

code	run	value
0-2	0	code
3	1	1
4-...	code & 0x3	((code + 9) >> 2) - run

Codes for intra block types:

code	run	value
0	0	0
1	0	1
2	1	1
3	0	2
4	2	1
5	0	3
6	0	4
7	0	5
8	3	1
9	4	1
10	1	2
11	1	3
12	0	6
13	0	7
14	0	8
15	0	9
16-...	code & 0x7	(code >> 3) - intra_run[run]

Codes for inter block types:

code	run	value
0	0	0
1	0	1
2	1	1
3	2	1
4	0	2
5	3	1
6	4	1
7	5	1
8	0	3
9	1	2
10	2	2
11	6	1
12	7	1
13	8	1
14	9	1
15	0	4
16-... (inter)	code & 0xF	(code >> 4) - inter_run[run]

Run correction values:

 intra_run = { 8, 2, 0, 0, 0, -1, -1, -1, [minus  ones] };
 inter_run = { 4, 2, 2, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, [zeroes] };

Macroblock transform and dequantization

Transform coefficients:

 13  17   1   7
 13   7  -1 -17
 13  -7  -1  17
 13 -17   1  -7

Dequantization is performed by multiplying every coefficient by the same value determined by quantizer. In case if inter blocks first coefficient may be quantized slightly differently:

For some intra blocks (TODO: condition):

   dc = 169 * 1538 * block[0]

For chroma blocks (?):

   dc = (svq3_dequant_coeff[Q] * (block[0] >> 3)) >> 1;

Quantizer table (from svq3.c)

 static const uint32_t svq3_dequant_coeff[32] = {
   3881,  4351,  4890,  5481,  6154,  6914,  7761,  8718,
   9781, 10987, 12339, 13828, 15523, 17435, 19561, 21873,
  24552, 27656, 30847, 34870, 38807, 43747, 49103, 54683,
  61694, 68745, 77615, 89113,100253,109366,126635,141533
 };

Dequantization formula (dc=0 if not defined otherwise):

 out = (coeff * svq3_dequant_coeff[Q] + dc + 0x80000) >> 20;