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Revision as of 23:11, 14 February 2006 by (talk) (add cursory data description details)
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Old specs can be found here: [1]

VC-1 is the codec microsoft is pushing for SMPTE standard. VC-1 is what wmv9 became and specs for it can be found here:

VC-1 Compressed Video Bitstream Format and Decoding Process [2]

VC-1 Bitstream Transport Encodings (specs for placing VC-1 in MPEG-2 Program and Transport streams) [3]

VC-1 Decoder and Bitstream Conformance [4]

Googling for might turn up sources for the reference decoder.

Data Format

This description assumes that the data to be decoded in WMV3 data coming in from a Microsoft ASF file. The video data should be packaged with "extradata" which is attached to the end of a BITMAPINFOHEADER structure and transported in the ASF file. The format of the extradata is as follows:

 2 bits    VC-1 Profile
 if (profile == 3)
   3  bits  Profile level
   2  bits  Chroma format (SRD does not care)
   3  bits  VC1_BITS_FRMRTQ_POSTPROC (? SRD does not care)
   5  bits  VC1_BITS_BITRTQ_POSTPROC (? SRD does not care)
   1  bit   VC1_BITS_POSTPROCFLAG (? SRD does not care)
   12 bits  Encoded width (actual width = (w + 1) * 2)
   12 bits  Encoded height (actual height = (h + 1) * 2)

There are 4 VC-1 profiles:

  • 0 simple profile
  • 1 main profile
  • 2 reserved
  • 3 advanced profile

If profile is advanced, the extradata carries a lot of setup information. For simple and main profiles, the relevant setup data is established outside of the decoder, e.g., the BITMAPINFO header of a Microsoft ASF file. This information provides the width and height that the decoder uses to set up its state.

The decoder computes the macroblock width and height as the ceiling of each dimension divided by 16:

 macroblock_width = (frame_width + 15) / 16
 macroblock_height = (frame_height + 15) / 16

The total number of macroblocks in a frame is defined as:

 total_macroblocks = macroblock_width * macroblock_height

If the level is marked as unknown during the initialization process, figure out what level the video belongs at. This is determined by the number of macroblocks in combination with the profile. The relevant table is vc1gentab.c:vc1GENTAB_LevelLimits[][]. The profile/level combination defines the following limits:

 max macroblocks/second
 max macroblocks/frame
 max peak transmission rate in kbps
 max buffer size in multiples of 16 kbits
 motion vector range

SRD maintains the following information about each macroblock:

 macroblock type, contains the following attributes:
   (attribute 1)
   1 MV
   2 MV
   4 MV

   (attribute 2)
   direct macroblock
   forward prediction
   backward prediction
   forward and backward prediction

   (attribute 3)
   MVs apply to fields
   bottom different than top
   field transform

 AC prediction status, one of the following attributes:
   AC prediction off
   AC prediction on
   no blocks can be predicted

 Block type, one of the following types:
   8x8 inter-coded
   8x4 inter-coded
   4x8 inter-coded
   4x4 inter-coded
   intra-coded, no AC prediction
   intra-coded, AC prediction from top values
   intra-coded, AC prediction from left values

 flag indicating whether overlap filter is active for this macroblock
 flag indicating whether macroblock is motion predicted only (no residual)
 byte indicating coded block pattern which indicates which of the 6
   sub-blocks are coded

 Quantizer information, this includes:
   quantizer step in the range 1..31
   quantizer half step, either 0 or 1
   flag indicating uniform or non-uniform quantizer

 Information for each of the 6 constituent blocks in the macroblock:
   block type (same choices as the macroblock attributes)
   flag indicating non-zero AC coeffs for intra, non-zero AC/DC for inter
   union between an intra block structure and an inter block structure
   intra structure:
     number of zero/non-zero AC coeffs
     quantized DC coeff
     quantized AC top row for prediction (7 values)
     quantized AC left column for prediction (7 values)
     bottom 2 rows (16 values) kept for overlap smoothing
   inter structure:
     number of zero/non-zero AC coeffs for 4 sub-blocks (Y blocks?)
     forward and backward motion vector structures, each includes:
       hybrid prediction mode, one of the following attributes:
         predict from left
         predict from top
         no hybrid prediction
       (x,y) motion vectors for each of the 4 Y blocks
       (x,y) differential motion vectors in 1/4 pel units
 (note: I am a little confused as to why each of the 6 sub-blocks stores the motion vector data for the entire macroblock)

The initializer then needs to computer how much space to allocate for each reference frame. The size of a frame determined by frame width and height, encoding profile, and interlacing. This size is used to allocate space for 4 different frames:

 reference new  (new/current I/P frame)
 reference old  (old I/P reference frame)
 reference B    (reconstructed B frame)
 reference NoIC (B reference before intensity compensation was applied)

Further, the initializer allocates space for 7 different bitplanes. Each bitplanes has 1 flag per each macroblock as enumerated by the max macroblocks per frame for the profile/level. The bitplanes are:


Allocate space for motion vector history. The number of entries in this array is macroblock_width * (macroblock_height + 1) (extra height is for interlaced field). Each entry is a motion vector history structure which contains the 4 Y block motion vectors for a particular macroblock. The individual motion vector structures are the same as in the intra structure which provides hybrid prediction, motion vectors, and diff MVs (again, 4 for each block?).

And that's it for the SRD "requirements gathering" process (vc1dec.c:vc1DEC_DecoderRequirements()). The function returns the number of bytes needed for the decoder's internal state. The client app is expected to allocate enough space for this state.

Next is the vc1dec.c:vc1DEC_DecoderInitialise() function. This sets up the positions and structures contained within the memory pool allocated for space.

Next is the vc1dec.c:vc1DEC_DecodeSequence() function which unpacks the sequence layer:

 2 bits     profile
 if (profile is simple or main)
   2 bits   VC1_BITS_RES_SM (? SRD does not care)
 if (profile is advanced)
   3 bits   level of advanced profile
   2 bits   chroma format (note that only format 1, YUV 4:2:0 is defined)
 3 bits     QFrameRateForPostProc ("see standard", SRD does not use)
 5 bits     QBitRateForPostProc ("see standard", SRD does not use)
 if (profile is simple or main)
   1 bit    loop filter flag
   1 bit    reserved, should be 0
   1 bit    multiresolution coding flag
   1 bit    reserved, should be 1, SRD calls it "RES_FASTTX"
   1 bit    fast U/V motion compensation
     note: must be 1 in simple profile
   1 bit    extended motion vectors
     note: must be 0 in simple profile
   2 bits   macroblock dequantization
   1 bit    variable sized transform
   1 bit    reserved, should be 0, SRD calls it "RES_TRANSTAB"
   1 bit    overlapped transform flag
   1 bit    sync marker flag
   1 bit    range reduction flag
   3 bits   maximum number of consecutive B frames
   2 bits   quantizer
 if (profile is advanced)
   1 bit    post processing flag
   12 bits  max coded width (actual width = (w + 1) * 2)
   12 bits  max coded height (actual height = (h + 1) * 2)
   1 bit    pulldown flag
   1 bit    interlaced
   1 bit    frame counter flag
 1 bit      frame interpolation flag
 if (profile is advanced)
   [lots more stuff to be filled in when advanced profile is needed]
 if (profile is simple or main)
   1 bit    reserved, should be 1, SRD calls it "RES_RTM_FLAG"

Finally, it is time to decode an actual frame (referred to as "unpacking the picture layer"). The decode process iterates through however many fields comprise the frame (1 or 2).

Choose from among 5 different zigzag table sets depending on profile and interlacing:

 if (picture format is interlaced frame)
   choose set 4
 if (picture is intra)
   choose set 0
   if (profile is simple or main)
     choose set 1
     if (picture format is progressive)
       choose set 2
       choose set 3

(unfinished) ... there is a lot more logic dealing with frame accounting; let's skip to the real meat: macroblock decoding! ...

Decode a macroblock:

 set the macroblock overlap filter flag, coding type, quantizer and halfstep
   parameters to the same as the picture
 clear the skipped flag
 set the CBP to 0 (no coded blocks)
 choose the quantizer (long list of logic, see
 for each of the 6 sub-blocks, set coded field to 0, clear down all MV data
 decide on non-uniform quantizer

 unpack an I or BI macroblock: