VC-1

• FourCCs: WMV3, WMV9, WMVA, WVC1, WMVP
• Company: Microsoft
• Samples:
  • WMV9: http://www.mplayerhq.hu/MPlayer/samples/V-codecs/WMV9/
  • WMVA: http://www.mplayerhq.hu/MPlayer/samples/V-codecs/WMVA/
  • WMVP: http://www.mplayerhq.hu/MPlayer/samples/V-codecs/WMVP/
  • WVP2: http://www.mplayerhq.hu/MPlayer/samples/V-codecs/WVP2/
• General info: http://www.microsoft.com/windows/windowsmedia/forpros/events/NAB2005/VC-1.aspx

Old specs can be found here: http://jovian.com/files/C24.008-VC9-Spec-CD1.pdf

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 http://www.smpte.org/smpte_store/standards/pdf/s421m.pdf

VC-1 Bitstream Transport Encodings (specs for placing VC-1 in MPEG-2 Program and Transport streams) http://www.smpte.org/smpte_store/standards/pdf/rp227.pdf

VC-1 Decoder and Bitstream Conformance http://www.smpte.org/smpte_store/standards/pdf/rp228.pdf

Googling for VC1_reference_decoder_release6.zip might turn up sources for the reference decoder.

See Understanding VC-1 for more information about the technical details of the format.

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 BITMAPINFOHEADER 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

The initializer then needs to compute 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:

 ACPRED
 SKIPMB
 MVTYPEMB
 DIRECTMB
 OVERFLAGS
 FORWARDMB
 FIELDTX

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.

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
 else
   if (profile is simple or main)
     choose set 1
   else
     if (picture format is progressive)
       choose set 2
     else
       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
 vc1iquant.c:vc1IQUANT_ChooseQuantizer())
 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:

(unfinished)