VC-1 overview

VC-1 is a video coding standard developed by Microsoft. It began as Windows Media Video 9. It is prevalent in ASF files downloaded from the internet. It is also supposed to be used on HD-DVDs.

Encapsulation

Most commonly, VC-1 data is found inside of Microsoft ASF files and identified with the FourCC 'WMV3'. Note that the FourCC 'WMV9' may not actually exist in the wild but the acronym gained prominence anyway due to the fact that this video codec was introduced as part of the Windows Media 9 tool suite. VC-1 video will probably be encapsulated in other types of containers and stream formats such as MPEG for HD-DVD transport.

Profiles And Levels

This table is cribbed wholesale from http://www.microsoft.com/windows/windowsmedia/forpros/events/NAB2005/VC-1.aspx

VC-1 has 3 profiles: simple, main, and advanced. Each has various levels. The combinations of profiles and levels represent trade-offs between encoding/decoding complexity, compression quality, and compressed image size.

Profile	Level	Maximum Bit Rate	Representative Resolutions by Frame Rate (Format)
Simple	Low	96 kilobits per second (Kbps)	176 x 144 @ 15 Hz (QCIF)
	Medium	384 Kbps	240 x 176 @ 30 Hz 352 x 288 @ 15 Hz (CIF)
Main	Low	2 megabits per second (Mbps)	320 x 240 @ 24 Hz (QVGA)
	Medium	10 Mbps	720 x 480 @ 30 Hz (480p) 720 x 576 @ 25 Hz (576p)
	High	20 Mbps	1920 x 1080 @ 30 Hz (1080p)
Advanced	L0	2 Mbps	352 x 288 @ 30 Hz (CIF)
	L1	10 Mbps	720 x 480 @ 30 Hz (NTSC-SD) 720 x 576 @ 25 Hz (PAL-SD)
	L2	20 Mbps	720 x 480 @ 60 Hz (480p) 1280 x 720 @ 30 Hz (720p)
	L3	45 Mbps	1920 x 1080 @ 24 Hz (1080p) 1920 x 1080 @ 30 Hz (1080i) 1280 x 720 @ 60 Hz (720p)
	L4	135 Mbps	1920 x 1080 @ 60 Hz (1080p) 2048 x 1536 @ 24 Hz

Coding Concepts

Colorspace

VC-1 codes a sequence of images in the YUV 4:2:0 colorspace.

Macroblocks, Blocks, and Sub-blocks

When VC-1 codes an image, it divides the image into macroblocks. Each 16x16 macroblock is comprised of 6 8x8 sample blocks (4 Y blocks, 1 U block, and 1 V block). Further, the coding method may divide an individual 8x8 block into 2 8x4 blocks, 2 4x8 blocks, or 4 4x4 blocks.

Transform Coding

VC-1 uses a variation of the Discrete Cosine Transform to convert blocks of samples into a transform domain to facilitate more efficient coding. The transform may operate on the full 8x8 block or any of the 3 supported sub-block sizes (8x4, 4x8, or 4x4). Unlike many codec standards preceding VC-1, the specification defines a bit-accurate transform method that all implementations are expected to conform to so as to minimize transform error.

Zigzag

After tranforming sample data into the transform domain, VC-1 reorders the transformed data in a zigzag pattern which makes certain successive coding techniques more effective. VC-1 has 31 different zigzag patterns depending on various parameters.

Quantization

Quantization is the compression step that potentially loses the most information in a lossy compression scheme such as VC-1. This codec features an impressive number of quantization modes.

Bitplane Coding

VC-1 uses a number of bitplanes which are simply maps of ones and zeros that specify properties for the macroblocks in an image. For example, a particular bitplane codes information about which macroblocks are not coded in a frame. These bitplanes are coded into the final bitstream using a number of methods.

Motion Compensation

VC-1 uses half-pel and quarter-pel interframe motion compensation.

Huffman Coding

Intensity Compensation

Intensity Compensation is special mode when reference frame luma and chroma data are scaled before using it in motion compensation.

Bitstream Packing

VC-1 bitstreams are packed as bits into bytes in left -> right order:

byte 0   byte 1   byte 2   byte 3   byte 4   ....

 byte 0    byte 1
abcdefgh  ijklmnop

Given the preceding bytestream/bitstream, a get_bit() operation to retrieve the next bit in the stream would return bit a. A get_bits(5) operation to request the next 5 bits would return 'bcdef'. The next get_bits(4) operation would return 'ghij'.

Setup Data / Sequence Layer

When VC-1 data is encapsulated inside of an ASF file it will be accompanied with setup data attached as the extradata of a BITMAPINFOHEADER data structure. In VC-1 parlance, this data is called the sequence layer. The format of this data is as follows:

• 2 bits: profile
• if profile is simple or main (0 or 1, respectively)
  • 2 bits: reserved, should be 0
• if profile is advanced (3)
  • 3 bits: level of advanced profile (values 5-7 are invalid)
  • 2 bits: chroma format (note that only format 1, YUV 4:2:0 is defined; other values are invalid)
• 3 bits: Q frame rate for post processing; unused
• 5 bits: Q bit rate for post proc; unused
• 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
  • 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
  • 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 = (width + 1) * 2)
  • 12 bits: max coded height (actual height = (height + 1) * 2)
  • 1 bit: pulldown flag
  • 1 bit: interlaced
  • 1 bit: frame counter flag
• 1 bit: frame interpolation flag
• if profile is advanced
  • (UNFINISHED: lots more stuff to be filled in when advanced profile is needed)
• if profile is simple or main
  • 1 bit: reserved, should be 1

In the case of simple or main profile data encapsulated in a general container format, the max coded width and height parameters will come from the container format rather than being encoded in the sequence layer. Observe that the total number of bits that comprise the sequence layer for simple or main profile data is 32 which, incidentally, ought to be the size of the extradata transmitted from the ASF container to a VC-1 decoder.

High Level Decoding Algorithm

For each encoded frame:

• unpack the frame information such as quantization parameters, bitplanes, and tables
• for each field
  • unpack each macroblock

Official Information

This Wiki aims to provide a complete, independent, and understandable description of the VC-1 format. Until such time, here are some external references on the format.

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