VC-1 Data Structures

Part of Understanding VC-1

This page is a discussion of the various data structures and constant enumerations employed in the SMPTE Reference Decoder (SRD) VC-1 reference implementation. These are mostly found in the file vc1types.h.

Macroblocks, Blocks, and Sub-blocks

A macroblock embodies a 16x16 block of pixels in a YUV 4:2:0 colorspace. A macroblock consists of 6 blocks: 4 8x8 Y blocks, 1 U block, and 1 V block. Further, these blocks maybe divided into sub-blocks of sizes 8x4, 4x8, or 4x4.

Intra Block

An intra block data structure requires the following information:

• number of non-zero AC coefficients
• quantized DC coefficient for prediction
• 7 quantized AC coefficients along the top row of the block, used for prediction
• 7 quantized AC coefficients along the left side of the block, used for prediction
• 16 samples representing the bottom 2 8-pixel rows of the block, maintained for overlap smoothing filters

Hybrid Prediction Modes

Note: enumerated numbers are defined in the SRD; specific numbers may or may not be relevant to independent implementation

• 0: predict from left
• 1: predict from top
• 2: no hybrid prediction

Sub-block Patterns

Note: enumerated numbers are defined in the SRD; specific numbers may or may not be relevant to independent implementation

• 0: 8x8 transform, coded
• 1: 8x4 transform, bottom subblock coded
• 2: 8x4 transform, top subblock coded
• 3: 8x4 transform, both subblocks coded
• 4: 4x8 transform, right subblock coded
• 5: 4x8 transform, left subblock coded
• 6: 4x8 transform, both subblocks coded
• 7: 4x4 transform, subblock pattern separate
• 8: 8x8 transform, coded, whole MB
• 9: 8x4 transform, bottom subblock coded, whole MB
• 10: 8x4 transform, top subblock coded, whole MB
• 11: 8x4 transform, both subblocks coded, whole MB
• 12: 4x8 transform, right subblock coded, whole MB
• 13: 4x8 transform, left subblock coded, whole MB
• 14: 4x8 transform, both subblocks coded, whole MB
• 15: 4x4 transform, subblocks pattern separate, whole MB

One more stray constant:

• 8: MB level threshold

Motion Vector

This is the information maintained for an individual motion vector:

• X offset
• Y offset
• a flag indicating whether the vector pertains to the top or bottom field of interlaced video

The (X, Y) vector is relative to the top-left coordinate of a block. The fractional pel resolution of the vectors depends on the motion vector coding mode.

Motion

This structure encapsulates motion vectors along with mode information.

• prediction mode, as enumerated in Hybrid Prediction Modes
• motion vector data structure
• differential motion vector data structure, represented in quarter-pel units

The SRD contains the following not accompanying this data structure:

/*
 * If Two Reference Images (NUMREF=1) then:
 *      Y=2*(YValue)+PredFlag
 *      PredFlag: 0=dominant 1=non-dominant
*/

Motion Vector History

This data structure consists of an array of 4 motion vector data structures which stores the motion vector history for 4 Y blocks, used for direct mode.