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.

Block Types

Note: enumerated numbers are defined in the SRD; specific numbers may or may not be relevant to independent implementation. 0: 8x8 inter-coded block 1: 8x4 inter-coded block 2: 4x8 inter-coded block 3: 4x4 inter-coded block 4: transform type not yet determined 5: intra-coded block, no AC prediction 6: intra-coded block, AC prediction of top row coefficients 7: intra-coded block, AC prediction of left column coefficients

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.

Inter Block

An intra block data structure requires the following information:

• an array of 4 integers representing the number of non-zero coefficients (DC and AC together) for up to 4 sub-blocks in the block
• 2 motion structures, 1 for backward prediction and 1 for forward prediction

Block

This is the information that the SRD maintains for an individual block:

• block type, as defined in Block Types section
• a flag indicating whether there are non-zero AC coefficients for an intra block, or non-zero DC or AC coefficients for an inter block
• a C union that contains either an intra or inter block data structure

Quantizer

This data structure maintains information about quantization parameters.

• quantizer step, range 0..31
• quantizer half-step, either 0 or 1
• a flag indicating whether the quantization is uniform or non-uniform

Macroblock Properties

These properties are associated with various macroblock coding options:

• a block is intra-coded, or has 1, 2, or 4 motion vectors associated
• a block predicts backwards from a previous frame, from a forward frame, from both a backward or forward frame, or uses "direct" mode
• a flag that indicates whether MVs apply to interlaced fields
• a flag that indicates that "bottom field different direction to top"
• a flag that indicates "field transform"

AC Prediction

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

• 0: AC prediction off
• 1: AC prediction on
• 2: AC prediction absent (no blocks to predict from?)

Macroblock

The SRD maintains the following information about an individual macroblock:

• macroblock type: this is a combination of properties enumerated in Macroblock Properties
• AC prediction status as enumerated in AC Prediction
• block type as enumerated in Block Types; might be "any"
• a flag indicating whether the overlap filter is active for this macroblock
• a flag indicating whether is motion predicted only (presumably, this means that a predicted block is formed by no other transform/addition occurs for the MB)
• a 6-bit flag vector that indicates which of the 6 constituent block in the MB are coded
• a 4-bit flag vector that indicates motion vector block pattern-- these flags are set if the differential MV for a respective Y block is not 0
• quantizer data structure for the macroblock
• 6 block structures

Picture Format

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

• 0: picture is a progressive frame
• 1: picture is an interlaced frame
• 2: picture consists of 2 interlaced fields
• 3: picture format has not been determined

Bitstream Profile

These are the supported profiles in the VC-1 coding scheme.

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

Profile Level Enumeration

• simple/main profiles:
  • 0: low
  • 1: medium
  • 2: high
• advanced profile:
  • 0..4: levels 0..4
• levels 5..7 are reserved
• 255 indicates that the level is unknown

Chroma Format

The SRD only supports one chroma format: YUV 4:2:0, which is format 1. Ostensibly, there are 2 bits in the bitstream to define chroma format. Modes 0, 2, and 3 are all reserved.

Color Primaries

• 0: color primaries are forbidden
• 1: ITU-R BT-709
• 2: unspecified
• 3: reserved
• 4: reserved
• 5: EBU Tech 3213
• 6: SMPTE C
• 7-255: reserved

Transfer Characteristics

These properties are encoded into the bitstream and describe the characteristics of the source bitstream:

• 0: forbidden
• 1: ITU-R BT-709
• 2: unspecified
• 3: reserved
• 4: reserved
• 5: reserved
• 6: reserved
• 7: SMPTE 240M
• 8-255: reserved

Matrix Coefficients

• 0: forbidden
• 1: ITU-R BT-709
• 2: unspecified
• 3: reserved
• 4: reserved
• 5: reserved
• 6: SMPTE 170M
• 7: SMPTE 240M
• 8-255: reserved

Quantizer Modes

• 0: quantizer implied by quantizer step size
• 1: quantizer explicitly signaled
• 2: non-uniform quantizer
• 3: uniform quantizer

Picture Types

• 0: I-frame-- intraframe/field
• 1: P-frame-- predicted frame/field
• 2: B-frame-- bi-directionally predicted frame/field
• 3: BI-frame-- ??? perhaps an I-frame upon which no other frames depend
• 4: skipped

Scaling

This enumeration defines whether there will be any scaling in the picture before display:

• 0: 1x1 = no scaling
• 1: 2x1 = horizontal scaling
• 2: 1x2 = vertical scaling
• 3: 2x2 = horizontal and vertical scaling

Motion Vector Ranges

• Range #0:
  • x component range = -64..63
  • y component range = -32..31
• Range #1:
  • x component range = -128..127
  • y component range = -64..63
• Range #2:
  • x component range = -512..511
  • y component range = -128..127
• Range #3:
  • x component range = -1024..1023
  • y component range = -256..255