Difference between revisions of "VC-1 Data Structures"

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* frame user data present flag
* frame user data present flag
* end-of-sequence marker present flag
* end-of-sequence marker present flag
== Start Codes ==
These are the various start codes that the SRD defines:
* 0x0A: end of sequence
* 0x0B: slice
* 0x0C: field
* 0x0D: frame header
* 0x0E: entry point header
* 0x0F: sequence header
* 0x1B: user-defined slice
* 0x1C: user-defined field
* 0x1D: user-defined frame header
* 0x1E: user-defined entry point header
* 0x1F: user-defined sequence header
== Component ==
The SRD defines a component as a single Y, U, or V plane and associates these members with the data structure:
* data: the raw bytes that comprise the Y, U, or V sample data
* bytes/line, a.k.a. stride
== Field ==
This data structure defines all of the parameters that pertain to a particular field:
* picture type
* conditional overlap filter mode
* quantization mode
* motion vector mode
* motion vector range
* block transform type
* post processing flag
* extended X differential motion vector range
* extended Y differential motion vector range
* number of reference fields (either 1 or 2)
* reference field (either last or last-but-one)
* motion vector VLC table (0..7)
* MB mode VLC table (0..7)
* block pattern 2 motion vector table (0..3)
* block pattern 4 motion vector table (0..3)
* inter-coded block coding pattern VLC table (0..3)
* AC coding set to use for intra-coded Y blocks (0..2)
* AC coding set to use for all inter-coded blocks or U and V intra (0..2)
* DC coding set (0..1)
* rows per slice (0 = no slicing used)
== Picture ==
The picture is the fundamental data unit in the VC-1 coding scheme. A picture can be one of the following things:
* a progressive frame
* an interlaced top field
* an interlaced bottom field
* an interlaced frame
The SRD maintains the following information about a picture:
* frame number (modulo 1<<32)
* picture format
* Y component data structure
* U component data structure
* V component data structure
* 2 field data structures
* picture resolution index
* top field first flag
* repeat first field flag
* range reduction used flag
* frame interpolation hint flag
* chrominance sample format flag
* repeat frame count
* pan scan parameters data structure
* post processing mode

Revision as of 20:24, 23 April 2006

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 an 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 Modes

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

Differential Motion Vector Ranges

  • 0: no extended DMV
  • 1: extended DMV horizontal/X
  • 2: extended DMV vertical/Y
  • 3: extended DMV horizontal & vertical

Macroblock Quantizer Step Sizes

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

  • 0: all macroblocks use PQUANT
  • 1: edge MBs use ALTPQUANT
  • 2: left/top MBs use ALTPQUANT
  • 3: top/right MBs use ALTPQUANT
  • 4: right/bottom MBs use ALTPQUANT
  • 5: bottom/left MBs use ALTPQUANT
  • 6: left MBs use ALTPQUANT
  • 7: top MBs use ALTPQUANT
  • 8: right MBs use ALTPQUANT
  • 9: bottom MBs use ALTPQUANT
  • 10: PQUANT vs. ALTPQUANT is selected per MB
  • 11: quantizer select per MB

Bitplane Coding Methods

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

  • 0: normal-2 method
  • 1: normal-6 method
  • 2: rowskip method
  • 3: colskip method
  • 4: diff-2 method
  • 5: diff-6 method
  • 6: uncompressed

Overlap Filter Modes

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

  • 0: disable overlap filter
  • 1: enable overlap filter for all macroblocks
  • 2: overlap filter is enabled for select macroblocks

Motion Vector Modes

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

  • 0: 1 motion vector, half-pel, bilinear interpolation
  • 1: 1 motion vector, half-pel, bicubic interpolation
  • 2: 1 motion vector, quarter-pel, bicubic interpolation
  • 3: mixed motion vectors, quarter-pel, bicubic interpolation
  • 4: intensity compensation

Intensity Compensation

The SRD used the following data structure to maintain information about intensity compensation:

  • a flag to indicate whether intensity compensation is enabled
  • IC scale
  • IC shift

B Fraction

This data structure tracks B-fraction data:

  • B-fraction numerator
  • B-fraction denominator
  • scalefactor: approximate numerator * 256 / denominator

Hypothetical Reference Decoder

The SRD contains data structures pertaining to a hypothetical reference decoder involving a leaky bucket algorithm.

Pan Scan Window

This data structure pertains to pan and scan windows:

  • horizontal offset in pixels
  • vertical offset in pixels
  • width in pixels
  • height in pixels

Pan Scan Parameters

This data structure contains a flag indicating whether pan and scan is present, and an array of 3 Pan Scan Window data structures (3 is the maximum supported).

Sequence And Layer Parameters

The SRD maintains all of the information for the sequence layer:

  • profile (simple, main, or advanced)
  • maximum coded width
  • maximum coded height
  • coded width
  • coded height
  • display width
  • display height
  • aspect width
  • aspect height
  • profile level
  • interface flag
  • frame rate numerator (0 unless otherwise specified)
  • frame rate denominator (the comments list 1000, 1001, and 32 as valid values, and claim that it is 0 if not specified, which sounds potentially problematic)
  • color format indicator flag
  • chroma format
  • color primaries
  • transfer characteristics
  • matrix coefficients
  • hypothetical reference decoder
  • loop filter flag
  • multi resolution coding flag
  • fast chrominance motion compensation flag
  • extended motion vector flag
  • extended differential motion vector flag
  • d-quant
  • VS transform flag
  • overlapped transform flag
  • sync marker flag
  • range reduction flag
  • max B-frames
  • quantizer mode
  • post processing flag
  • frame counter flag
  • pull down flag
  • PsF (???)
  • Q framerate for post processing
  • Q bitrate for post processing
  • pan scan flag
  • reserved RTM flag
  • frame interpolation flag
  • range scale Y (comment: scale value times 8)
  • range scale UV (comment: scale value times 8)
  • number of pan scan windows
  • broken link flag
  • closed entry flag
  • refdist (???) flag
  • frame user data present flag
  • end-of-sequence marker present flag

Start Codes

These are the various start codes that the SRD defines:

  • 0x0A: end of sequence
  • 0x0B: slice
  • 0x0C: field
  • 0x0D: frame header
  • 0x0E: entry point header
  • 0x0F: sequence header
  • 0x1B: user-defined slice
  • 0x1C: user-defined field
  • 0x1D: user-defined frame header
  • 0x1E: user-defined entry point header
  • 0x1F: user-defined sequence header

Component

The SRD defines a component as a single Y, U, or V plane and associates these members with the data structure:

  • data: the raw bytes that comprise the Y, U, or V sample data
  • bytes/line, a.k.a. stride

Field

This data structure defines all of the parameters that pertain to a particular field:

  • picture type
  • conditional overlap filter mode
  • quantization mode
  • motion vector mode
  • motion vector range
  • block transform type
  • post processing flag
  • extended X differential motion vector range
  • extended Y differential motion vector range
  • number of reference fields (either 1 or 2)
  • reference field (either last or last-but-one)
  • motion vector VLC table (0..7)
  • MB mode VLC table (0..7)
  • block pattern 2 motion vector table (0..3)
  • block pattern 4 motion vector table (0..3)
  • inter-coded block coding pattern VLC table (0..3)
  • AC coding set to use for intra-coded Y blocks (0..2)
  • AC coding set to use for all inter-coded blocks or U and V intra (0..2)
  • DC coding set (0..1)
  • rows per slice (0 = no slicing used)

Picture

The picture is the fundamental data unit in the VC-1 coding scheme. A picture can be one of the following things:

  • a progressive frame
  • an interlaced top field
  • an interlaced bottom field
  • an interlaced frame

The SRD maintains the following information about a picture:

  • frame number (modulo 1<<32)
  • picture format
  • Y component data structure
  • U component data structure
  • V component data structure
  • 2 field data structures
  • picture resolution index
  • top field first flag
  • repeat first field flag
  • range reduction used flag
  • frame interpolation hint flag
  • chrominance sample format flag
  • repeat frame count
  • pan scan parameters data structure
  • post processing mode