H.264 STRUCTURE |
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Extended profile
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Main profile |
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SP and |
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SI slices |
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B slices |
Interlace |
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Data |
Weighted |
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Partitioning |
CABAC |
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Prediction |
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I slices
P slices
CAVLC
Slice Groups
and ASO
Redundant
Slices
Baseline profile
Figure 6.3 H.264 Baseline, Main and Extended profiles
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NAL |
RBSP |
NAL |
RBSP |
NAL |
RBSP |
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header |
header |
header |
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Figure 6.4 Sequence of NAL units
6.3.3 Coded Data Format
H.264 makes a distinction between a Video Coding Layer (VCL) and a Network Abstraction Layer (NAL). The output of the encoding process is VCL data (a sequence of bits representing the coded video data) which are mapped to NAL units prior to transmission or storage. Each NAL unit contains a Raw Byte Sequence Payload (RBSP), a set of data corresponding to coded video data or header information. A coded video sequence is represented by a sequence of NAL units (Figure 6.4) that can be transmitted over a packet-based network or a bitstream transmission link or stored in a file. The purpose of separately specifying the VCL and NAL is to distinguish between coding-specific features (at the VCL) and transport-specific features (at the NAL). Section 6.7 describes the NAL and transport mechanisms in more detail.
6.3.4 Reference Pictures
An H.264 encoder may use one or two of a number of previously encoded pictures as a reference for motion-compensated prediction of each inter coded macroblock or macroblock
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H.264/MPEG4 PART 10 |
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Table 6.1 H.264 slice modes |
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Slice type |
Description |
Profile(s) |
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I (Intra) |
Contains only I macroblocks (each block or |
All |
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macroblock is predicted from previously coded |
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data within the same slice). |
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P (Predicted) |
Contains P macroblocks (each macroblock |
All |
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or macroblock partition is predicted from one |
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list 0 reference picture) and/or I macroblocks. |
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B (Bi-predictive) |
Contains B macroblocks (each macroblock or macroblock |
Extended and Main |
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partition is predicted from a list 0 and/or |
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a list 1 reference picture) and/or I macroblocks. |
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SP (Switching P) Facilitates switching between coded streams; contains |
Extended |
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P and/or I macroblocks. |
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SI (Switching I) Facilitates switching between coded streams; contains SI |
Extended |
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macroblocks (a special type of intra coded macroblock).
partition. This enables the encoder to search for the best ‘match’ for the current macroblock partition from a wider set of pictures than just (say) the previously encoded picture.
The encoder and decoder each maintain one or two lists of reference pictures, containing pictures that have previously been encoded and decoded (occurring before and/or after the current picture in display order). Inter coded macroblocks and macroblock partitions in P slices (see below) are predicted from pictures in a single list, list 0. Inter coded macroblocks and macroblock partitions in a B slice (see below) may be predicted from two lists, list 0 and list 1.
6.3.5 Slices
A video picture is coded as one or more slices, each containing an integral number of macroblocks from 1 (1 MB per slice) to the total number of macroblocks in a picture (1 slice per picture) The number of macroblocks per slice need not be constant within a picture. There is minimal inter-dependency between coded slices which can help to limit the propagation of errors. There are five types of coded slice (Table 6.1) and a coded picture may be composed of different types of slices. For example, a Baseline Profile coded picture may contain a mixture of I and P slices and a Main or Extended Profile picture may contain a mixture of I, P and B slices.
Figure 6.5 shows a simplified illustration of the syntax of a coded slice. The slice header defines (among other things) the slice type and the coded picture that the slice ‘belongs’ to and may contain instructions related to reference picture management (see Section 6.4.2). The slice data consists of a series of coded macroblocks and/or an indication of skipped (not coded) macroblocks. Each MB contains a series of header elements (see Table 6.2) and coded residual data.
6.3.6 Macroblocks
A macroblock contains coded data corresponding to a 16 × 16 sample region of the video frame (16 × 16 luma samples, 8 × 8 Cb and 8 × 8 Cr samples) and contains the syntax elements described in Table 6.2. Macroblocks are numbered (addressed) in raster scan order within a frame.