H.263、H.264与H.265编解码标准详解：FFmpeg应用

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H.263、H.264和H.265是国际电信联盟（ITU-T）制定的一系列视频编码标准，用于低比特率通信，尤其是在电视会议、网络视频传输和早期的移动电话视频通话等场景中。这些标准在信息技术领域具有重要地位，它们通过提高压缩效率和图像质量，减少了数据传输的带宽需求，从而推动了实时视频通信的发展。 H.263标准最初是在1993年至1996年间由ITU-T Study Group 15负责制定，其目标是提供一种适合于低比特率环境的视频编码方案。这个标准的批准遵循了世界电信标准化大会（WTSC）每四年一次的决议流程，WTSC决议1（1993年赫尔辛基会议）规定了推荐标准的审批程序。 H.263标准在1996年3月19日得到了WTSC的认可，标志着它作为通信技术中的一个里程碑。H.263的成功促进了后续版本的更新和发展，包括H.264（Advanced Video Coding, AVC），以及更高级别的H.265（High Efficiency Video Coding, HEVC）。H.264在高清视频和网络流媒体中被广泛应用，而H.265则进一步提升了编码效率，尤其对于4K甚至8K超高清视频提供了更好的支持。 ffmpeg是一个开源的跨平台多媒体处理工具包，它不仅包含了多种编解码器，包括H.263、H.264和H.265，还提供了视频转换、流媒体处理、音频处理等功能。使用ffmpeg，开发人员能够方便地实现不同格式之间的转换，优化视频质量，并进行高效的视频处理。通过ffmpeg，H.263、H.264和H.265标准得以在实际应用中得到广泛集成和优化。总结来说，H.263、H.264和H.265是通信行业的重要视频编码技术，它们与ffmpeg的结合使得高效、高质量的网络视频传输成为可能，对现代通信系统和在线娱乐产生了深远影响。随着技术的进步，这些标准也在不断演进，以适应不断发展的数字媒体需求。

Recommendation H.263 (03/96) 3

3.4.4 PB-frames mode

A PB-frame consists of two pictures being coded as one unit. The name PB comes from the name of picture types in

Recommendation H.262 where there are P-pictures and B-pictures. Thus a PB-frame consists of one P-picture which is

predicted from the previous decoded P-picture and one B-picture which is predicted from both the previous decoded

P-picture and the P-picture currently being decoded. The name B-picture was chosen because parts of B-pictures may be

bidirectionally predicted from the past and future pictures. With this coding option, the picture rate can be increased

considerably without increasing the bit rate much.

3.5 Bit rate

The transmission clock is provided externally. The video bit rate may be variable. In this Recommendation no

constraints on the video bit rate are given; constraints will be given by the terminal or the network.

3.6 Buffering

The encoder shall control its output bit stream to comply with the requirements of the hypothetical reference decoder

defined in Annex B. Video data shall be provided on every valid clock cycle. This can be ensured by MCBPC stuffing

(see Tables 4 and 5) or, when forward error correction is used, also by forward error correction stuffing frames (see

Annex H).

The number of bits created by coding any single picture shall not exceed a maximum value specified by the parameter

BPPmaxKb which is measured in units of 1024 bits. The minimum allowable value of the BPPmaxKb parameter

depends on the largest source picture format that has been negotiated for use in the bit stream (see Table 1). An encoder

may use a larger value for BPPmaxKb than as specified in Table 1, provided the larger value is first negotiated by

external means, for example Recommendation H.245.

TABLE 1/H.263

BPPmaxKb for each of the source picture formats

3.7 Symmetry of transmission

The codec may be used for bidirectional or unidirectional visual communication.

3.8 Error handling

Error handling should be provided by external means (for example Recommendation H.223). If it is not provided by

external means (for example in Recommendation H.221) the optional error correction code and framing as described in

Annex H can be used.

A decoder can send a command to encode one or more GOBs of its next picture in INTRA mode with coding parameters

such as to avoid buffer overflow. A decoder can also send a command to transmit only non-empty GOB headers. The

transmission method for these signals is by external means (for example Recommendation H.245).

Source format BPPmaxKb

sub-QCIF 64

QCIF 64

CIF 256

4CIF 512

16CIF 1024

4 Recommendation H.263 (03/96)

3.9 Multipoint operation

Features necessary to support switched multipoint operation are included in Annex C.

4 Source coder

4.1 Source format

The source coder operates on non-interlaced pictures occurring 30 000/1001 (approximately 29.97) times per second.

The tolerance on picture frequency is ± 50 ppm.

Pictures are coded as luminance and two colour difference components (Y, C

and C

). These components and the

codes representing their sampled values are as defined in ITU-R Recommendation 601.

• Black = 16;

• White = 235;

• Zero colour difference = 128;

• Peak colour difference = 16 and 240.

These values are nominal ones and the coding algorithm functions with input values of 1 through to 254.

There are five standardised picture formats: sub-QCIF, QCIF, CIF, 4CIF and 16CIF. For each of these picture formats,

the luminance sampling structure is dx pixels per line, dy lines per picture in an orthogonal arrangement. Sampling of

each of the two colour difference components is at dx/2 pixels per line, dy/2 lines per picture, orthogonal. The values of

dx, dy, dx/2 and dy/2 are given in Table 2 for each of the picture formats.

TABLE 2/H.263

Number of pixels per line and number of lines for each of the H.263 picture formats

For each of the picture formats, colour difference samples are sited such that their block boundaries coincide with

luminance block boundaries as shown in Figure 2. The pixel aspect ratio is the same for each of these picture formats

and is the same as defined for QCIF and CIF in Recommendation H.261: (4/3)

(288/352). The picture area covered by

all picture formats except the sub-QCIF picture format has an aspect ratio of 4:3.

Picture Format

Number of pixels

for luminance

(dx)

Number of lines

for luminance

(dy)

Number of pixels

for chrominance

(dx/2)

Number of lines

for chrominance

(dy/2)

sub-QCIF 128 96 64 48

QCIF 176 144 88 72

CIF 352 288 176 144

4CIF 704 576 352 288

16CIF 1408 1152 704 576

Recommendation H.263 (03/96) 5

T1519560-95/d02

Luminance sample

Chrominance sample

Block edge

FIGURE 2/H.263

Positioning of luminance and chrominance samples

FIGURE 2/H.263...[D02] = 3 CM (118%)

All decoders shall be able to operate using sub-QCIF. All decoders shall also be able to operate using QCIF. Some

decoders may also operate with CIF, 4CIF or 16CIF. Encoders shall be able to operate with one of the formats sub-QCIF

and QCIF. The encoders determine which of these two formats are used, and are not obliged to be able to operate with

both. Some encoders can also operate with CIF, 4CIF or 16CIF. Which formats can be handled by the decoder is

signalled by external means, for example Recommendation H.245. For a complete overview of possible picture formats

and video coding algorithms, refer to the terminal description, for example Recommendation H.324.

NOTE – For CIF, the number of pixels per line is compatible with sampling the active portions of the luminance and

colour difference signals from 525- or 625-line sources at 6.75 and 3.375 MHz respectively. These frequencies have a simple

relationship to those in ITU-R Recommendation 601.

Means shall be provided to restrict the maximum picture rate of encoders by having a minimum number of

non-transmitted pictures between transmitted ones. Selection of this minimum number shall be by external means (for

example, Recommendation H.245). For the calculation of the minimum number of non-transmitted pictures in PB-

frames mode, the P-picture and the B-picture of a PB-frames unit are taken as two separate pictures.

4.2 Video source coding algorithm

The source coder is shown in generalized form in Figure 3. The main elements are prediction, block transformation and

quantization.

4.2.1 GOBs, macroblocks and blocks

Each picture is divided into groups of blocks (GOBs). A Group of Blocks (GOB) comprises of k

16 lines, depending

on the picture format (k = 1 for sub-QCIF, QCIF and CIF; k = 2 for 4CIF; k = 4 for 16CIF). The number of GOBs per

picture is 6 for sub-QCIF, 9 for QCIF, and 18 for CIF, 4CIF and 16CIF. The GOB numbering is done by use of vertical

scan of the GOBs, starting with the upper GOB (number 0) and ending with the lower GOB. An example of the

arrangement of GOBs in a picture is given for the CIF picture format in Figure 4. Data for each GOB consists of a GOB

header (may be empty) followed by data for macroblocks. Data for GOBs is transmitted per GOB in increasing GOB

number.

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