"H.264 / MPEG-4 AVC (H.264) is the latest, most promising video compression standard after the MPEG-2 video compression standard. H.264 is made from ITU-T and ISO / IEC The latest international video coding standard developed by the joint development group. Through this standard, the compression efficiency under the same image quality is more than 2 times higher than the previous standard, so H.264 is generally considered to be the most influential industry. standard.
First, H.264 development history
H.264 is called H.26L during the 1997 Video Coding Experts Group, and is known as MPEG4 Part10 (MPEG4 AVC) or H.264 (JVT) after ITU and ISO collaborative research. .
H.264 advanced technology background
The main objective of the H.264 standard is to provide more excellent image quality over the same bandwidth compared to other existing video coding standards.
However, H.264 is compared with previous international standards such as H.263 and MPEG-4, the biggest advantage is reflected in the following four aspects:
1. Each video frame is separated into a block consisting of pixels, so the process of encoding the video frame can reach the level of block.
2. Some original blocks of video frames are spatially predictive, conversion, optimization, and entropy encoding (variable long coding) for some original blocks of video frames.
3. The method of temporary storage is employed for different blocks of the continuous frame, so that only the part thereof in the continuous frame is encoded. This algorithm is accomplished by exercise prediction and motion compensation. For some specific blocks, searching for the search to determine the motion vector of the block in one or more frames, and thus the main block is predicted in the rear coding and decoding.
4. The residual block in the video frame is encoded by the remaining spatial redundancy technology. For example, for the source block and the corresponding prediction block, the conversion, optimization, and entropy encoding are again used.
H.264 characteristics and advanced advantages
H.264 is a new generation of digital video compression formats after the International Standardization Organization (ISO) and International Telecommunications Union (ITU), which retains the advantages and essence of compression technology in the past, and other compression techniques cannot match. Many advantages.
1. Low bit flow (Low bit): Compared to compression techniques such as MPEG2 and MPEG4 ASP, the amount of data after the compression of H.264 is only 1/8 of MPEG2, 1/3 of MPEG2, and 1/3 of MPEG2. Obviously, the use of H.264 compression technology will greatly save users' download time and data traffic charges.
2. High quality image: H.264 provides continuous, smooth high quality images (DVD quality).
3. The fault tolerance is strong: H.264 provides the necessary tools that address errors such as packet loss that occurs in unstable network environments.
4. Network adaptability: H.264 provides Network Adaptation Layer, enabling H.264 files to easily transfer (eg, Internet, CDMA, GPRS, WCDMA, CDMA2000, etc.) on different networks (such as Internet, CDMA, GPRS, WCDMA, CDMA2000, etc.). Be
Second, H.264 standard overview
The H.264 and previous standards are also mixed encoding modes for DPCM. However, it uses the "return to basic" simple design, no numerous options, more compressed performance than H.263 ++; strengthen the adaptability of various channels, using "network friendly" structure and grammar, It is conducive to the processing of errors and packet loss; the application target range is wider to meet the needs of different rates, different resolutions, and different transmission (storage) applications.
Technically, it concentrates on the advantages of past standards and absorbs the experience of accumulation in standard development. H.264 can save up to 50 in most code rates when using the best encoder similar to the above-described encoding method than H.263 V2 (H.263 +) or MPEG-4 simple class (Simple Profile) % Code rate. H.264 can continue to provide higher video quality at all yaw rates. H.264 can work in low delay mode to accommodate real-time communication applications (such as video conferencing), while working well without delay restrictions, such as video storage and server-based video stream applications . The H.264 provides tools for processing packets in the package transfer network, as well as tools for handling bit error in the transcriptable wireless network.
On the system level, H.264 proposes a new concept, conceptual segmentation between video coding layer, VCL and network extraction layer (NAL), the former is the core of video content The representation of the compressed content is a representation of the presentation by a particular type of network, such a structure facilitates the package and better priority control of information. The system encoding block diagram of H.264 is shown in Figure 1.
Figure 1 H.264 system block diagram
Third, the key technology of the H.264 standard
1. Intra prediction code
The intra encoding is used to reduce the space redundancy of the image. In order to increase the efficiency of the H.264 intra coding, the spatial relevance of the adjacent macroblock is taken into a given frame, and the adjacent macroblocks typically contain similar properties. Therefore, when encoding a given macroblock, first, according to the surrounding macroblock prediction (typically based on the macroblock in the upper left corner, because the macroblock has been encoded), then the predicted value and the actual value The value is encoded so that the code rate can be greatly reduced relative to the frame encoding directly.
H.264 provides 6 modes for 4 × 4 pixel macroblock prediction, including 1 DC prediction and 5 direction predictions, as shown in FIG. In the figure, the A to I of the adjacent block has been encoded, which can be used to predict if we select mode 4, then, A, B, C, D4 pixels are predicted to equal equivalents with E. Values, E, F, G, and H4 pixels are predicted to equal value to F, and the flat regions containing very little spatial information in the image, and H.264 also supports 16 × 16 intra-intra-code encoding. Be
Figure 2 intra encoding mode
2. Inter-frame prediction code
Inter-frame prediction coding utilizes time redundancy in the continuous frame to perform motion estimation and compensation. H.264 Motion Compensation Supports most of the key features in previous video coding standards, and flexibly adds more features, in addition to supporting P frame, B frame, H.264 also supports a new flow of flow Frame --SP frame. After the SP frame is included in the code stream, it can quickly switch between the similar content but there is a stream of different yaw rates, while supporting random access and fast playback mode.
The Motion estimation of H.264 has the following four characteristics.
(1) macroblock segmentation of different sizes and shapes
Motion compensation for each 16 × 16 pixel macroblock can be used in different sizes and shapes, and H.264 supports seven modes, as shown in Figure 4. The motion compensation of the small block mode improves performance, reducing the block effect, and improves the quality of the image.
(2) High-precision sub-pixel motion compensation
In H.263, a half pixel precision is used, while in H.264, a motion estimate of 1/4 or 1/8 pixel precision can be employed. In the case where the same accuracy is required, H.264 uses a 1/4 or 1/8 pixel precision, the residual is smaller than the residual of the H.263 using a half pixel precision motion. Thus, in the same accuracy, H.264 is smaller than the code rate required in the frame encoding.
(3) Multi-frame prediction
H.264 provides an optional multi-frame prediction feature, and 5 different reference frames can be selected when encoded, providing better error correction performance, so that video image quality can be improved. This feature is primarily applied to the following occasions: cyclical motion, translation, transform the camera in two different scenes.
(4) Go to block filters
H.264 defines a filter that adaptively remove block effects, which can handle the horizontal and vertical block edges in the prediction loop, greatly reduces the square effect.
3. Integer transformation
In terms of transformation, H.264 uses a transformation of DCT based on 4 × 4 pixel blocks, but uses an integer-based spatial transform, there is no anti-transform, because there is a problem with error, and the transformation matrix is like Figure 5 shows. Compared to floating point operations, integer DCT transformations cause some additional errors, but because quantification after the DCT transformation also has quantization errors, the quantization error caused by integer DCT transformation is not large. In addition, the integer DCT transformation also has the advantage of reducing the amount of computational calculation and complexity, which is conducive to the design of the DSP transplantation.
4. Quantify
32 different quantization steps can be selected in H.264, which is similar to 31 quantization steps in H.263, but in H.264, the step size is incorporated in 12.5%, and Not a fixed constant. In H.264, there are two ways to read the transform coefficient: zigzag scan and double scan. In most cases, a simple zigzag scan is used; double scan is only used in blocks of smaller quantization levels to help improve coding efficiency.
5. Entropy encoding
The final step of video encoding processing is entropy encoding, two different entropy encoding methods in H.264: General Variable Long Coding (UVLC) and text-based adaptive binary arithmetic coding (CABAC). In H.263 and other standards, different VLC code tables are used depending on the data types, motion vectors such as transform factors, motion vectors, etc.. The UVLC code table in H.264 provides a simple method, regardless of what type of data, which is specified, all use unified variant length coding tables. Its advantage is simple; the disadvantage is that a single code table is derived from the probability statistical distribution model. It does not consider the correlation between encoded symbols, and the effect is not very good when the medium is high. Therefore, an optional CABAC method is also provided in H.264. Arithmetic coding enables the code and decoding of the probability model of all syntax elements (transform factors, motion vectors). In order to improve the efficiency of the arithmetic coding, the basic probability model can be adapted to the statistical characteristics change with the video frame by the process of modeling. Content modeling provides the conditional probability estimation of encoding symbols, using the appropriate content model, the correlation between the symbols can be removed by selecting the corresponding probability model of the encoded symbols currently to encode symbols, and different syntax elements are usually maintained. Different models.
IV, H.264 Application in Video Conference
At present, most of the video conferencing systems use H.261 or H.263 video coding standard, while H.264 appears such that H.264 can reduce the rate of 50% higher than H.263. In other words, even with 384kbit / s bandwidth, users can enjoy high quality video services up to 768kbit / s under H.263. H.264 not only helps save huge expenses, but also improve resource efficiency, while making video conferencing services to commercial quality have more potential customers.
At present, there have been a few vendors' video conferencing products to support the H.264 protocol, and the manufacturers are committed to popularizing the new industry standards in H.264. With other video conferencing programs, we will experience the advantages of H.264 video services. "
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