H.264, like the previous standard, is a hybrid coding framework. The AVS video standard uses a similar technical framework to H.264, including transform, quantization, entropy coding, intra prediction, inter prediction, and loop filtering. And other modules. The differences in their core technologies include the following:
First, transform and quantify
H.264 uses block-based transform coding for residual data to remove the spatial redundancy of the original image, so that the image capability is concentrated on a small part of the coefficient, and the DC coefficient value is generally the largest, which can improve the compression ratio and enhance the resistance. Interference ability. The previous standard generally adopts the DCT transform. The disadvantage of this transform is that there will be a mismatch phenomenon. The original data will have a difference after being transformed by the transform and the inverse transform, and the calculation amount is also large because it is a real number operation. H.264 uses an integer transform based on 4&TImes; 4 blocks.
AVS uses 8&TImes; 8 integer transforms, which can be implemented without mismatch on 16-bit processors. The high-resolution video image de-correlation is more efficient than the 4&TImes;4 transform, using 64-level quantization, which can adapt to different application and service requirements for code stream and quality.
Second, intra prediction
Both H.264 and AVS techniques use intra prediction to predict the current block with adjacent pixels, using multiple prediction modes that represent spatial domain textures. The luminance prediction of H.264 has 4 prediction modes of 4 and TImes; 4 blocks and 16×16 blocks. For 4×4 blocks: plus a DC prediction from -135 degrees to +22.5 degrees, there are a total of 9 prediction directions; 16×16 blocks: There are 4 prediction directions. The chrominance prediction is an 8×8 block with four prediction modes, similar to the four modes of intra 16×16 prediction, where DC is mode 0, horizontal is mode 1, vertical is mode 2, and plane is mode 3.
Third, inter prediction
H.264 inter-prediction is a prediction mode that utilizes coded video frames and block-based motion compensation. The difference from previous standard interframe prediction is the wider block size range, the use of sub-pixel motion vectors, and the use of multiple reference frames.
H.264 has 8 macroblocks and sub-macroblock partitions of 16×16, 16×8, 8×16, 8×8, 8×4, 4×8 and 4×4, while AVS is only 16×16, 16 There are 4 macroblock division modes of ×8, 8×16 and 8×8.
H.264 supports the prediction of inter macroblocks and slices using a plurality of different reference frames. In AVS, P frames can utilize up to 2 frames of forward reference frames, and B frames use one frame before and after.
Fourth, entropy coding
H.264 has developed information-based entropy coding efficiency, one is to use uniform variable length coding (UVLC) for all symbols to be coded, and the other is to use content-based adaptive binary arithmetic coding (CABAC, Context-Adaptive Binary Arithmetic Coding) greatly reduces block coding correlation redundancy and improves coding efficiency. The UVLC calculation complexity is low, mainly for applications with strict coding time. The disadvantage is low efficiency and high code rate. CABAC is an efficient entropy coding method with a coding efficiency 50% higher than UVLC coding.
AVS entropy coding uses adaptive variable length coding techniques. In the AVS entropy encoding process, all syntax elements and residual data are mapped into a binary bit stream in the form of an exponential Golomb code.
The advantage of using the index Columbus code is: on the one hand, its hardware complexity is relatively low, the code can be parsed according to the closed formula, no need to look up the table; on the other hand, it can flexibly determine the K-order index Columbus according to the probability distribution of the coding elements. Code coding, if K is chosen properly, the coding efficiency can approach the information entropy.
The block transform coefficients of the prediction residuals are scanned to form (level, run) pairs, and level and run are not independent events, but there is a strong correlation. In AVS, level and run are combined by two-dimensional joint coding, and according to The current probability distribution trends of level and run, adaptively change the order of the index Columbus code.
In addition, there are no SI or SP frames in AVS. It can be said that AVS is developed on the basis of H.264, absorbing the essence of H.264, but in order to bypass the trouble of patents, it has to give up some core algorithms of H.264. In exchange for the cost, the coding efficiency is greatly reduced, and the complexity is greatly reduced.
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