高效视频编码中变换跳过模式的快速选择

Fast transform skip mode decision for high efficiency video coding

目的新一代高效视频编码(HEVC)标准采用了灵活的块结构和大量新颖的编码工具,有效提高了视频编码效率。变换跳过模式作为一种新增加的模式,可以有效地提高编码效率,但是也显著地增加了编码的复杂度,增加了实时编码应用的难度。因此提出一种针对变换跳过模式的提前剪枝算法。方法通过分析不同率失真代价下是否选择变换跳过模式的残差块的分布情况,最终选取率失真代价的平方根作为阈值,并建立了量化参数与阈值之间的模型。之后可以根据量化参数提前计算得到阈值大小,减少变换跳过模式的编码次数,从而降低变换跳过模式的复杂度。结果由于最终只需要对少量的块进行变换跳过模式编码,并且使用模型得到经验阈值并不会额外增加复杂度,因此能减少编码器的计算复杂度。实验结果表明,与标准编码器相比,该算法对于不同场景的标准测试序列平均峰值性噪比和平均比特率变化都非常小,平均减少了70%的变换跳过模式编码的次数。结论该变换跳过模式的剪枝算法,选取率失真代价的平方根作为阈值,根据本文模型获取不同量化参数下的经验阈值,对是否需要进行变换跳过模式提前判断。实验结果表明,该算法能在保证视频编码质量的前提下有效地降低由于加入变换跳过模式增加的编码复杂度。

Objective The latest video coding standard high efficiency video coding （HEVC） adopts a more flexible struc- ture and new coding tools compared with other coding standards. The adoption of new technologies results in entirely unre- lated prediction residual blocks. An inferior energy concentration can also be obtained by using several methods, such as DCT transform in frequency domain, in less-correlative pixel residual blocks. Among these new coding tools, transform skip mode can enhance the coding efficiency effectively while increasing the coding complexity considerably. Therefore, the difficulty of practical real-time coding is increased. To address the problem, a fast algorithm that reduces the transform skip mode in advance is proposed. Method The square root of the rate distortion cost is selected as the threshold value by analyzing whether the distribution of the residual block has selected the transform skip modeas the best mode at different rates of distortion costs. To save bits, the HEVC standard defines whether the coefficients of transform units are all zero after transform skip and quantization. The DCT/DST transform is directly chosen as the best mode. A larger quantization param- eter can mean a larger percentage of all zero blocks after transform skip mode. Therefore, an exponential model of the rate distortion cost and quantization parameter is established. In the actual coding process, thresholds can be calculated in ad- vance based on the quantization parameters. These thresholds are used to reduce the number of transform units that need to check the transform skip mode. Therefore, the complexity of the transform skip mode is reduced. Result Only a small num- ber of blocks need to check the transform skip mode, and the thresholds are obtained by offline training. Thus, no addi- tional complexity is added, and the computational complexity of the encoder is reduced. Experimental results show that compared with the standard encoder, the fast algorithm has minimal effect on PSNR and bit rate for standard test sequences that include different scenes. On average, about 70% of the transform units do not need to check the transform skip mode. The exponential model established in this paper fits very well, with a square of determination coefficient larger than 0. 95. Conclusion The algorithm selects the square root of the rate distortion cost as the threshold to reduce the transform skip mode and obtains the threshold by using the exponential model and quantization parameter. The transform skip mode can be reduced in advance. Experimental results indicate that the fast algorithm can reduce the coding complexity effective- ly because of the addition of transform skip mode with negligible performance loss. The proposed algorithm can be applied to real-time situations. It can reduce the time of transform skip mode significantly and can be further optimized. The relationship between the coding efficiency and coding time can be balanced dynamically by establishing the model of performance loss and pruning percentage in further research.