Index Map Prediction by 2-Neighbor Joint Transition Probability in Palette Coding
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摘要: 采用4-邻域模板匹配对索引图进行非局部预测是调色板编码的一种典型技术.通过实验分析发现，1个4-邻域模板存在数量众多的干扰模板，并且不能有效捕获边缘反走样区域的颜色转移特征，从而提出一种包含4个子模板的2-邻域匹配模板结构来刻画前景物体或文字边缘在其左上角、左下角、右上角和右下角的特定颜色转移模式；同时，将模板预测建模为一种可通过查表操作实现的转移概率；进而，提出一种2-邻域联合转移概率的索引图预测方法.实验表明：该算法的预测准确率为97.70%，比多级预测算法(MSP)和局部方向预测算法(LDP)分别平均提高了4.50%和2.27%，尤其适用于包含大量字符和由计算机生成的几何图元的复杂屏幕内容编码.并且，计算复杂度与LDP相当，明显低于MSP，可应用实时性需求较高的、基于调色板-索引图的屏幕内容预测编码中.Abstract: Using a 4-neighbor template to perform the nonlocal prediction on the index map is one of typical palette coding techniques. By analyzing the experimental results, it is found that one 4-neighbor template usually has a large number of interference templates and cannot effectively capture the color transition features in the edges’ anti-aliasing area. Therefore, a 2-neighbor template is proposed which includes four subtemplates to represent particular color transition modes of the foreground objects and the text edges at their upper left corners, lower left corners, upper right corners, as well as lower right corners. Meanwhile, the template prediction is further modeled into a transition probability that can be implemented by table lookup operations. An index map prediction method is further addressed using the 2-neighbor joint transition probability. Experimental results show that the prediction accuracy of the proposed method is 97.70%, which is separately 4.50% and 2.27% higher than that of the multi-stage prediction (MSP) method and that of the local directional prediction (LDP) method. It is especially suitable for the complex screen content coding with a large number of characters and computer-generated geometrical primitives. Moreover, the computational complexity of the proposed method is equivalent to that of the LDP method, and obviously lower than that of the MSP. The proposed method can be applied into the palette-index map based screen content predictive coding with high real-time demand.
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