目的 屏幕图像编码要求压缩效率高、实时性好、复杂性适中。调色板编码是屏幕图像编码的主流方法之一，但是其索引图的预测编码效率仍有待提高。为此提出一种基于局部方向相关性的索引图快速预测编码算法。方法 实验统计发现两个相邻的索引值在0.93的平均概率下具有相同的方向性，本文称之为索引图的“局部方向相关性”，进而提出采用一个2×3的模板进行初始方向预测，在其预测失败的情况下再采用一个3×4的方向模板进行二次方向预测。结果 在19个标准测试视频序列和3个测试图像上进行了大量实验，实验结果表明，本文算法的预测准确率为95.43%，比典型的多级预测算法（MSP）平均提高2.48%，尤其对于包含文字字符和大量几何图元的复杂场景视频更为有效，并且计算复杂度显著低于MSP算法，能够较好地满足屏幕图像编码的要求。结论 本文提出的索引图的预测算法，发掘了索引图的局部方向相关性，加快了预测速度，可应用在屏幕图像文本/图形块的调色板编码中。

Objective Screen image coding requires high coding efficiency, real-time performance, and moderate computational complexity. Palette coding is a state-of-art screen content coding method, but its predictive coding efficiency of index maps needs improvement. This study proposes a fast prediction algorithm of index maps based on the local directional correlation. Method Experiments show that two neighboring indexes have the same directionality with a probability of 0.93. We define this "the local directional correlation" of an index map. We then use a 2×3 template to perform an initial direction prediction. If the initial prediction fails, we use a 3×4 template to perform the second-round direction prediction. Result We conducted extensive experiments on 19 standard test video sequences and 3 test images. Experimental results showed that the prediction accuracy of our algorithm reached 95.43%, which was a 2.48% average increase over typical multi-stage prediction algorithms, and was particularly suitable for videos with text characters, complex scenes, and multiple geometric elements. Moreover, the computational complexity was significantly lower than that of MSP. Thus, this algorithm satisfied the requirements of screen image coding. Conclusion This study presented a prediction algorithm of index maps, which exploited the local directional correlation of index maps and accelerated the prediction speed. The proposed algorithm was found to be applicable to the palette-based coding of text/graphics blocks in screen images.