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. 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. 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. 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.