Hash based intra string copy for HEVC based screen content coding

Block matching used in traditional hybrid coding, like intra block copy (IBC), is not always efficient to code similar or identical pattern within a picture, since they use 2-D pattern of only a few fixed sizes. To improve the coding efficiency of such repeated pattern with a flexible size, an intra string copy method is proposed in this paper. The proposed string copy is flexible in the sense that it allows string matching of variable sizes and shapes. For the matched strings, the relative distance from reference sample position to current sample position is coded as well as the matching length. To enable fast encoder search, hash function is employed to quickly identify the reference pixels in the previously coded areas by hash lookup table. Moreover, the string matching is applied not only the previously coded CUs but also the current CU. The simulation results report that the proposed method achieves 12.3% and 11% BD-rate savings for 1080P RGB and YUV respectively in full frame IBC condition.