Improved transform skip mode for HEVC screen content coding

Transform skip modes (TSM) have previously been proposed for HEVC, where a transform on the prediction residual is skipped in one or both directions. TSM can improve coding efficiency when the prediction errors have minimal correlation in one or both directions. In this paper, improvements are proposed to the 1D transform skip mode design. Specifically, a unified dynamic range control method is proposed for the horizontal-only and vertical-only 1D TSM. It is also proposed to modify the scan order of the transform coefficients and to apply coefficient flipping to some 1D TSM cases. Finally, a number of efficient encoder complexity reduction methods are considered for the proposed 1D TSM. Experimental results show that the proposed method can achieve up to 4.7% gain in coding efficiency with limited encoder complexity increase. The proposed method also significantly improves visual quality, especially in areas rich in text and graphics.