Improved separable transform skip condition for non-regular intra-prediction for screen contents

In this paper, we investigate the separable transform skip condition in the current ECM which selectively permits the identity matrix to be used as a separable transform kernel. Using the identity matrix for transform kernel is equivalent to skipping transform for the selected direction of horizontal or vertical. By noting its significantly lower utilization under the blending intra prediction (BIP) mode (i.e. DIMD, OBIC, TIMD) in the current ECM compared to the regular intra prediction while the transform skip is effective in encoding screen contents, we investigate its separable transform skip (STS) condition for enhancing the usage of separable transform skip specifically for the BIP mode. In our experiment with screen content video sequences (classes F and TGM), our new modified STS condition achieves BDBR gain of -0.02%, 0.00%, and 0.01% in Y, Cb, and Cr components, respectively. For the natural content videos (classes B, C, D, E), it does not have negative impact since it shows BDBR gain of -0.00%, 0.03%, and -0.01%, respectively for Y, Cb, and Cr components.