A Fast Lossless Implementation of the Intra Subpartition Mode for VVC

Lossy compression is the main target of the upcoming video coding standard Versatile Video Coding (VVC). However, lossless coding is supported in VVC by utilizing a certain encoder configuration. Particularly, the Transform Skip Mode (TSM) is always selected at the block level to bypass the transform stage (together with a QP that results in the same output as input at the quantization stage). Consequently, the Intra Subpartition (ISP) coding mode cannot be used for lossless coding, considering that its combination with TSM is not supported in VVC because it does not provide a significant coding benefit for the lossy common test conditions. For this reason, it is proposed to enable such a combination for the benefit of lossless coding. Besides, the encoder search has been optimized to improve the trade-off between compression benefit and encoder run-time. Experimental results show a 0.71% coding gain with a corresponding encoder run-time of 111%.