Design of the intra subpartition mode in VVC and its optimized encoder search in VTM

The Intra Subpartition (ISP) mode is one of the intra prediction tools incorporated to the new Versatile Video Coding (VVC) standard. ISP divides a luma intra-predicted block along one dimension into 2 or 4 smaller blocks, called subpartitions, that are predicted using the same intra mode. This paper describes the design of this tool and its encoder search implementation in the VVC Test Model 7.3 (VTM-7.3) software. The main challenge of the ISP encoder search is the fact that the mode pre-selection based on the sum of absolute transformed differences typically utilized for intra prediction tools is not feasible in the ISP case, given that it would require knowing beforehand the values of the reconstructed samples of the subpartitions. For this reason, VTM employs a different strategy aimed to overcome this issue. The experimental tool-off tests carried out for the All Intra configuration show a gain of 0.52% for the 22-37 Quantization Parameter (QP) range with an assoc iated encoder runtime of 85%. The results are improved to a 1.06% gain and an 87% encoder runtime in the case of the 32-47 QP range. Analogously, for the tool-on case the results for the 22-37 QP range are a 1.17% gain and a 134% encoder runtime and this improves in the 32-47 QP range to a 1.56% gain and a 126% encoder runtime.