Efficient arbitrary ratio downscale transcoding for HEVC

The arbitrary ratio transcoding usually introduces coding block grid misalignment, which results in difficulties to utilize the decoding information in coding blocks of source videos during the encoding phase. To reduce the large computation load of HEVC downscale transcoding in such situation, we propose an efficient transcoding method that we refer the decoded coding unit (CU) partitioning to accelerate partition decision, which takes up the most complexity in the encoding phase as well as the whole transcoding. First, we predict CU depth in pixel level according to decoded partitioning of source videos. Then, we propose adaptive rules to determine CU partitioning of target videos based on the prediction, so that we can make early CU splitting or pruning decision without complex recursive search. Experiments demonstrate that the proposed method achieves about 74% time reduction on average with acceptable BD-rate increase in the encoding phase compared to the encoder in reference software HM13.0.