Evaluating the CU-tree algorithm in an HEVC encoder

Abstract: CU-tree (Coding Unit tree) is an algorithm for adaptive QP (quantization parameter). It runs in the lookahead and decreases the QP of blocks that are heavily referenced by future blocks, taking into account the quality of the prediction and the complexity of the future blocks, approximated by the inter and intra residual. In this study, CU-tree is implemented in c65, an experimental HEVC encoder used internally by Ericsson. The effects of CU-tree are evaluated on the video clips in the HEVC Common test conditions and the performance is compared across c65, x265 and x264. The results are similar across all encoders, with average PSNR (peak signal-to-noise ratio) improvements of 3-10% depending on the fixed QP offsets that are replaced. The runtime is not impaired and improvements to visual quality are expected to be even greater. The algorithm works better at slow speed modes, low bitrates and with source material that is well suited for inter prediction.