Optimizing Macroblock Tree Algorithm for Real-Time Video Encoding in HW

Macroblock-Tree [1] (also referred as MB-Tree) is a popular and well-established algorithm used to optimally distribute bits across macroblocks within a frame to achieve high compression in video coding. MB-tree’s implementation in x264 [2] encoder brings 10-12% BD-rate PSNR improvement. In x265 [4], a different version of same algorithm is implemented, called CU-Tree. MB-Tree is based on philosophy of assigning more bits (higher quality) to blocks which are most referred in inter frame estimation. This algorithm involves highly computational blocks like motion estimation, intra estimation and cost-propagation. Since this algorithm works in a look-ahead manner (generally as part of rate-control module) to the encoder, it poses a big challenge for real-time implementation like broadcasting and online video transmissions. To reap its benefits in real-time encoding we propose steps to reduce its complexity up to half, without compromising on objective video quality. The results are provided with high performance real-time 4Kp60 H.264 HW encoder inside AMD-Xilinx’sOR Zynq UltraScale+ MPSoC [6].