Low-complexity block size decision for HEVC intra coding using binary image feature descriptors

We present a novel low-complexity algorithm for feature-based block size decision in HEVC intra coding. Our approach evaluates a set of point pairs within a coding unit (CU) in order to determine whether a CU should be further split into smaller sub CUs for coding. We apply a modified version of the Binary Robust Independent Elementary Feature (BRIEF) descriptor, which in its original version is usually applied to describe local image properties in the context of image/video analysis. While the elements of the original BRIEF descriptor describe which pixel of a pixel pair has the higher value, the modified descriptor evaluates whether the difference between the pixel values is above a pre-defined threshold, which is determined by training. If a certain number of pixel pair differences exceed their corresponding threshold the CU is split into its four sub CUs. Furthermore, we restrict the feature point pairs to be located within different potential sub CUs. For an adaptive training approach, we achieved in our experiments an average encoding time reduction of 58% compared to the HEVC reference software HM12.0 with an average rate increase of 3.8% at equal quality and an average encoding time reduction of 65% with an average rate increase of 5.98% at equal quality for an offline training approach.