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Understanding in-loop filtering in the HEVC video standard Mihir Mody - June 21, 2013 1.0 Introduction High Efficiency Video Coding (HEVC) is a video compression standard, a successor to H.264/MPEG- 4 AVC (Advanced Video Coding), jointly developed by the ISO/IEC Moving Picture Experts Group (MPEG) and ITU-T Video Coding Experts Group (VCEG) as ISO/IEC 23008-2 MPEG-H Part 2 and ITU-T H.265. HEVC promises half bit-rate compared to current de-facto standard H.264 at a similar video quality and is expected to be deployed in a wide variety of video applications ranging from cell phones, broadcast, set-top box, video conferencing, video surveillance, automotive, etc. The figure below shows the block diagram of the HEVC Video decoder with loop filtering, as shown with loop filtering highlighted. As shown, it is a cascading of two stages - namely de-blocking filtering (DBLK) and Sample adaptive offset (SAO) filtering to remove blocking artifacts causing during video encoding. The next two sections describe these two stages in detail. Figure 1: HEVC Video Decoder Block diagram 2.0 DB-Blocking Filtering (DBLK) This section explains de-blocking filtering as described in the HEVC Video standard [1]. De-block filter operation can broadly be divided into two parts: 1. Boundary Strength (aka BS) Computation on filter edge 2. Actual Filter operation Throughout the de-block filter operation the following convention has been used in the literature: Q pixels belong to the right side and P pixels belong to the left side of the vertical filter edge. Similarly, Q belongs to pixels below the horizontal edge and P above it for the horizontal filter edge. Figure 2: Neighbor pixels naming convention in in-loop de-blocking filtering 2.1 Filter Edge Strength (aka BS) BS [hor/ver][xpos][ypos] is computed on an 8x8 grid and takes values 0, 1 2 as shown in the figure below: Figure 3: Boundary Strength (BS) diagram showing edges on 8x8 grid and its numbering Although