H.265, also known as HEVC (High Efficiency Video Coding) is a new video compression standard, developed by the Joint Collaborative Team on Video Coding (JCT-VC). The JCT-VC brings together video coding experts from around the world, producing a single standard that is approved by two standards bodies; the ITU-T Study Group 16 (VCEG) and the ISO/IEC JTC 1/SC 29/WG 11 (Motion Picture Experts Group, or MPEG).
HEVC was developed with the goal of providing twice the compression efficiency of the previous standard, H.264 / AVC. What does twice the efficiency mean?
- At an identical level of visual quality, HEVC enables video to be compressed to a file that is about half the size (or half the bit rate) of AVC.
- Or, when compressed to the same file size or bit rate as AVC, HEVC delivers significantly higher quality.
How can HEVC encode video files twice as efficiently as previous video coding standards?
- Most of the power of video compression standards comes from a technique known as motion compensated prediction. Blocks of pixels are encoded by making reference to another area in the same frame (intra-prediction), or in another frame (inter-prediction). Where H.264/AVC defines macroblocks up to 16×16 pixels, HEVC can describe a much larger range of block sizes, up to 64 x 64 pixels.
- HEVC allows predicted blocks to be coded in different block sizes than the residual error. Each top level coding unit (or CTU) is first coded as a prediction quad-tree, where at each depth the encoder decides whether to encode with merge/skip, inter, or intra coding. The residual from those predictions is then coded with a second quad-tree which can optionally have greater depth than the prediction quad-tree. For instance, this allows the residual error from a 32×32 inter coded coding unit (CU) to be represented by a mixture of 16×16, 8×8, and 4×4 transforms.
- HEVC can encode motion vectors with much greater precision, giving a better predicted block with less residual error. There are 35 intra-picture directions, compared with only 9 for H.264/AVC.
- HEVC includes Adaptive Motion Vector Prediction, a new method to improve inter-prediction.
- An improved deblocking filter
- Sample Adaptive Offset – an additional filter that reduces artifacts at block edges