HEVC / H.265 Explained
What is HEVC / H.265?
High Efficiency Video Coding (HEVC), also known as H.265, is a new video compression standard, developed by the Joint Collaborative Team on Video Coding (JCT-VC). The JCT-VC brings together image and video encoding experts from around the world, producing a single standard that is approved by two standards bodies;
- ITU-T Study Group 16 – Video Coding Experts Group (VCEG) – publishes the H.265 standard as ITU-T H.265, and
- ISO/IEC JTC 1/SC 29/WG 11 Motion Picture Experts Group (MPEG) – publishes the HEVC standard as ISO/IEC 23008-2.
The initial version of the H.265/HEVC standard was ratified in January, 2013.
HEVC was developed with the goal of providing twice the compression efficiency of the previous standard, H.264 / AVC. Although compression efficiency results vary depending on the type of content and the encoder settings, at typical consumer video distribution bit rates HEVC is typically able to compress video twice as efficiently as AVC. End-users can take advantage of improved compression efficiency in one of two ways (or some combination of both);
- 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 better visual 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