erlemental_elemental 30/08/2013 09:12 Page 1
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he most recent nonproprietary video compression standard, High-Efficiency Video Codec (HEVC), also known as H.265, is expected to become the video standard of choice for the next decade. As with each generation of video compression technology before it, HEVC promises to reduce the overall cost of delivering and storing video assets while maintaining or increasing the quality of experience for the viewer. Comparison of AVC/H.264 (left) and HEVC/H.265 (right) compressed image
HEVC: Architected for the most efficient video compression techniques
Without sacrificing video quality, HEVC can reduce the size of a video file or bit stream by as much as 50% compared to AVC/H.264 or as much as 75% compared to MPEG-2 standards. This results in reduced video storage and transmission costs and also paves the way for higher definition content to be delivered for high-quality-of-experience consumer consumption. Designed to evolve the video compression industry, HEVC intends to: l Deliver an average bit rate reduction of 50% for a fixed video quality compared to H.264 l Deliver higher quality at same bit rate l Define a standard syntax to simplify implementation and maximise interoperability l Remain network friendly—i.e. wrapped in MPEG Transport Streams While H.264 featured seven profiles (defined sets of coding tools used to create a
4:2:2 and 4:4:4 chroma sampling, Multiview Video Coding (MVC) and Scalable Video Coding (SVC). HEVC’s Main profile supports a bit depth of 8 bits per color, while Main 10 supports 8 bits or 10 bits per color. Because of the additional bit depth option, Main 10 has the potential to provide better video quality than Main. Finally, Main Still Picture profile allows for a single still picture to be encoded with the same constraints as Main profile. The HEVC spec also defines 13 levels, which are sets of constraints that indicate the required decoder performance to playback a bit stream of the specified profile. The levels are split into two tiers: Main, which includes levels 1 - 3.1, and High, which includes levels 4 - 6.2 and is designed for highly demanding applications. HEVC levels share a number of similarities with the levels of H.264, with a key difference being the addition of levels 6, 6.1 and 6.2, which define requirements to support 8K resolution video.
Intra Prediction and Coding
In the H.264 standard, nine modes of prediction exist in a 4 x 4 block for intra prediction within a given frame and nine modes of prediction exist at the 8 x 8 level. It’s even fewer at the 16 x 16 block level, dropping down to only four modes of prediction. Intra prediction attempts to estimate the state of adjacent blocks in a direction that minimizes the error of the estimate. In HEVC, a similar technique exists, but the number of possible modes is 35 - in line with the additional complexity of the codec. This creates a dramatically higher number of decision points involved in the analysis, as there are nearly two times the number of spatial intra-prediction sizes in HEVC as compared to H.264 and nearly four times the number of spatial intra-prediction directions.
Inter Prediction and Coding
H.264 uses block-based motion compensation with adjustable block size and shape to look for temporal redundancy across frames in a video. Motion compensation is often noted as the most demanding portion of the encoding process. The degree to which it can be implemented intelligently within the decision space has a major impact on the efficiency of the codec. HEVC takes this to a new level. HEVC replaces the H.264 macroblock structure with a more efficient, but also complex, set of treeblocks. Each treeblock can be larger (up to 64x64) than the standard 16x16 macroblock, and can be efficiently partitioned using a quadtree. This system affords the encoder a large amount of flexibility to use large partitions when they predict well and small partitions when more detailed predictions are needed. This leads to higher coding efficiency, since large prediction units (up to and including the size of the treeblock) can be cheaply coded when they fit the content. By the same token, when some parts of the treeblock need more detailed predictions, these can also be efficiently described.
How HEVC is Different
The primary goal of the new HEVC standard is to provide the tools necessary to transmit the smallest amount of information necessary for a given level of video quality. While there are a Expected compression bit rates at time of standardisation number of differences compliant bit stream), the HEVC spec between H.264 and HEVC, two stand out: currently supports three: Main, Main 10, and increased modes for intra prediction and Main Still Picture. Future profile extensions for refined partitioning for inter prediction. HEVC will likely include increased bit depth,
Keith Wymbs, VP marketing, Elemental, gives expert advice on the technical and market implications of HEVC’s adoption in the content creation and delivery market.
HEVC Demystified
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Application Impact
There are several cases where the improved