Coding tree unit (CTU) is the basic processing unit of the

High Efficiency Video Coding High Efficiency Video Coding (HEVC), also known as H.265 and MPEG-H Part 2, is a video compression standard designed as part of the MPEG-H project as a successor to the widely used Advanced Video Coding (AVC, H.264, or MPEG-4 Part 10). In compar ...

(HEVC) video standard and conceptually corresponds in structure to

macroblock The macroblock is a processing unit in image and video compression formats based on linear block transforms, typically the discrete cosine transform (DCT). A macroblock typically consists of 16×16 samples, and is further subdivided into transform ...

units that were used in several previous video standards. CTU is also referred to as largest coding unit (LCU). A CTU can be between 16×16 pixels and 64×64 pixels in size with a larger size usually increasing coding efficiency. The first video standard that uses CTUs is HEVC/H.265 which became an

ITU-T The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...

standard on April 13, 2013.

History

Macroblock encoding methods have been used in digital video coding standards since

H.261 H.261 is an ITU-T video compression standard, first ratified in November 1988. It is the first member of the H.26x family of video coding standards in the domain of the ITU-T Study Group 16 Video Coding Experts Group (VCEG, then Specialists Gro ...

which was first released in 1988. However, for

error correction In information theory and coding theory with applications in computer science and telecommunication, error detection and correction (EDAC) or error control are techniques that enable reliable delivery of digital data over unreliable communica ...

and

signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...

the standard 16x16 macroblock size is not capable of getting the kind of bit reductions that

information theory Information theory is the scientific study of the quantification (science), quantification, computer data storage, storage, and telecommunication, communication of information. The field was originally established by the works of Harry Nyquist a ...

and coding theory suggest are theoretically and practically possible.

Technical details

HEVC replaces

s, which were used with previous video standards, with CTUs which can use larger block structures of up to 64×64 pixels and can better sub-partition the picture into variable sized structures. HEVC initially divides the picture into CTUs which are then divided for each luma/chroma component into coding tree blocks (CTBs). A CTB can be 64×64, 32×32, or 16×16 with a larger pixel block size usually increasing the coding efficiency. CTBs are then divided into one or more coding units (CUs), so that the CTU size is also the largest coding unit size. * The arrangement of CUs in a CTB is known as a

quadtree A quadtree is a tree data structure in which each internal node has exactly four children. Quadtrees are the two-dimensional analog of octrees and are most often used to partition a two-dimensional space by recursively subdividing it into four q ...

since a subdivision results in four smaller regions. * CUs are then divided into prediction units (PUs) of either intra-picture or inter-picture prediction type which can vary in size from 64×64 to 4×4. To limit worst-case memory bandwidth when applying motion compensation in the decoding process, prediction units coded using inter-picture prediction are restricted to a minimum size of 8×4 or 4×8 if they are predicted from a single reference (uniprediction) or 8×8 if they are predicted from two references (biprediction). * To code the prediction residual, a CU is divided into a quadtree of DCT transform units (TUs). TUs contain coefficients for spatial block transform and quantization. A TU can be 32×32, 16×16, 8×8, or 4×4 pixel block sizes.

Standardization

At the July 2012 HEVC meeting it was decided, based on proposal JCTVC-J0334, that HEVC level 5 and higher would be required to use CTB sizes of either 32×32 or 64×64. This was added to HEVC in the Draft International Standard as a level limit for the Log2MaxCtbSize variable. Log2MaxCtbSize was renamed CtbSizeY in the October 2012 HEVC draft and then renamed CtbLog2SizeY in the January 2013 HEVC draft.

Coding efficiency

The design of most video coding standards is primarily aimed at having the highest coding efficiency. Coding efficiency is the ability to encode video at the lowest possible bit rate while maintaining a certain level of video quality. HEVC benefits from the use of larger CTB sizes. This has been shown in PSNR tests with a HM-8.0 HEVC encoder where it was forced to use progressively smaller CTU sizes. For all test sequences when compared to a 64×64 CTU size it was shown that the HEVC bit rate increased by 2.2% when forced to use a 32×32 CTU size and increased by 11.0% when forced to use a 16×16 CTU size. In the Class A test sequences, where the resolution of the video was 2560×1600, when compared to a 64×64 CTU size it was shown that the HEVC bit rate increased by 5.7% when forced to use a 32×32 CTU size and increased by 28.2% when forced to use a 16×16 CTU size. The tests showed that large CTU sizes become even more important for coding efficiency with higher resolution video. The tests also showed that it took 60% longer to decode HEVC video encoded at 16×16 CTU size than at 64×64 CTU size. The tests showed that large CTU sizes increase coding efficiency while also reducing decoding time. The tests were conducted with the Main profile of HEVC based on equal peak signal-to-noise ratio (PSNR).

References

External links

H.265/HEVC standardHEVC overviewHEVC coding efficiencyITU-T Recommendation H.265 - High Efficiency Video Coding
{{Compression methods IEC standards ISO standards ITU-T recommendations Lossy compression algorithms MPEG Video compression