Know the HEVC Standard

What is HEVC?

HEVC (High Efficiency Video Coding) or also called H.265 is the next generation Video compression standard is currently being standardized and is said to be ratified in Jan 2013. HEVC promises up to 50% gains over H.264 with the same or better visual quality. However, gains come at the higher encoding and decoding complexity of the algorithms in the standard. With the encoding processes significantly higher in complexity than the decoding process.

Some salient features of the standard include:

Quad-tree structured coding of coding units of size up to 64x64
Prediction tree (motion compensation block sizes from 64x64 to 4x4)
Transform tree (transform block sizes from 64x64 to 4x4)
Improved spatial intra prediction
More angles of prediction and support for blocks from 32x32 to 4x4
Improved interpolation for motion compensation
MV competition and merge options to reduce MV coding overheads
Mode dependent scans
Improved in-loop deblocking filter
Ability to apply sample-adaptive offsets to reduce coding noise 
Improved Entropy Coding techniques 
Parallel Entropy Coding option

Coding trees and blocks in HEVC

Coding Tree Unit (CTU)

In HEVC, each frame is divided into Coding Tree Units (CTUs). Each CTU can be of size 64x64 or 32x32 or 16x16. The size of the coding tree unit is a parameter that is encoded in the sequence set. A CTU is logical unit i.e. a logical partition of a frame of video.

A logical unit consists of pixel components Y, Cb, Cr. For the sake of simplicity we'll consider each pixel to be in 4:2:0 color sample format so a 64x64 CTU will have one 64x64 Y samples , 32x32 Cb samples and 32x32 Cr samples. Each CTU is composed of 3 Coding Tree Blocks (CTBs). In other words, each CTU will have one Y CTB, one Cb CTB and one Cr CTB. A CTB is a real unit of data in memory as opposed to a CTU which is a logical unit.

Each CTB is further divided into Coding Blocks (CB). The division can be into blocks of size 64x64 or 32x32 or 16x16 or 8x8 in a quad tree format . Each CU will have one Y CB, one Cb CB and one Cr CB. Each CB cab further be divided into Prediction Blocks (PBs) and Transform Blocks (TBs).

Comparison of compression ratios across compression standards