HEVC Definitions and high-level syntax

Copyright © 2013 LOGTEL
Lecture 2 - High Level Syntax

Ref
HEVC Encoder
Motion
Est.
Motion
Comp.
+
Intra
Pred.
Intra
Inter
-
Residual
T & Q CABAC
Bit-Stream
Q-1
& T-1
+
+
MVs
Intra
Est.
Mode
SAO Params
Est.
Deblk.
&SAO
Filter Control
Reconstructed
RefRefRef.
DPB
RefRefRef
Input Video
SAO params
Quantized
residuals
Reference samples Intra/Inter
Decision
MVs/Intra modes

Bitstream Structure
VPSVPS SPSSPS PPSPPS
Slice
Header
Slice
Header
Slice DataSlice Data
Picture #1
* * * *
Slice
Header
Slice
Header
Slice DataSlice Data
Picture #k
Slice
Header
Slice
Header
Slice DataSlice Data * * * *
Notes
•VPS/SPS/PPS can be either a part of bitstream or can be transmitted
by other means (e.g. out-of-band).
•SPS/PPS are accessed through referencing to facilitate with applications
where SPS/PPS are signaled out-of-band
•VPS/SPS/PPS/Slice are encapsulated in NAL unit with 16-bits header

High-Level Syntax (VPS/SPS(
 VPS – dedicated to convey information that is common for multiple
layers, i.e. each layer refers same VPS
 SPS – contain information which applies to all slices of a video sequence
and is fixed within this sequence:
 Profile, level, picture size, number sub-layers
 Enabling flags
 Restrictions
 Temporal scalability control
 Visual usability information (VUI)
Note: there is duplication of some information between SPS and VPS

High-Level Syntax (PPS, Slice Header(
 PPS – conveys information which could change from picture to picture
 Reference list size
 Initial QP
 Enabling flags
 Tiles/Wavefronts
 Slice Header - conveys information that can change from slice to slice
 POC, Slice type
 Reference picture lists
 Prediction weights
 Tiles Entry points

Selected Picture Types (IDR, CRA)
 IDR - pictures following the IDR in decoding order cannot use
pictures decoded prior to the IDR as reference:
 CRA – pictures following the CRA in both decoding and presentation
order cannot use pictures decoded prior the CRA as reference:
11 2 3 4
0 2 3 11 4
0
CRA
CRA
Decoding order:
Presentation order:
Leading pictures

11 2 3 4
0 2 3 11 4
0
CRA
CRARADLRASL
Selected Picture Types: (RADL, RASL)
Leading pictures - following in decoding order but preceding in
presentation order. Leading pictures are divided into two types:
• RADL (random access decodable leading) – can be correctly
decoded if decoding starts with the current CRA
• RASL (random access skipped leading) - can’t be correctly
decoding if decoding starts with the current CRA and therefore
this picture should be skipped.
Decoding order:
Presentation order:
Leading pictures

Picture/Slice Syntax
Coding Tree Block (CTB):
Picture/Slice/Tile is partitioned into square coding tree blocks (CTBs). The
size of CTB is signaled in SPS (16x16, 32x32, 64x64).
Luma CTB covers a square picture area of N ×N samples and the
corresponding chroma CTBs cover each (N/2) × (N/2) samples (in 4:2:0
format).
Coding Block (CB):
Each CTB can be further partitioned into multiple coding blocks (CBs).
The size of the CB can range from the same size as the CTB to the
minimum size (8×8).
The minimum CU size (maximal depth of coding block tree) is signaled in
SPS

Copyright © 2013 LOGTEL 9
Picture/Slice Syntax (cont.)

CTU/CU Syntax
Coding Tree Units (CTU):
The luma CTB and the two chroma CTBs, together with the associated
syntax, form coding tree unit (CTU). The CTU is the basic processing
unit similar to MB in prior standards.
Coding Unit (CU):
The luma CB and the chroma CBs, together with the associated syntax,
form a coding unit (CU). Each CU can be either Intra or Inter predicted.
Actually CU is the basic unit for compression.

CTU/CU Syntax (2(
64x64 CTU
smallest CU (SCU(

CTU/CU Syntax (3) - traversing
All CUs in a CTU are encoded (traversed( in Z–Scan order:
64x64 CTU
The above figure was granted by Benjamin Bross
“Relax it's only HEVC”, WBU-ISOG Forum, European Broadcast Union, Geneva, Switzerland, November
28, 2012

CTU/CU Syntax (4(
Formally CTU specifies quad-tree traversed in ‘inorder’.
Note: unlike to prior standards where MB header is followed by data, in HEVC ‘headers are
dispersed’:
CTU
Header
CTU
Header
CU DataCU Data
CTU
CU HdrCU Hdr * * * * CU DataCU DataCU HdrCU Hdr

CTU/CU Syntax (5(
Prediction Block (PB):
Each CB is partitioned in 1, 2 or 4 prediction blocks (PBs).
Prediction Unit (PU):
The luma PB and the chroma PBs, together with the associated syntax, form a
prediction unit (PU).
2Nx2N NxN only if CB is smallest CB
2Nx2N NxN 2NxN Nx2N
INTRA
INTER

CTU/CU Syntax (6(
Inter Assymetric Partitions: conditioned by amp_enabled_flag in SPS
nLx2N nRx2N 2NxnU 2NxnD
2NxnU 2NxnD nRx2NnLx2N
When assymetric partitions are beneficial:

CTU/CU Syntax (7(
Transform Block (TB( :
Each luma CB can be quadtree partitioned into one, four or larger number
of TBs. The number of transform levels is controlled by
max_transform_hierarchy_depth_inter and
max_transform_hierarchy_depth_intra.
Example:
CB divided into two TB levels (the block #1 is split into four blocks):
00
22 33
1,21,2 1,31,3
1,01,0 1,11,1
0 2 3
1,0 1,1 1,2 1,3

CTU/CU Syntax (8(
CTU Header: CU partitioning, sample adaptive offset (SAO) parameters.
CU Header: PU and TU partitioning
CTU
Header
CTU
Header
CU DataCU Data
CTU
CU HdrCU Hdr * * * * CU DataCU DataCU HdrCU Hdr
Notes:
The smallest luma PB size is 4 × 8 or 8 × 4 samples (4x8 and 8x4 are
permitted only for uni-directional predictions).
Chroma PBs mimic corresponding luma partition with the scaling factor 1/2
(for 4:2:0) until 8x8 sizes.

HEVC Definitions and high-level syntax

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