Complexity Reduction Algorithm
for Intra Mode Selection in
H.264/AVC Video Coding
BY
AMRUTA KULKARNI
STUDENT ID : 1000666836
UNDER SUPERVISION OF
DR. K.R. RAO
H.264/MPEG-4 Part 10 or AVC
 Is an advanced video compression standard,
developed by ITU-T Video Coding Experts Group
together with ISO/IEC Moving Picture Experts
Group.
 It is widely used Video Codec in mobile applications,
internet( YouTube, flash players), set top box, TV
etc.
 A H.264 encoder converts the video into a
compressed format(.264) and a decoder converts
compressed video back into an uncompressed
format.
How does H.264 codec work ?
 An H.264 video encoder carries out prediction,
transform and encoding processes to produce a
compressed H.264 bit stream.
 A decoder carries out a complementary process by
decoding, inverse transform and reconstruction to
output a decoded video sequence.
H.264 encoder block diagram[7]
Profiles in H.264[9]
Prediction
 The H.264 encoder forms a prediction of the current
macro block –
a) Based on the current frame using intra
prediction/spatial prediction.
b) Based on the previous frames that have already
been coded using inter prediction.
Intra prediction
 Supports the following macro block sizes :
a) 16x16 (for Luma) –
 Mode 0 (vertical): extrapolation from upper samples (H).
 Mode 1 (horizontal): extrapolation from left samples (V).
 Mode 2 (DC): mean of upper and left-hand samples (H+V).
 Mode 3 (Plane): a linear “plane” function is fitted to the upper and left-
hand side samples H and V. This works well in areas of smoothlyvarying luminance.
8x8 (for Chroma) –
 DC , Plane , Horizontal Vertical
b)
Intra prediction
c) 4x4 (for Luma) modes for intra prediction-[2]
Introduction
 The H.264/AVC intra-prediction is conducted for all
types of blocks such as 4x4 luma blocks, 16x16 luma
blocks, and 8x8 chroma blocks.
 The residual between the current block and its
prediction is then transformed, quantized, and
entropy coded.
 To obtain the best mode among these modes, the
H.264/AVC encoder performs the rate-distortion
optimization (RDO) technique for each macro block.
RDO procedure for one macro block
 Set macro block parameters : QP(quantization parameter) and





Lagrangian multiplier (λ)
Calculate : λMODE = 0.85⋅2(QP-12)/3
Then calculate cost, which determines the best mode Cost = D
+ λMODE ⋅ R,
D – Distortion
R - bit rate with given QP
Distortion (D) is obtained by SSD (Sum of Squared
Differences) between the original macro block and its
reconstructed block.
Bit rate(R) includes the bits for the mode information and
transform coefficients for macro block
Quantization parameter (QP) can vary from (0-51)
RDO (Rate distortion optimization)
 Considering the RDO procedure for intra mode selection in
H.264/AVC, the number of mode combinations in one macro
block is N8x(16xN4 + N16)
 N8 – number of modes of an 8x8 chroma block – 4 modes
 N4 – number of modes of an 4x4 luma block - 9 modes
 N16 – number of modes of an 16x16 luma block – 4 modes
N8x(16xN4 + N16) = 4x(16x9 + 4)
= 592
 Thus, to select the best mode for one Macro block in the intra
prediction, the H.264/AVC encoder carries out 592 RDO
calculations. As a result, the complexity of the encoder increases
extremely.
Introduction
 This project uses the baseline profile, as it provides
simplicity in implementation.
 The important features are –
a) I and P slice coding
b) Enhanced error resilience such as FMO (Flexible macro
block ordering) and Arbitrary slice ordering(ASO) and
redundant slices (RS)
c) Context adaptive variable length coding (CAVLC)
 Baseline profile is primarily used for low-cost
applications , for data loss robustness.
 The joint model (JM 17.2) implementation of the H.264
encoder is used in this project.
Complexity reduction algorithm for intra
prediction
 This project will implement the complexity reduction
algorithm for all the 3 block sizes
1) 16x16 luma
2) 4x4 luma
3) 8x8 chroma
Algorithm
 The proposed intra mode selection algorithm for a 16x16
luma block is summarized as follows:
 Step 1 - Examine sizes of adjacent blocks: if both blocks (upper block
and left block) are 16x16, go to Step 2, otherwise go to Step 4.
 Step 2 - Examine modes of adjacent blocks: if both modes are same, go
to Step 3, otherwise select the best mode for a 16x16 luma block, which
results in the minimum SATD (sum of absolute transformed
differences) between two adjacent modes of modeA and modeB.
 Step 3 - If both adjacent modes are DC mode, go to Step 4, otherwise
select the best mode for a 16x16 luma block, which results in the
minimum SATD between the adjacent mode and DC mode.
Algorithm
 Step 4 - Let ΔV be a vertical difference between upper boundary pixels
of the current block and boundary pixels of the upper block, and ΔH be
a horizontal difference between left boundary pixels of the current
block and boundary pixels of the left block as follows.
 ΔV = Σ |u(i)-q(i)| for i =0 to 15.
 ΔH = Σ |l(i)-r(i)| for i =0 to 15.
Where u(i) -> upper block boundary pixels
q(i) -> upper boundary pixels of current block
l(i) -> boundary pixels of the left block
r(i) -> left boundary pixels of the current block
T2 -> Threshold level 2 (T2 = 32)
Algorithm
 Obtain candidate modes as follows by using two difference values, ΔV
and ΔH: if |ΔV − ΔH | is smaller than 2T2, candidate modes are DC
mode and plane mode; if (ΔV − ΔH) is larger than T2, candidate modes
are DC mode and horizontal mode; if (ΔV − ΔH) is smaller than −T2,
candidate modes are DC and vertical mode, where T2 is a positive
value. Finally, select the best mode between each candidate mode by
choosing the mode with minimum SATD.
Calculation of ΔV & ΔH [8]
References:
1.
2.
3.
4.
5.
ITU-T Rec. H.264 | ISO/IEC 14496-10: Information Technology –
Coding of Audio-visual Objects, Part 10: Advanced Video Coding
2002.
T.Wiegand, G.Sullivan, G.Bjontegaard et al,: “Overview of the
H.264/AVC Video Coding Standard.” IEEE Trans. Circuits and Syst.
for Video Technol., Vol. 13. pp 560-576, July 2003 .
Z.Chen, Z. Zhou,Y.He, et al: “Fast Integer Pixel and Fractional Pixel
Motion Estimation for JVT.” Doc. #JVT-F017,Dec 2002.
B.Hsieh, B.Huang, et al,:“Fast Motion Estimation for H.264/MPEG4 AVC by Using Multiple Reference Frame Skipping Criteria.” VCIP
2003,Proceedings of SPIE, Vol. 5150, pp 1551-1560, Oct 2003.
A.Puri et al. “Video coding using the H.264/MPEG-4 AVC
compression standard”, Signal Processing: Image Communication,
vol.19, pp 793-849, Oct 2004.
References
6.
7.
8.
9.
H.264/AVC JM software: http://iphome.hhi.de/suehring/tml/
An overview of the H.264 encoder: www.vcodex.com
J. Kim, D. Kim, et al, “Complexity reduction algorithm for intra mode
selection in H.264/AVC video coding” J. Blanc-Talon et al. (Eds.):
ACIVS 2006, LNCS 4179, pp. 454 – 465, 2006.Springer-Verlag Berlin
Heidelberg 2006.
I. Richardson, “The H.264 advanced video compression standard” –
second edition. Wiley 2010