FAST MODE DECISION ALGORITHM FOR INTRA
PREDICTION IN HEVC
INTERIM REPORT
LANKA NAGA VENKATA SAI SURYA TEJA
STUDENT ID: 1000916473
MAIL ID [email protected]
DATE : 04/24/2014
UNDER THE GUIDANCE OF
DR. K. R. RAO
EE 5359 MULTIMEDIA PROCESSING
UNIVERSITY OF TEXAS AT ARLINGTON
ACRONYMS:
BD- Bitrate - Bjøntegaard Delta Bitrate
BD- PSNR
CU
-
Bjøntegaard Delta Peak Signal-to-Noise Ratio
- Coding Unit
DCT - Discrete Cosine Transform
DST
- Discrete Sine Transform
HEVC - High Efficiency Video Coding
JCT- VC- Joint Collaborative Team on Video Coding
LCU - Largest Coding Unit
MPM - Most Probable Mode
PSNR - Peak Signal-to-Noise Ratio
PU
- Prediction Unit
QP
- Quantization Parameter
RDOQ - Rate Distortion Optimization Quantization
RDO - Rate- Distortion Optimization
RMD - Rough Mode Decision
SSIM
- Structural Similarity Index
TU
- Transform Unit
PROPOSAL: To improve the coding efficiency of intra frame coding, up to 34 intra prediction
modes are defined in High Efficiency Video Coding (HEVC) [1]. The best mode among these
pre-defined intra prediction modes is selected by rate-distortion optimization (RDO) for each
block. This project proposes a new method to reduce the candidates in RDO process, as it will be
time-consuming if all directions are tested in the RDO process. Also in this project, it provides
20% and 28% time savings in intra high efficiency and low complexity cases on average
compared to the default encoding scheme in HM 13.0 [5] with almost the same coding
efficiency. Also based on PSNR, BD- PSNR and BD- Bitrate analysis of fast mode decision
algorithm for intra prediction [] in HEVC can be done by comparing with the default encoding
scheme in HM 13.0 [5].
Index Terms: video coding, HEVC, intra prediction
INTRODUCTION: HEVC standard [2] provides a highly flexible hierarchy of unit
representation which consists of three block concepts: coding unit (CU), prediction unit (PU),
and transform unit (TU). This separation of the block structure is helpful for each unit of
optimization. CU is a macroblock-like unit of region splitting which is always square and its size
can be from 8x8 luma samples up to the largest coding units (LCUs). This concept allows
recursive splitting into four equal sized blocks, starting from LCU. This process gives a contentadaptive coding tree structure comprised of CU blocks. Figure 4 shows coding tree structure [3].
The PU is used only for the CU which is the leaf node in the Quadtree structure and the size of
two PUs are 2Nx2N and NxN. The third block concept transform unit size cannot exceed that of
the CU. Figure 1 shows the block diagram of H.264 encoder [20].
Figure 1: Block Diagram of H.264 Encoder [20]
Figure 2- Block diagram of HEVC encoder [15]
Figure 2 shows block diagram of HEVC encoder [15] in which each picture is partitioned into
blocks of different sizes and the same is conveyed to the decoder. In the given sequence intra
prediction is applied to the very first picture which uses spatial redundancy of the picture while
for rest of the frames temporal redundancy is exploited using inter prediction. Since encoder
needs to exhaust all the combinations of CU, PU and TU to find the optimal solutions, it is very
time-consuming. The encoder will not tolerate it if all the directions are employed in the ratedistortion optimization process. To reduce the computational complexity of the encoder, a fast
intra mode decision [4] was adopted in HM 13.0 [5]. Figure 3 shows block diagram of HEVC
decoder [15].
Figure 3- Block diagram of HEVC decoder [15]
(c)
(d)
Figure 4- Coding tree structure [3]
OVERVIEW OF INTRA PREDICTION IN HEVC: In H.264, intra prediction [6][7][8][9] is
based on spatial extrapolation of samples from previously decoded image blocks, followed by
integer discrete cosine transform (DCT) [10] based coding. HEVC utilizes the same principle,
but further extends it to efficiently represent wider range of textural and structural information in
images. HEVC contains several elements improving the efficiency of intra prediction over earlier
solutions. The introduced methods can model accurately different structures as well as smooth
regions with gradually changing sample values. Figure 4 shows the intra prediction modes of
HEVC [7] and figure 5 shows the intra prediction modes of H.264 [21].
Figure 5- HEVC intra prediction modes [7]
Figure 6: H.264 intra prediction modes [21]
Prediction size
64x64
32x32
16x16
8x8
4x4
Total Modes
Total Intra Angular modes
HEVC/H.265(64x64)
H.264/AVC(16x16)
5
34
34
34
17
64x(5+34+34+34+17)=7936
NA
NA
4
9
9
16x(16x9+4x9+4)=2944
Table 1: Comparing HEVC Intra luma prediction modes for 64x64 LCU with H.264/AVC Intra
modes for a 64x64 image region [11]
METHOD PROPOSED FOR FAST MODE DECISON ALGORITHM FOR INTRA
PREDICTION: The fast intra prediction consists of three steps.
1 - Hadamard Transformed Coefficients Of Residual Signal [13]
2 - Progressive Mode Search [13]
3 - Early RDOQ Termination [13]
By combining these three steps, fast mode decision algorithm can be performed. The unified
intra in HM13.0 first determines the first N best candidate modes selected by a rough mode
decision (RMD) process where all modes are tested by minimum absolute sum of Hadamard
transformed coefficients of residual signal and the mode bits in the rough mode decision. Instead
of the total intra prediction modes decision, the RD optimization is only applied to the N best
candidate modes selected by the rough mode decision where all modes are compared in this
decision. However, computation load of the encoder is still very high. On the other side, the intra
prediction modes are always correlated among the neighbors which are not considered in HM
13.0. Therefore, there is still some room for further reducing the encoder complexity.
To further relieve the computation load of the encoder, it is important to reduce the candidates
for RDO process and make full use of the information of its neighboring blocks. In this project,
check for less number of best RMD modes for RDO, and the most probable mode (MPM) is
always included in the candidates for RDO.
TEST SEQUENCES USED:
[1] BQSquare_416x240_60 [16]
[2] BQMall_ 832x480_60 [16]
[3] KristenAndSara_1280x720_60 [16]
EXPERIMENTAL RESULTS OF BQMall_832x480_60: Tables 2 and 3 demonstrate the
implementation results test sequence BQMall_832x480_60 with number of frames 30.
Analysis of results for default method with number of frames 30:
QP
BITRATE
PSNR
ENCODING TIME
(kbps)
(avg)
(sec)
dB
24
23836.5280
41.1352
519.561
28
16075.4080
38.6716
515.1675
32
10704.5280
36.2715
426.831
34
8506.4960
34.9803
407.187
Table 2 : Implementation results for default method with number of frames 30 for
BQMALL__832x480_60
Analysis of results for HM13.0 using algorithm with number of frames 30 :
QP
BITRATE
PSNR
ENCODING TIME
(kbps)
(avg)
(sec)
dB
24
24074.8932
40.9944
396.944
28
16236.1620
38.5300
383.799
32
10811.5732
36.1225
312.44
34
8591.5609
34.8425
297.246
Table 3: Implementation results for modified method with number of frames 30 for
BQMALL__832x480_60
Tables 4 and 5 demonstrate the implementation results test sequence BQMall_832x480_60 with
number of frames 10.
Analysis of results for default method with number of frames 10 :
QP
BITRATE
PSNR
ENCODING TIME
(kbps)
(avg)
(sec)
dB
24
23754.8160
41.137
176.624
28
16010.5920
38.6744
161.116
32
10612.8000
36.2830
151.866
34
8447.7600
34.9856
134.619
Table 4: Implementation results for default method with number of frames 10 for
BQMALL__832x480_60
Analysis of results for HM 13.0 using algorithm with number of frames 10
QP
BITRATE
PSNR
ENCODING TIME
(kbps)
(avg)
(sec)
dB
24
24046.1849
40.9975
134.234
28
16227.4525
38.5344
120.837
32
10718.928
36.1320
112.380
34
8532.2376
34.8473
98.271
Table 5: Implementation results for modified method with number of frames 10 for
BQMALL__832x480_60
Encoding Time Vs QP Graphs: Figures 7 and 8 illustrate the Encoding time and QP plot for
test sequence BQMall_832x480_60 for number of frames 10 and 30 respectively.
For number of frames 10
200.00
180.00
T
i
m
e
(
s
e
c
)
E
n
c
o
d
i
n
g
160.00
140.00
120.00
Encoding Tme(sec) for
unmodifed method
100.00
80.00
Encoding Time (sec) for modified
method
60.00
40.00
20.00
0.00
24
28
32
34
QP
Figure 7: RD plot for BQMall_832x480_60 with number of frames 10
For number of frames 30
600
t 500
i
400
m
e 300
Encoding Tme(sec) for unmodifed
method
(
s
200
e
c
100
Encoding Time (sec) for modified
method
)
E
n
c
o
d
i
n
g
0
24
28
32
34
QP
Figure 8: RD plot for BQMall_832x480_60 with number of frames 30
Graph Between PSNR (avg) vs Bitrate: Figures 9 and 10 illustrate the Bitrate-PSNR plot for
test sequence BQMall_832x480_60 for frame sizes 10 and 30 respectively.
For number of frames 10
Chart Title
42
P 41
S
40
N
39
R
38
For default method
For proposed method
(
)
37
a
v 36
g 35
d 34
B
0
5000
10000Bitrate
15000
(kbps)20000
25000
30000
Figure 9: RD plot for BQMall_832x480_60 with number of frames 10
For Number of Frames 30
Chart Title
42
P
41
S
N 40
R 39
For default method
(
38
For proposed method
)
a 37
v
36
g
35
d
B 34
0
5000
10000 15000 20000
Bitrate (kbps)
25000
30000
Figure 10: RD plot for BQMall_832x480_60 with number of frames 30
CONCLUSION:
For the modified method compared to unmodified HM13.0,
•
0.1-0.5 dB loss in the PSNR
•
11-15 kbps increase in the bit-rate
•
20-28 % reduction in encoding time
WQVGA – SD sequences of 10 frames and 30 frames each were tested from QPs 24, 28, 32,34.
visual quality was maintained.
FUTURE WORK: BD-PSNR and BD-bitrate can be compared for default method and modified
method. Also this modified method can be tested using test sequences BQSquare_416x240_60
and KristenAndSara_1280x720_60 for different QP values and number of frames of 10 and 30.
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