DCT-Domain Inverse Motion Compensation (IMC)
• Extract the Prediction Block in the DCT-Domain
4
xˆ   qi1 xi qi 2
X1
i  1,  ,4
h
X̂
i 1
4
Xˆ   Qi1 X i Qi 2
X2
w
X3
in DCT domain
X4
Reference frame
E
Current block
i 1
0
q11  
0
Ih 
0
, q12  
0
 Iw
8x8
0

0 
8x8
• Previous Algorithms
– Pre-compute Qij [Chang & Messerschmitt, 93]
– Factorize Qij [Merhav & Bhaskaran, 97]
– Approximate Qij [Assuncao & Ghanbari, 98]
– Use the shared information in
each macro-block [Song & Yeo, 00]
X1
h
X3
X2
w
X̂
X̂
X4
1
Windowing Effect (in 1-D)
x1(n)
n
0
y(n)= x1(n) + x2(n)
x2(n)
+
=
n
0
0
n
Pixel Domain
DCT Domain
f
0
X1(n)
+
0
f
X2(n)
=
0
f
Y(n)= X1(n) + X2(n)
2
Local Bandwidth Constrained DCT Domain IMC
• Neighboring Pixels Are Highly Correlated in Images
L3
w
8-w
L1
L2
DCT coefficients
L1 = [5, 0, 1, 0, 0, 0, 0, 0]
L2 = [3, 2, 0, 1, 0, 0, 0, 0]
L3 = [x, x, x, x, 0, 0, 0, 0]
x: element needs computation
3
Experimental Results
Time to convert a P frame to an I frame at bit rate of 4 Mb/s and 1 Mb/s
(Unit: seconds)
Video
Sequence
“foreman”
Encoded at 4 Mb/s
Encoded at 1 Mb/s
The original
The proposed
The original
The proposed
algorithm (Chang 93)
algorithm
algorithm (Chang 93)
algorithm
0.3137
0.2387
0.2512
0.1324
“coastguard”
0.2374
0.1700
0.1912
0.0937
“mobile”
0.3487
0.2513
0.2983
0.1550
“stefan”
0.2057
0.1370
0.1636
0.0743
• Reduce the Computing Time by 25-30% at 4 Mb/s,
45-55% at 1Mb/s
• Average PSNR Degradation is Less Than 0.3 dB
• Similar results for B frames
• Can Work on Top of Other Fast Methods
S. Liu, and A. C. Bovik, “Local Bandwidth Constrained Fast Inverse Motion Compensation for DCT-Domain
Video Transcoding ”, IEEE Trans. On Circuits and Systems for Video Technology, revised
4
DCT-Domain IMC
Pre-compute
Qij
[Chang & Messerschmitt, 93]
Local Bandwidth
Constrained IMC
[Liu & Bovik, 01]
Exploit shared MB
information
[Song & Yeo, 00]
Factorize
Qij
[Merhav & Bhaskaran, 97]
Approximate
Qij
Look-up-table
method
[Assuncao & Ghanbari, 98]
[Liu & Bovik, 01]
5
Look-Up-Table (LUT) Based DCT Domain IMC
• Model DCT Coefficients as Laplacian Distribution [Smoot & Rowe, 96]
p( x ) 

exp( 
1
| x |),   E[| X |] ,
 
2
• The Value of  is Estimated as 0.0284
• Let TH  2  100 , Pre-compute
2

the Results for Coefficients
with
| x | TH
• 800KB Memory Required
• LUT Can Be Shared by Multiple Processes
Values of DCT AC coefficients
6
Experimental Results
Table: The average time to convert one P or B frame to an I frame
(Unit: Second)
Video
sequence
Foreman
Coastguard
Mobile
Stefan
Chang’s method
P frame
0.3137
0.2374
0.3487
0.2057
B frame
0.4738
0.3417
0.4136
0.3667
LUT based method
P frame
0.0931
0.0912
0.1462
0.0780
B frame
0.1423
0.1190
0.2000
0.1416
• Saves More Than 50% of
Computing Time
• Reduces Jerkiness
S. Liu, and A. C. Bovik, “Look-Up-Table Based DCT
Domain Inverse Motion Compensation ”, ICIP 2001
7