Encoding Stored Video forStreaming Applications
IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS FORVIDEO TECHNOLOGY, VOL. 11, NO. 2, FEBRUARY 2001
I.-Ming Pao
Ming-Ting Sun
Outline
Introduction
Method
Simulation Result
Conclusion
Introduction
Digital video applications have becomeincreasingly popular.
There are several video standards establishedfor different purposes.
e.g, MPEG-1, MPEG-2, H.263…
Introduction
Basic building blocks
Introduction
Real-time Visual Communication
delay sensitive
processes need to be done in constraint time
rate control scheme is not suitable
Nonreal-time Visual Communication
delay tolerable
pre-loaded time
decoder buffer
rate control scheme is suitable
Buffer and Pre-loading
Introduction
Streaming video applications
Video sequences are
encoded off-line
Stored in a server
Pre-load before playback
e.g, VOD
Problem
Bit allocation and video quality
Minimum distortion under the rate constraint
Introduction
Contribution of this paper :
1.Propose a sliding-window rate-control scheme.
2.A quantized DCT coefficient selection scheme.
3.Improve video quality for video streaming.
Outline
Introduction
Method
Simulation Result
Conclusion
Global View
Generate encoded bitstream
Sliding-window encoding scheme
Consider the constraints
buffer-size
pre-loading time
DCT coefficient selection
Run-length coding
Sliding-Window Encoding Scheme
Use future frames to improve video quality.
Set window size W to encode video frame.
frames : i, i+1, …, i+W-1
let frame i be the current frame
This proposed encoder better than real-time’sfor the same bitrate[20].
[20] I.-M. Pao and M.-T. Sun, “A rate-control scheme for streaming video encoding,”in Proc.32nd Asilomar Conf. Signals, Systems and Computers, vol. 2, Asilomar, CA, Nov. 1998, pp.1616–1620.
Sliding-Window Encoding Scheme
Bit allocation
rate and distortion scheme[18]
low distortion or high-rate case
high distortion or low-rate case
[18] J. Ribas-Corbera and S. Lei, “Rate control in DCT video coding for low-delay videocommunications,” IEEE Trans. Circuits Syst. Video Technol., vol. 9, pp. 172–185, Feb. 1999.
Sliding-Window Encoding Scheme
Bit allocation
mathematical modeling[18]
[18] J. Ribas-Corbera and S. Lei, “Rate control in DCT video coding for low-delay videocommunications,” IEEE Trans. Circuits Syst. Video Technol., vol. 9, pp. 172–185, Feb. 1999.
Sliding-Window Encoding Scheme
Target number of bits for encoding frame
R × time
Buffer-size and Pre-loading TimeRequirement
Why need buffer ?
store the excess bits waiting to be decoded
e.g, bits of future frames
Why need pre-loading time?
the delay before playback
Buffer-size and Pre-loading TimeRequirement
Buffer variation expression :
B0
p0
Buffer Size G
Buffer-size and Pre-loading TimeRequirement
A.Finding decoder buffer size and pre-loadingtime given a video bitstream
underflow
overflow
Buffer-size and Pre-loading TimeRequirement
B.Generating a video bitstream given decoderbuffer size and pre-loading time
To prevent the buffer-underflow :
To prevent the buffer-overflow :
Bit Allocation with Constraints
Step 0 : Initialization :
initialize the bit-count regulator .
Step 1 : Compute the Proposed Target Bits for
Frame :
compute the ideal target number of bits for frame i(i = 0, 1, 2, 3, ...).
avoid the underflow and overflow constraints.
Bit Allocation with Constraints
Step 2 : Macroblock-Layer Rate-Control :
distribute to the macroblocks in the ith frame.
find DCT coefficient selection for each macroblock.
encode bitstream
Step 3 : Update Bit-Count Regulator :
update regulator :
if there are more frames to be encoded, go to Step 1,or else stop.
DCT Coefficient Selection
Quantize the DCT coefficients
rate-distortion sense and macroblock level.
quantizer step-sizes(Q) largely determine the rate-distortion tradeoff.
DCT Coefficient Selection
Run-length coding with LAST
(LAST, RUN, LEVEL)
(0, 4, 6)
bitstream
DCT Coefficient Selection
There are not optimal for all video sequencesby
limited quantizer selections and
predetermined run-length codeword.
The encoder can adjust the quantizedcoefficient’s level.
a marginal distortion increase but
a significant bit-rate reduction.
DCT Coefficient Selection
This paper use Lagrange multiplier method forrate-distortion optimization in
selecting the quantization parameter (QP) and
adjusting the quantized DCT coefficients (LEVEL).
the best combination of QP and LEVELs will be thelowest cost in the rate-distortion sense.
DCT Coefficient Selection
Goal is to find the minimum distortion underthe rate constraint :
for every 8 X 8 block
the optimal QP for the macroblock
the LEVEL for each coefficient
DCT Coefficient Selection
The constrained problem converts to anunconstrained problem through the Lagrangemultiplier λ (≥ 0).
.
the problem becomes the determination of theLEVELs of the coefficients.
Better
Outline
Introduction
Method
Simulation Result
Conclusion
Simulation Result
Different bitrates :
32, 64, and 128 kbits/s
Different types of video sequences :
large facial movement
head and shoulder
camera panning
Compare with TMN8
Outline
Introduction
Method
Simulation Result
Conclusion
Conclusion
Better video quality than TMN8 in highmotion-activity frames and scene-changeframes.
Require more buffer size and pre-loading timethan TMN8.