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Transcript of Videos Mei-Chen Yeh. Outline Video representation Basic video compression concepts – Motion...
Videos
Mei-Chen Yeh
Outline
• Video representation• Basic video compression concepts– Motion estimation and compensation
Some slides are modified from Prof. Chu’s slides.
Videos
• A natural video stream is continuous in both spatial and temporal domains.
• A digital video stream sample pixels in both domains.
Video processing
YCbCr
YCbCr
Video signal representation (1)
• Composite color signal– R, G, B– Y, Cb, Cr
• Why Y, Cb, Cr?– Backward compatibility (back-
and-white to color TV)– The eye is less sensitive to
changes of Cb and Cr components
YRC
YBC
BGRY
r
b
114.0587.0299.0
Luminance(Y)
Chrominance(Cb + Cr)
Video signal representation (2)
• Y is the luma component and Cb and Cr are the blue and red chroma components.
Y Cb Cr
Sampling formats (1)
4:4:4 4:2:2 (DVB) 4:1:1 (DV)
Slide from Dr. Ding
Sampling formats (2)
4:2:0 (VCD, DVD)
TV encoding system (1)
• PAL– Phase Alternating Line, is a color encoding system used in
broadcast television systems in large parts of the world.• SECAM– (French: Séquential Couleur Avec Mémoire), is an analog
color television system first used in France.• NTSC– National Television System Committee, is the analog
television system used in most of North America, South America, Burma, South Korea, Taiwan, Japan, Philippines, and some Pacific island nations and territories.
TV encoding system (2)
Other frame formats
• CIF (352x288)Common Interchange Format
• QCIF (176x144)Quarter Common Interchange Format
Uncompressed bitrate of videos
Video Type
Pixels per Frames
Image Aspect Ratio
Frames per Second
Bits/pixel Uncompressed Bitrate
NTSC 480 483 4:3 29.97 16 111.2 Mb/s
PAL 576 576 4:3 25 16 132.7 Mb/s
CIF 352 288 4:3 14.98 12 18.2 Mb/s
QCIF 176 144 4:3 9.99 12 3.0 Mb/s
HDTV 1280 720 16:9 59.94 12 622.9 Mb/s
HDTV 1920 1080 16:9 29.97 12 754.7 Mb/s
Slide from Dr. Chang
Outline
• Video representation• Basic video compression concepts– Motion estimation and compensation
Video compression
• A video is a sequence of correlated frames.• Compression: remove redundancy– Make use of the temporal correlation!– The previous reconstructed frame is used to
generate a prediction for the current frame.
Video compression
• Intra coding: Compress one frame without referring to other frames– Each pixel has to be compressed– DCT → Quantization → Entropy coding
• Inter coding: Compress one frame depending on the previously encoded frame– Only compress moving objects and new objects– Background can be ignored
~ Image compression
Example
1. Compress and in frame 1.
2. Compress the motion of in remaining frames. Direction and magnitude
1 2 3 4
Slide from VC Lab
Motion compensation (1)• Objects tend to move between frames
Slide from Prof. Bernd Girod
Motion compensation (2)• Block-based motion compensation– Divide a frame into blocks– Search the previous reconstructed frame for the
most matching block– Motion vector: the offsets between block
locations
search area
Motion compensation: Example
Slide from Prof. Bernd Girod
Motion-compensated decoder
CodedBitstream VLD
1Q IDCT
MotionCompensation
PreviousFrame memory
Reconstructedframe
MV
Residual
Slide from VC Lab
Motion-compensated encoder
Slide from VC Lab
MotionCompensated
Predictor
DCT + Q
iDCT + iQ
MV = (dx, dy)①
② ③
④
Residuals
PreviousFrame Buffer
Three types of coded picture
• I frame (Intraframe)– Encoded without any temporal prediction
• P frame (Forward Predicted Frame)– Encoded using motion prediction from the
previous I or P frame• B frame (Bidirectionally Predicted Frame)– Encoded using interpolated motion prediction
between the previous I or P frames and the next I or P frames
Example: A typical Group of Picture (GOP) in MPEG-2
Summary
• Video compression concepts– Cb, Cr sampling
– Motion compensation and estimation• The encoder mimics the behavior of the decoder• Unbalanced computing: searching for a good prediction
can be time consuming
– GOP structure