View
243
Download
2
Category
Preview:
Citation preview
1
VIDEO COMPRESSION FUNDAMENTALS
Pamela C. Cosman
2
Compressing Digital Video
Exploit spatial redundancy within frames (like JPEG: transforming, quantizing, variable length coding)
Exploit temporal redundancy between frames Only the sun has changed position between these 2 frames
Previous Frame Current Frame
3
Simplest Temporal Coding - DPCM
Frame 0 (still image) Difference frame 1 = Frame 1
– Frame 0 Difference frame 2 = Frame 2
– Frame 1 If no movement in the scene,
all difference frames are 0. Can be greatly compressed!
If movement, can see it in the difference images
0 1 2 3
4
Difference Frames
Differences between two frames can be caused by Camera motion: the outlines of background or
stationary objects can be seen in the Diff Image Object motion: the outlines of moving objects can
be seen in the Diff Image Illumination changes (sun rising, headlights, etc.) Scene Cuts: Lots of stuff in the Diff Image Noise
5
Difference Frames
If the only difference between two frames is noise (nothing moved), then you won’t recognize anything in the Difference Image
But, if you can see something in the Diff Image and recognize it, there’s still correlation in the difference image
Goal: remove the correlation by compensating for the motion
6
7
8
9
Types of Motion
Translation: simple movement of typically rigid objects
Camera pans vs. movement of objects
Frame n Frame n+1
Rotation: spinning about an axis Camera versus object
rotation Zooms –in/out
Camera zoom vs. object zoom (movement in/out)
Frame n+1 (Rotation)
Frame n+2 (Zoom)
Frame n
10
Describing Motion
Translational Move (object) from (x,y) to (x+dx,y+dy)
Rotational Rotate (object) by (r rads) (counter/clockwise)
Zoom Move (in/out) from (object) to increase its size by
(t times)
Which is easiest? Which are we most likely to encounter?
11
Motion Estimation
Determining parameters for the motion descriptions
For some portion of the frame, estimate its movement between 2 frames- the current frame and the reference frame
What is some portion? Individual pixels (all of them)? Lines/edges (have to find them first) Objects (must define them) Uniform regions (just chop up the frame)
12
General Idea
For a region PC in the current frame, find a region PR in the search window in reference frame so that Error(PR,PC) is minimized
Issues: Error measures, search techniques, choice of search window, choice of reference frame, choice of region PC
Current
Frame
Portionofinterest
PC
Searchwindow
Reference
Frame
13
Block-based Motion Estimation PC is a block of pixels (in the current frame) The search window is a rectangular segment
(in the reference frame)
T=1 (reference) T=2 (current)
14
Motion Vectors A motion vector (MV) describes the offset between
the location of the block being coded (in the current frame) and the location of the best-match block in the reference frame
T=1 (reference) T=2 (current)
15
Motion Compensation
The blocks being predicted are on a grid
The blocks used for prediction are NOT
16
9
8765
43211
5
9
34
6 7 8
2
1413
121110
13
14
15
15 16
1011
12
Motion Vector Search
1. Mean squared error Select a block in the
reference frame to minimize
Σ(b(Bref)-b(Bcurr))2
2. Mean abs. error Select block to
minimize
Σ|b(Bref)-b(Bcurr)|
Given error measure, how to efficiently determine best-match block in search window? Full search: best results,
most computation Logarithmic search –
heuristic, faster Hierarchical motion
estimation
16
17
Motion Vector Search Full search: Evaluate
every position in the search window
Logarithmic Search: First examine positions marked 1.
Choose best of these (lowest error measure) and examine positions marked 2 surrounding it
Choose the best of these, and examine the positions marked 3
Final result = best of these
18
Hierarchical Motion Estimation Use an averaging filter on the image, then
downsample by a factor of 2 Conduct a search on the downsampled
image (only ¼ of the size) Given the results of the search on the
downsampled image, return to the full resolution image and refine the search there
19
Motion Compensation
The standards do not specify HOW the encoder will find the motion vectors (MVs)
The encoder can use exhaustive/fast search, MSE /MAE/other error metric, etc.
The standard DOES specify The allowable syntax for specifying the MVs What the decoder will do with them
What the decoder does is to grab the indicated block from reference frame, and glue it in place
20
Standard specifies bit stream
ENCODER DECODER
bit stream
Standard defines this
not thisnot this
The video compression standards define syntax and semantics for the bit stream between encoder and decoder
Encoder is not specified by MPEG except that it produces a compliant bit stream
Compliant decoder must interpret all legal MPEG bit streams
This allows future encoders of better performance to remain compatible with existing decoders.
Also allows for commercially secret encoders to be compatible with standard decoders
Today’s Ho-Hum Encoder
Tomorrow’s Nifty Encoder
Very secret Encoder
Today’s Decoder
Today’s decoder still works!
21
Motion Compensation Example
Frame n-1
MOTION COMPENSATED
Frame n
(0,0) (-16,0) (5,0) (0,0)
(0,0) (16,7) (5,2) (0,0)
(20,-24) (0,0) (-20,-18) (0,0)
Frame n
22
Objects versus Macroblocks Real moving objects will not coincide with
boundaries of macroblocks
If encoder sends MV=(MotX,MotY), object well coded, but background poorly coded
If encoder sends MV=(0,0), background well coded, but moving object poorly coded
Either approach is valid
background
moving object
Prediction error
Moving object well encoded with motion vector
Prediction error
Background well encoded (no motion vector)
23
Motion Compensation
This glued together frame is called
the motion compensated frame The encoder can also form the difference between
the motion compensated frame and the actual frame.
This is called the motion compensated difference frame
This difference frame formed using MC should have less correlation between pixels than the difference frame formed without using MC
Motion Compensated Difference Frames
24
Suppose we are doing lossless coding Encoder has sequence of frames: …, F(n-2), F(n-1) Next: encode F(n) Past frames have been losslessly encoded, so the
decoder knows F(n-1) perfectly already Encoder sends the motion vectors for frame F(n)
relative to frame F(n-1), to form motion compensated frame M(n) Encoder knows M(n), Decoder knows M(n)
25
Motion Compensation Example
F(n-1)
MOTION COMPENSATED Frame
(0,0) (-16,0) (5,0) (0,0)
(0,0) (16,7) (5,2) (0,0)
(20,-24) (0,0) (-20,-18) (0,0)
F(n)
M(n)
Encoding Difference Frames
Encoder forms motion compensated diff frame:
MCD(n) = F(n) – M(n) Encoder losslessly
encodes MCD(n) Decoder can then do
F(n) = MCD(n) + M(n) → knows F(n) exactly
With no motion compensation encoder could do frame diff: FD(n) = F(n) – F(n-1)
Encoder losslessly encodes FD(n)
Decoder can then do
F(n) = FD(n) + F(n-1) → knows F(n) exactly
26
If successive frames are very similar: fewer bits to send Motion Vectors + MCD(n) instead of FD(n)fewer bits to send FD(n) instead of F(n)
27
Motion compensated difference frames Decoder knows F(n-1) and, once you send the
motion vectors, it knows M(n)
Reference Frame F(n-1)
Original Frame F(n)
Difference Image FD(n)=F(n)-F(n-1)
Motion compensated frame M(n)
Motion compensated difference image MCD(n) =F(n) – M(n)
Send Motion Vectors
Send FD(n)
Send MCD(n)
Motion Compensated Difference Frames
28
But we are NOT doing lossless coding Encoder has sequence of frames: …, F(n-2), F(n-1) Next: encode F(n) Past frames have been lossy encoded, so the
decoder has versions …, G(n-2), G(n-1) Encoder knows …, G(n-2), G(n-1) also Encoder sends the motion vectors for frame F(n)
relative to frame G(n-1), to form motion compensated frame M(n)
Encoding Difference Frames
Encoder forms motion compensated difference frame: MCD(n) = F(n) – M(n)
Encoder lossy encodes MCD(n) Call the decoder version MCD*(n) If the decoder received MCD(n) exactly,
could do: F(n) = MCD(n) + M(n) But with MCD*(n), decoder can do
G(n) = MCD*(n) + M(n) → knows F(n) approximately
29
30
Motion estimation philosophy
Goal of motion estimation is NOT to provide a careful analysis of the actual motion
Goal is to achieve a given quality of representation of the video while globally minimizing the bit rate required to send The motion information The prediction error information
Most of the time, for a given representation quality fewer bits to send MV+MCD(n) instead of sending FD(n) fewer bits to send FD(n) instead of sending F(n) itself.
31
Motion Compensation for Chrominance Luminance is highly correlated, more so than
chrominance The “best” motion vectors are available by
searching in the luminance plane Motion vectors for chrominance are not
computed separately, simply scaled as needed
32
Motion Estimation/Compensation Summary At the encoder:
For each block in the frame being coded, examine the search window(s) in the reference frame to find the best match block (do this for luminance only)
Form the MC difference image = original image minus motion compensated image
Scale the motion vectors for the chrominance, form the motion compensated chrominance frames, and form chrominance difference image
33
Motion Estimation/Compensation Summary At the decoder:
Decode the reference frames (Y,Cr,Cb) For each block in a temporally coded Y frame,
use the motion vector to select a block from the reference frame and glue it in place
Add the Y difference image For each block in temporally coded Cr,Cb frames,
first scale the motion vector, then do the previous 2 steps with Cr and Cb data
34
Progress of Video Compression
35
Progress of Video Compression
36
Progress of Video Compression
37
Temporal Location of Reference The reference frame need not occur before
the temporally coded frames which use it
Why? Scene changes, allow better matches
38
Flavors of Motion Estimation
1. Forward predicted blocks: the best-match block occurs in the reference frame before the block’s frame
2. Backward predicted blocks: the best-match block occurs in the reference frame after the block’s frame
3. Interpolatively predicted blocks: the best-match block is the average of the best-match blocks from reference frames before & after
The motion compensation direction can be selected independently for each block in a frame.
39
MPEG Frame Types
Intra (I) pictures: coded by themselves, as still images. No temporal coding. No motion vectors.
40
MPEG Frame Types
Forward Motion Compensated predicted (P) pictures – forward motion compensated from the previous I or P frame
41
MPEG Frame Types Motion Compensated interpolated (B) pictures –
forward, backward, and interpolatively motion compensated from previous/next I/P frames
42
Motion Vector Coding
How are the motion vectors actually encoded for transmission to the decoder? Start by taking the difference between the current
motion vector and the most recent previous one of the same type (forward/backward/interpolative)
Encode the difference using variable length coding
Horizontal and vertical components coded separately
43
MPEG Frame Structure Terminology A block contains 8x8 pixels
The DCT unit A macroblock (MB) contains 4 blocks from
the luminance, plus the corresponding chrominance blocks 4 blocks from each of Cr/Cb if 4:4:4 format 2 blocks from each of Cr/Cb if 4:2:2 format 1 block from each of Cr/Cb if 4:1:1 or 4:2:0 format The motion compensation unit
44
MPEG Frame Structure Terminology A slice is a collection of macroblocks, tracing
in a raster scan from upper left to lower right The resynchronization unit
A picture is a frame, either progressive (non-interlaced) or interlaced The primary coding unit
A Group of Pictures (GOP) contains ≥ 1 frame. The unit for random access into the sequence
45
MPEG GOP Structure
A Group of Pictures (GOP) may contain All I pictures I & P pictures only I, P, & B Pictures
A common GOP format for 30 frames/sec: I-picture spacing 15 frames (1/2 second) P-picture spacing 3 frames (1/10 second)
I B B P B B P B B P B B P B B I1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
46
Frame Ordering
Display order (encoder input order):
But consider coding dependencies: Frame 2 (B) needs frame 4 (P) to be decoded first, etc. So better transmit frame 4 before frame 2
B B I B B P B B P B B P B B P B B I
-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
I B B P B B P B B P B B P B B I B B
1 -1 0 4 2 3 7 5 6 10 8 9 13 11 12 16 14 15
47
Types of Coding Modes
What if the best-match block in the reference frame is a great match? Then the motion vector is all you need to send
What if it is a terrible match? Then don’t use the motion vector at all, just code
the block by itself, with something like JPEG (called intra mode coding)
What if it is a so-so match? Then you can send the MV, and also send the
frame difference information for that macroblock
48
Coding Mode I (Inter-Coding)
CurrentFrame
PreviousFrame
Motion Vector
MacroBlock
Inter coding refers to coding with motion vectors
49
Coding Mode II (Intra-Coding)
CurrentFrame
PreviousFrame
MacroBlock
INTRA coding refers to coding without motion vectors The MB is coded all by itself, in a manner similar to JPEG
50
I-Picture Coding
Two possible coding modes for macroblocks in I-frames Intra- code the 4 blocks with the current
quantization parameters Intra with modified quantization: scale the
quantization matrix before coding this MB All macroblocks in intra pictures are coded Quantized DC coefficients are losslessly
DPCM coded, then Huffman as in JPEG
51
I-Picture Coding
8
8
88
Use of macroblocks modifies block-scan order:
Quantized coefficients are zig-zag scanned and run-length/Huffman coded as in JPEG
Very similar to JPEG except (1) scaling Q matrix separately for each MB, and (2) order of blocks
P-Picture Coding: many coding modes
Motion compensated coding: Motion Vector only
Motion compensated coding: MV plus difference macroblock
Motion compensation: MV & difference MB with modified quant. scaling
52
DCTMotion Vector
MV = (0,0), just send difference block
MV=(0,0), just send diff block, with modified quantization scaling
Intra: the MB is coded with DCTs (no difference is computed)
Intra with modified quantization scaling
53
How to choose a coding mode? MPEG does not specify how to choose mode
Full search = try everything…the different possibilities will lead to different rate/distortion outcomes for that macroblock
• Intra
• MV, no difference
• MV, plus difference
rate
distortion •
•
••
•
54
How to choose a coding mode?
Tree search: use a decision tree For example:
First find the best-match block in the search window. If it’s a very good match, then use motion compensation. Otherwise, don’t.
If you decided to use motion compensation, then need to decide whether or not to send the difference block as well. Make decision based on how good a match it is.
If you decided to send the difference block, then have to decide whether or not to scale the quantization parameter… check the current rate usage…
55
B-Picture Coding
B pictures have even more possible modes: Forward prediction MV, no difference block Forward prediction MV, plus difference block Backward prediction MV, no difference block Backward prediction MV, plus difference block Interpolative prediction MV, no difference block Interpolative prediction MV, plus difference block Intra coding Some of above with modified Quant parameter
56
Group of Pictures
IIIII…: Every picture is intra-coded. Fully decodable without reference to any other picture Editing is straightforward Requires about 2.5 more bit rate than bidirectional
IBBPBBPB…: Forward and bidirectional Best compression factor Needs large decoder memory Hard to edit Most useful for final delivery of post-produced material
(e.g., broadcast) because no editing requirement
57
Group of Pictures
IPPPPIPP…: Forward predicted only. Needs less decoder memory
IBIBIB…: bidirectional compromise Some of the bit rate advantage of bidirectional coding Not nearly the full latency penalty of bidirectional Editable with moderate processing.
For example, if the video after a B picture needs to be deleted, the B frame would not be decodable.
Solution is to decode the B frame first, re-encode it using forward prediction only. Some quality loss.
Recommended