Video Compression

1
Video Compression

• Video Sequence of frames
• Each Frame 2-D Array of Pixels
• Video 3-D data
• 2-D Spatial, 1-D Temporal
• Video has both
• Spatial Redundancy (within each frame
  Intra-Frame)
• Temporal Redundancy (between Frames
  Inter-Frame)

2
Video Coding Applications

• Broadcast TV
• Consumer Playback Apps. (ex. Interactive Games)
• Desktop Video (ex. Authoring/Editing)
• Video Conferencing/Video Telephony

3
Major Video Coding Techniques

• Intel Indeo
• Motion JPEG
• XYZ Algorithm
• H.261/H.263
• MPEG (Motion Pictures Expert Group)

4
Intel Indeo

• Interpolate Video Data (ex. 128 x 240 ? 256 x
  480)
• Subsample Color components 41
• Use 16 bit codebook
• Use Frame differencing

5
Motion JPEG

• Each Frame treated separately
• Temporal Redundancy not exploited
• Baseline JPEG used to encode each frame
• Good Support for Random Access to Frames
• (Not a standard but used in many systems)

6
XYZ Algorithm

• Uses spatial-temporal redundancy
• X, Y Spatial Components Z Temporal
  Component
• Divide each Frame into 8 x 8 block
• Video Cube Corresponding blocks of 8
  consecutive Frames
• Apply 3-D DCT to video cubes
• Quantize, Entropy code the coefficients

7
XYZ - Algorithm
8
Motion Estimation Techniques

• Gradient Techniques
• Pel-recursive Techniques
• Block-matching Techniques
• Frequency-domain Techniques
• (Block-matching techniques most widely used)

9
Terminology

• Reference Frame Frame in past (or future) used
  to predict in current frame
• Current Frame Frame which is being analyzed to
  derive motion vectors
• Motion vector The displacement of the closest
  matching block in reference frame for a block in
  current frame
• Motion Estimator Process of determining the
  values of motion vectors for each frame

10
Block-Matching technique (BMT)

• Motion of a macroblock of pixels within a frame
  is estimated
• Ignores rotational motion
• Assumes all pixels within macroblock to have the
  same uniform motion
• Range of motion vector is constrained by Search
  Window

11
Outline of BMT

• For each macroblock in current Frame Do
• Search in the reference frame within search area
• Find the closest matching macroblock
• Determine the displacement (motion vector)

12
Finding Best Matching Block
13
Finding Motion Vector
14
Distortion Measures

• Mean Square Error
• Mean Absolute Error
• Cross Correlation Function
• Pixel Difference Classification

15
Motion Vector Search Algorithms

• Exhaustive Search
• 3-step Search
• 2-Dimensional Search
• Conjugate Direction Search
• Hierarchical Motion Estimation

16
Three-Step Search Algorithm
The vector chosen has location x6, y4
17
2-D Logarithm Search
The motion vector chosen is x6, y4
18
Conjugate Direction Search
19
Hierarchical Motion Estimation