Motion representation

Global:Entire motion field isrepresented by a fewglobal parameters

Block-based:Entire frame is dividedinto blocks, and motionin each block ischaracterized by a fewparameters.

Pixel-based:One MV at each pixel,with some smoothnessconstraint betweenadjacent MVs.

Region-based:Entire frame is dividedinto regions, eachregion correspondingto an object or subobjectwith consistentmotion, represented bya few parameters.

Typical camera movements

Backward and forward motion estimation

Block matching

Typically, the displaced frame difference, computed on a block-by-block basis, is minimized. How?

- exhaustive search, or (much faster)

- gradient-based, multiresolution, or log search

Real MV may not always be multiples of pixels. To allow subpixel MV, the search step size must be less than 1 pixel, e.g. ½ pixel (the image has to be interpolated…)

Mesh-based motion estimation

Using a triangular or a quadrilateral mesh

Block-based vs mesh-based

In mesh-based, motion in each element is interpolated from nodal MVs

In mesh-based: two problems

– Set up the mesh in the anchor frame, so that the mesh conforms with object boundaries: mesh generation

– Given a mesh in the anchor frame, determine nodal positions in the target frame: motion estimation

Block-based vs mesh-based

Video coding: outline

• Predict a new frame from a previous frame and only code the prediction error

• Prediction error will be coded using the DCT method• Those regions that cannot be predicted well will be coded

directly using DCT• Work on each macroblock (MB) (16x16 pixels) independently

for reduced complexity – Motion compensation done at the MB level – DCT coding of error at the block level (8x8 pixels)

GOP

Coder and decoder