motion representation
DESCRIPTION
Motion representation. Pixel-based: One MV at each pixel, with some smoothness constraint between adjacent MVs. Global: Entire motion field is represented by a few global parameters. Region-based: Entire frame is divided into regions, each region corresponding - PowerPoint PPT PresentationTRANSCRIPT
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