Then these vectors are appropriately scaled, if the parity of the predicted field is opposite to that of the predicted field. The prediction error is computed using the reconstructed pictures. The combination with the smallest MSE is selected. 5.2 Motion Compensation Motion compensation is performed differently for field coding and for frame coding. General formulas for frame and field coding are listed below.
Temporal interpolation is performed by averaging.
In frame prediction macroblocks there is one vector per macroblock. Vectors measure displacements on a frame sampling grid. Therefore an odd-valued vertical displacement causes a prediction from the fields of opposite parity. Vertical half pixel values are interpolated between samples from fields of opposite parity. Chrominance vectors are obtained directly by using the formulae above.