Patent ID: 8005151

Claim:
A method of filtering samples in a decoder to minimise coding artifacts introduced at a block boundary in a block-based video encoder, the method having the steps of: computing a boundary strength parameter, wherein the boundary strength parameter comprises a value from 0 to 4; checking content activity on every line of samples said block boundary; determining whether the filtering process will modify the sample values on the line of samples based on content activity thresholds, said content activity thresholds being dependent on an average quantization parameter and a selected filter offset parameter; determining whether a block on either side of the block boundary was coded using either intra or inter prediction; providing a pair of filter modes, i) the first filter mode, a strong filtering mode consisting of a selection of a 3-tap filter sub-mode and an alternative 5-tap filter sub-mode with the selection between these two sub-modes made separately on each side of the block boundary based on the sample values on each line of samples across said block boundary, ii) the second filter mode, a non-recursive default filtering mode; and selecting one of the pair of filter modes for filtering the samples according to the computed boundary strength parameter, wherein the second filter mode provides filtered output values based solely on unfiltered samples used as input data, and wherein the following equations are used to compute the filtered sample values at the first position from the block boundary on either side, denoted P 0 and Q 0 , for the chrominance component on block boundaries upon which the second filter mode is applied: P 0 =p 0 +Clip(−( C 0+1),( C 0+1),(((( q 0 −p 0 )<<2)+( p 1 −q 1)+4)>>3)) O 0 =q 0 −Clip(−( C 0+1),( C 0+1),(((( q 0 −p 0 )<<2)+( p 1 −q 1)+4)>>3)).