Adaptive Loop Filter with virtual boundaries and multiple sample sources

A method for implementing an adaptive loop filter (ALF) in a video system is provided. A video coder receives data for a block of pixels to be encoded or decoded as a current block of a current picture of a video. The video coder receives a current sample of the current block. The video coder applies a filter to the current sample to generate a correction value. Neighboring samples from two or more different sources are used as inputs to the filter. When a first neighboring sample is within a virtual boundary, the first neighboring sample is used as an input to the filter. When the first neighboring sample is beyond the virtual boundary, the first neighboring sample is precluded as an input to the filter. The video coder adds the correction value to the current sample as a filtered sample of the current block.