Image matting is performed on an image having a specified foreground region, a background region and an unknown region by selecting a kernel size based on a size of the unknown region. The matting processing is performed using the selected kernel size to provide an alpha matte that distinguishes a foreground portion from a background portion in the unknown region. Further, in some implementations, a trimap of the image may be segmented and matting processing may be performed on each segment using a kernel size appropriate for that segment.