1. A method comprising: receiving a trimap of an image that specifies a foreground region, a background region and an unknown region for the image; segmenting the trimap into a plurality of segments, each segment including a portion of the unknown region and at least one of a portion of the foreground region or a portion of the background region; solving, by a processor, the portion of the unknown region in each segment in a first local pass to provide a first alpha matte that distinguishes a foreground region from a background region in the portion of the unknown region; solving, by the processor, the unknown region for the entire image as a linear system to refine the first alpha matte to obtain a second alpha matte; and refining the second alpha matte to obtain a third alpha matte by solving the portion of the unknown region in each segment wherein the solving for each segment employs a kernel size based on a size of the unknown region of the segment that is smaller than a kernel size used in the first local pass.