1. A method comprising: determining a first image comprising a first plurality of picture elements for a first plurality of locations in the first image; determining a second image comprising a second plurality of picture elements for a second plurality of locations in the second image; determining an alpha map specifying a plurality of operations for a plurality of locations that correspond to the first plurality of locations in the first image and the second plurality of locations in the second image; applying the plurality of operations specified in the alpha map to generate a third image comprising a third plurality of picture elements for a third plurality of locations in the third image, a picture element in the third plurality of picture elements being generated in an operation in the plurality of operations, the operation being for a location in the plurality of locations, the location in the plurality of locations corresponding to a first location in the first plurality of locations in the first image and a second location in the second plurality of locations in the second image; wherein the method is performed by one or more devices comprising a processor.