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