1. A computer implemented method of additively generating a collage of images, said method comprising: seeding a first image into a viewing region to start said collage; providing a plurality of trial layout arrangements with an additional image added to said collage; generating an occlusion cost for a trial layout arrangement of said plurality of trial layout arrangements, wherein said occlusion cost represents image saliency occluded and viewing region saliency occluded by said trial arrangement; and adding an image to said collage, wherein said collage is updated to reflect one of said trial layout arrangements for which said occlusion cost has been generated.