1. A method of scaling a composite overlay image, wherein the composite overlay image comprises a plurality of pixels and each pixel comprises at least one color channel and at least one alpha channel, and further wherein at least one pixel of the composite overlay image has an alpha value of zero and a colour value that is not well defined, said method comprising: (a) identifying a set of pixels of the composite overlay image as pixels to be dilated; (b) dilating the color channels of the identified pixels to form a dilated composite overlay image; and (c) scaling the dilated composite overlay image.