Projective Alpha Colour and its Application

Image compositing is combining two or more images by overlaying them. For this to be meaningful, some of the image areas need to be less than perfectly opaque, so the rearmost images can be seen. When Porter and Duff wrote their 1984 paper on Image Compositing, they used a four channel colour model (r,g,b,a). The extra channel, called alpha, represented the opacity of the colour (r,g,b). We have recently shown that this is mathematically a projective space, which extends the range of use of the alpha colour model, including to applications beyond compositing. This published work will be described. We also have some very recent unpublished results and these too will be presented.