Methods, systems, and apparatus, including computer program products feature identifying a first plurality of pixels in a raster image as foreground pixels and a distinct second plurality of pixels in the raster image as background pixels. Each of the foreground and background pixels have a respective color value. The color values of the foreground pixels and the color values of the background pixels are used to solve for a color model of opacity. The color model of opacity is used to determine an opacity value for a pixel in the raster image.