1. A method of computing an image from a computer graphics model that describes at least one polygon that is visible in the image, the method comprising selecting between a first and second algorithm for computing a visual effect of the polygon, wherein the first algorithm computes an update value for the polygon and pixel values for spatially successive pixels in the polygon by successively updating the pixel values using the update value and wherein the second algorithm computes said pixel values for each pixel in the polygon individually, without said updating, the first algorithm being selected if a size of the polygon exceeds a threshold, the second algorithm being selected if the size does not exceed the threshold.