FOG SIMULATION FOR PARTIALLY TRANSPARENT OBJECTS

A method for simulating fog in 3D graphics rendering applications correctly computes fogged pixel colors even in graphics scenes where two surfaces overlap and the frontmost surface is partially transparent. The method computes the fog for each surface according to the following formula: .function.F atop A, where .function. is the amount of fog, F is the color of the fog, and A is the color of the pixel being fogged. Each fogged surface can be rendered independently to a separate image layer, called a fogged image layer. The graphics rendering system can then simulate the motion of a fogged image layer by moving the fogged layer in an (x,y) plane without re-computing the fogged pixels, or by moving the fogged layer in the z-direction and independently re-computing the moving fogged layer with a new value for the amount of fog applied to the image layer. The method applies to scenes where there are several fogged layers and to scenes that simulate fire and glow with pixels that are totally transparent but have non-zero color values.