A method and a computer system for performing a computer simulation in which virtual contaminants are rendered on a rendered scene of the computer simulation. A memory module is sued for loading thereinto a model defining a virtual environment of the computer simulation, the virtual environment defining a plurality of zones comprising a first additive zone and a second subtractive zone, the second subtractive zone being at least partially enclosed within the first additive zone. An image generator module is used for adding a virtual contaminant over the first additive zone, removing, at least partially, the virtual contaminant within the second subtractive zone leaving a remaining virtual contaminant in the first additive zone and rendering an image for display comprising the remaining virtual contaminant in the virtual environment using a visual texture of the virtual contaminant. The computer simulation may be a vehicle computer simulation.