A method and a computer system for modeling, in a virtual environment of a computer simulation, virtual contaminants in a scene to be rendered. A processing module, using a graphical user interface on a display device, is used for define, in a model, a first additive zone of the scene over which a virtual contaminant is to be added, defining, in the model, a second subtractive zone of the scene over which the virtual contaminant is to be at least partially removed, the second subtractive zone being at least partially enclosed within the first additive zone and a memory module is used for storing the model, the model being made available through a storage module for rendering the virtual contaminants on the scene in the computer simulation. A preview mode may be used for launching the computer simulation at a rate lower than the expected rate of the computer simulation.