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  In many instances, accumulated snow is not thick enough to completely obscure the underlying surface, appearing instead as a light ``dusting'' of flakes. This phenomena often occurs in areas of low snowfall, high instability, or on surfaces with microtexture bumps, such as tree bark . 
Since it is not practical to model dusting as thick 3D objects, we use already computed snow occlusion percentages to generate procedural noise textures of the appropriate averaged dusting density. Dusting textures are semi-transparent, textured polygons oriented to float slightly in front of the original model. 
Comparison of flake dusting on a real sign (left) and a computer generated sign (right).
Click to enlarge.