this might belong in the general theory forum, but since i'd like to do this in gl, here it is. what kind of processing goes into the kind of volumetric lighting effect that half-life 2 uses for spotlights? is it purely a 2d overlay, or is like a 2d billboarding effect in 3d? also, how is its projection calculated based on the current viewpoint? my second question is unrelated, but is it less taxing in opengl to transform the texture coordinates of an object to simulate rotation than it would be to actually rotate that object?