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Intersections and the Alpha Channel

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This is more of an intersection question I suppose. The easiest way for me to explain is to provide an example. I have an application that basically performs intersections on an active scene (w/ terrain, entities, etc.). As I am sure you know, the trees and bushes that populate the terrain are composed of one or two textured polygons. When I perform an intersection on a tree (for instance), I would like to obtain the color of the intersected pixel. This color would allow me to determine the alpha value (transparency). Using this alpha value (along with an alpha threshold that I supply) I would either return the intersion point, or ignore it if the alpha is below some threshold. In a nut shell, I would like to ignore the areas of a tree of bush that are transparent. Does anyone know to do this? Any algorithm, code or refence would be appreciated. Thanks,

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