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What is backprojection mathematically?

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Hello everybody, I have found several papers on the net using backprojection to display softshadows (e.g. A fast Shadow Algorithm for Area Light Sources Using Backprojection by George Drettakis...). However, I don't understand what exactly is computed. I know that it is used to determine how much of the Lightsource is covered by the current shadowmap-sample, but I would really like to know what is going on mathematically to understand the concept. Has anybody yet worked with backprojection and could explain the mathematical functionality to me? Thanks a lot, Icebraker

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oh I think this is drawing only backward oriented objects into the shadow map. You change this with the cull state.
So instead of backface culling they do frontface culling

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Thanks for your reply,

however, I don'T know how to implement the backprojection. I don't think that only changing backface-culling to frontface-culling will work because this does not give me the amount of the lightsource that is visible. Or did I misunderstand the concept? Could anybody please post some code/Foraulas which describe how backprojection is implemented?

Thx a lot

Icebraker

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