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pixel area of a polygon

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Anyone know of a good way to determine the area of a polygon in pixels when the camera is looking straight at the polygon? Of course, the distance from the camera would have an effect. I''ve tried just rendering the polygon and then using glReadPixels, but I''m having problem with it because somehow, it''s not reading back the rendered polygon (instead it gives me basically a snapshot of my desktop as viewed from the window). I''ve also looked into feedback mode, but it seems to only return vertex information (it would be helpful if it actually returned rendered pixels.) Anyone know of a good, accurate way of determining this?

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a way that''ll give u an exact amount of pixels rendered is to use stencil. ie clear the stencil buffer to 0 and then draw the polygon with stencil set to 1. the read back the stencil buffer pixels and count howmany are set to one. its not super fast but its very accurate

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