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OpenGL nv_occlusion_query for collisions

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Hi, I read this paper about CULLIDE ( and I'd like to implement it in my little app, but I didn't understand a few points mainly about the use of opengl. Basically I didn't understand how can I check if for instance 2 bounding box collide or not, here an example: Free Image Hosting at CULLIDE proceeds in two passes like this: The algorithm uses occlusion queries to test if an object is "fully visible" or not. To test if the object B is fully visible against A, CULLIDE renders A into the frame buffer. Next, it sets the depth function to GL GEQUAL and disables depth writes. The object B is rendered using an occlusion query(nv_occlusion_query). If the pixel pass count returned by occlusion query is zero, then the object B is fully visible against A. Then it goes the other way around meaning that it test if A is fully visible against B. CULLIDE states that A, B don't collide if A and B are fully visible. Now I'm not sure why CULLIDE inverts the depth function like that, my guess is because is the only way to check (given the # of pixel pass) if a primitive is fully visible. Also I'd like to know why it disables the depth write: my guess is that is to prevent the the depth to be updated while testing for the possible occlusion. Thank you Libe

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