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You setup a query object, draw your bounding volumes, then check the results of the query objects to see which volumes had any visible pixels.
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OpenGL Query Objects are the native OpenGL way of performing these tests.Īgain, these automatically work with the hardware's hierarchical Z buffer implementation.
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I'm not personally sure which is the best variant for this use case because of the items below.Ī more general question: In a previous section (and also here), the Occlusion Query term is described as "rendering a simplified bounding-volume of an object and comparing it's depth results to the Z-buffer, returning the amount of pixels that are visible." What functions in OpenGL are associated with this Occlusion Query concept? It's just an octree, but there's already a ton of different approaches to building octrees. Is the Octree structure the same Octree that is used for general frustum culling and rendering? Or is it a specialized Octree made just for the occlusion culling technique?īoth yes and no. explicitly writing depth values in your fragment shader). Though there are some things you might do that would disable that optimization (e.g. There's nothing special you have to do to use them. Note that hierarchical Z buffers are a feature of the hardware on literally all GPUs you're going to care about these days. If the check succeeds, you step into the next mip level and repeat.
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If that fails, you know that every sample in the full mip level will also fail, so you have no reason to check further. You start by reading from the smallest mip level. The lower mip levels allow you to instead query a single sample to determine the depth of a 2x2 region of the next mip level. If you have only the full resolution image, you must query each individual sample in the depth buffer to determine the maximum depth of an area. How does having a "Z-pyramid" contribute? Why do we need multiple resolutions of the Z-buffer? In the book, it's displayed as follows (left side):