OpenGL extension NV.occlusion_query
This module customises the behaviour of the OpenGL.raw.GL.NV.occlusion_query to provide a more Python-friendly API
Overview (from the spec)
The HP_occlusion_test extension defines a mechanism whereby an application can query the visibility of an object, where "visible" means that at least one pixel passes the depth and stencil tests.
The HP extension has two major shortcomings.
  • It returns the result as a simple GL_TRUE/GL_FALSE result, when in fact it is often useful to know exactly how many pixels passed.
  • It provides only a simple "stop-and-wait" model for using multiple queries. The application begins an occlusion test and ends it; then, at some later point, it asks for the result, at which point the driver must stop and wait until the result from the previous test is back before the application can even begin the next one. This is a very simple model, but its performance is mediocre when an application wishes to perform many queries, and it eliminates most of the opportunites for parallelism between the CPU and GPU.
This extension solves both of those problems. It returns as its result the number of pixels that pass, and it provides an interface conceptually similar to that of NV_fence that allows applications to issue many occlusion queries before asking for the result of any one. As a result, they can overlap the time it takes for the occlusion query results to be returned with other, more useful work, such as rendering other parts of the scene or performing other computations on the CPU.
There are many situations where a pixel count, rather than a boolean result, is useful.
  • If the visibility test is an object bounding box being used to decide whether to skip the object, sometimes it can be acceptable, and beneficial to performance, to skip an object if less than some threshold number of pixels could be visible.
  • Knowing the number of pixels visible in the bounding box may also help decide what level of detail a model should be drawn with. If only a few pixels are visible, a low-detail model may be acceptable. In general, this allows level-of-detail mechanisms to be slightly less ad hoc.
  • "Depth peeling" techniques, such as order-independent transparency, would typically like to know when to stop rendering more layers; it is difficult to come up with a way to determine a priori how many layers to use. A boolean count allows applications to stop when more layers will not affect the image at all, but this will likely be unacceptable for performance, with minimal gains to image quality. Instead, it makes more sense to stop rendering when the number of pixels goes below a threshold; this should provide better results than any of these other algorithms.
  • Occlusion queries can be used as a replacement for glReadPixels of the depth buffer to determine whether, say, a light source is visible for the purposes of a lens flare effect or a halo to simulate glare. Pixel counts allow you to compute the percentage of the light source that is visible, and the brightness of these effects can be modulated accordingly.
The official definition of this extension is available here: