OpenGL extension ARB.occlusion_query
This module customises the behaviour of the
OpenGL.raw.GL.ARB.occlusion_query to provide a more
Overview (from the spec)
This extension defines a mechanism whereby an application can query
the number of pixels (or, more precisely, samples) drawn by a
primitive or group of primitives.
The primary purpose of such a query (hereafter referred to as an
"occlusion query") is to determine the visibility of an object.
Typically, the application will render the major occluders in the
scene, then perform an occlusion query for the bounding box of each
detail object in the scene. Only if said bounding box is visible,
i.e., if at least one sample is drawn, should the corresponding object
The earlier HP_occlusion_test extension defined a similar mechanism,
but it had two major shortcomings.
- It returned the result as a simple GL_TRUE/GL_FALSE result, when in
fact it is often useful to know exactly how many samples were
- It provided 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 opportunities for parallelism between the CPU and GPU.
This extension solves both of those problems. It returns as its
result the number of samples that pass the depth and stencil tests,
and it encapsulates occlusion queries in "query objects" that allow
applications to issue many 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/sample count, rather than a
boolean result, is useful.
- Objects that are visible but cover only a very small number of
pixels can be skipped at a minimal reduction of image quality.
- Knowing exactly how many pixels an object might cover may help the
application decide which level-of-detail model should be used. If
only a few pixels are visible, a low-detail model may be
- "Depth peeling" techniques, such as order-independent transparency,
need to know when to stop rendering more layers; it is difficult to
determine a priori how many layers are needed. A boolean result
allows applications to stop when more layers will not affect the
image at all, but this will likely result in unacceptable
performance. Instead, it makes more sense to stop rendering when
the number of pixels in each layer falls below a given threshold.
- Occlusion queries can replace glReadPixels of the depth buffer to
determine whether (for example) 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
The official definition of this extension is available here:
|EXTENSION_NAME = 'GL_ARB_occlusion_query'|
GL_CURRENT_QUERY_ARB = GL_CURRENT_QUERY_ARB
GL_QUERY_COUNTER_BITS_ARB = GL_QUERY_COUNTER_BITS_ARB
GL_QUERY_RESULT_ARB = GL_QUERY_RESULT_ARB
GL_QUERY_RESULT_AVAILABLE_ARB = GL_QUERY_RESULT_AVAILABLE_ARB
GL_SAMPLES_PASSED_ARB = GL_SAMPLES_PASSED_ARB
__file__ = '/home/mcfletch/pylive/OpenGL/GL/ARB/occlusion_query.pyc'
__name__ = 'OpenGL.GL.ARB.occlusion_query'
__package__ = 'OpenGL.GL.ARB'
glBeginQueryARB = <OpenGL.platform.baseplatform.glBeginQueryARB object at 0x591b510>
glDeleteQueriesARB = OpenGL.lazywrapper.lazy( 'glDeleteQueriesARB' )
glEndQueryARB = <OpenGL.platform.baseplatform.glEndQueryARB object at 0x591b610>
glGenQueriesARB = OpenGL.lazywrapper.lazy( 'glGenQueriesARB' )
glGetQueryObjectivARB = <OpenGL.wrapper.glGetQueryObjectivARB object at 0x591c248>
glGetQueryObjectuivARB = <OpenGL.wrapper.glGetQueryObjectuivARB object at 0x591c3f8>
glGetQueryivARB = <OpenGL.wrapper.glGetQueryivARB object at 0x591c098>
glIsQueryARB = <OpenGL.platform.baseplatform.glIsQueryARB object at 0x591b4d0>