OpenGL extension NV.occlusion_query
This module customises the behaviour of the
OpenGL.raw.GL.NV.occlusion_query to provide a more
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:
|EXTENSION_NAME = 'GL_NV_occlusion_query'|
GL_CURRENT_OCCLUSION_QUERY_ID_NV = GL_CURRENT_OCCLUSION_QUERY_ID_NV
GL_PIXEL_COUNTER_BITS_NV = GL_PIXEL_COUNTER_BITS_NV
GL_PIXEL_COUNT_AVAILABLE_NV = GL_PIXEL_COUNT_AVAILABLE_NV
GL_PIXEL_COUNT_NV = GL_PIXEL_COUNT_NV
__file__ = '/home/mcfletch/pylive/OpenGL/GL/NV/occlusion_query.pyc'
__name__ = 'OpenGL.GL.NV.occlusion_query'
__package__ = 'OpenGL.GL.NV'
glBeginOcclusionQueryNV = <OpenGL.platform.baseplatform.glBeginOcclusionQueryNV object at 0x78f6f90>
glDeleteOcclusionQueriesNV = <OpenGL.platform.baseplatform.glDeleteOcclusionQueriesNV object at 0x7976410>
glEndOcclusionQueryNV = <OpenGL.platform.baseplatform.glEndOcclusionQueryNV object at 0x79765d0>
glGenOcclusionQueriesNV = <OpenGL.platform.baseplatform.glGenOcclusionQueriesNV object at 0x79763d0>
glGetOcclusionQueryivNV = <OpenGL.platform.baseplatform.glGetOcclusionQueryivNV object at 0x7976650>
glGetOcclusionQueryuivNV = <OpenGL.platform.baseplatform.glGetOcclusionQueryuivNV object at 0x79766d0>
glIsOcclusionQueryNV = <OpenGL.platform.baseplatform.glIsOcclusionQueryNV object at 0x7976510>