OpenGL extension ARB.pixel_buffer_object
This module customises the behaviour of the
OpenGL.raw.GL.ARB.pixel_buffer_object to provide a more
Overview (from the spec)
This extension expands on the interface provided by the
ARB_vertex_buffer_object extension (and later integrated into OpenGL
1.5) in order to permit buffer objects to be used not only with vertex
array data, but also with pixel data. The intent is to provide more
acceleration opportunities for OpenGL pixel commands.
While a single buffer object can be bound for both vertex arrays and
pixel commands, we use the designations vertex buffer object (VBO)
and pixel buffer object (PBO) to indicate their particular usage in
a given situation.
Recall that buffer objects conceptually are nothing more than arrays
of bytes, just like any chunk of memory. ARB_vertex_buffer_object
allows GL commands to source data from a buffer object by binding the
buffer object to a given target and then overloading a certain set of
GL commands' pointer arguments to refer to offsets inside the buffer,
rather than pointers to user memory. An offset is encoded in a
pointer by adding the offset to a null pointer.
This extension does not add any new functionality to buffer objects
themselves. It simply adds two new targets to which buffer objects
can be bound: GL_PIXEL_PACK_BUFFER and GL_PIXEL_UNPACK_BUFFER. When a
buffer object is bound to the GL_PIXEL_PACK_BUFFER target, commands
such as glReadPixels pack (write) their data into a buffer object.
When a buffer object is bound to the GL_PIXEL_UNPACK_BUFFER target,
commands such as glDrawPixels and glTexImage2D unpack (read) their
data from a buffer object.
There are a several approaches to improve graphics performance
with PBOs. Some of the most interesting approaches are:
- Streaming texture updates: If the application uses
glMapBuffer/glUnmapBuffer to write its data for glTexSubImage into
a buffer object, at least one of the data copies usually required
to download a texture can be eliminated, significantly increasing
texture download performance.
- Streaming draw pixels: When glDrawPixels sources client memory,
OpenGL says the client memory can be modified immediately after the
glDrawPixels command returns without disturbing the drawn image.
This typically necessitates unpacking and copying the image prior
to glDrawPixels returning. However, when using glDrawPixels with
a pixel pack buffer object, glDrawPixels may return prior to image
unpacking because future modification of the buffer data requires
explicit commands (glMapBuffer, glBufferData, or glBufferSubData).
- Asynchronous glReadPixels: If an application needs to read back a
number of images and process them with the CPU, the existing GL
interface makes it nearly impossible to pipeline this operation.
The driver will typically send the hardware a readback command
when glReadPixels is called, and then wait for all of the data to
be available before returning control to the application. Then,
the application can either process the data immediately or call
glReadPixels again; in neither case will the readback overlap with
the processing. If the application issues several readbacks
into several buffer objects, however, and then maps each one to
process its data, then the readbacks can proceed in parallel with
the data processing.
- Render to vertex array: The application can use a fragment
program to render some image into one of its buffers, then read
this image out into a buffer object via glReadPixels. Then, it can
use this buffer object as a source of vertex data.
The official definition of this extension is available here:
|EXTENSION_NAME = 'GL_ARB_pixel_buffer_object'|
GL_PIXEL_PACK_BUFFER_ARB = GL_PIXEL_PACK_BUFFER_ARB
GL_PIXEL_PACK_BUFFER_BINDING_ARB = GL_PIXEL_PACK_BUFFER_BINDING_ARB
GL_PIXEL_UNPACK_BUFFER_ARB = GL_PIXEL_UNPACK_BUFFER_ARB
GL_PIXEL_UNPACK_BUFFER_BINDING_ARB = GL_PIXEL_UNPACK_BUFFER_BINDING_ARB
__file__ = '/home/mcfletch/pylive/OpenGL/GL/ARB/pixel_buffer_object.pyc'
__name__ = 'OpenGL.GL.ARB.pixel_buffer_object'
__package__ = 'OpenGL.GL.ARB'