OpenGL extension NV.pixel_data_range
This module customises the behaviour of the
OpenGL.raw.GL.NV.pixel_data_range to provide a more
Overview (from the spec)
The vertex array range extension is intended to improve the
efficiency of OpenGL vertex arrays. OpenGL vertex arrays' coherency
model and ability to access memory from arbitrary locations in memory
prevented implementations from using DMA (Direct Memory Access)
Many image-intensive applications, such as those that use dynamically
generated textures, face similar problems. These applications would
like to be able to sustain throughputs of hundreds of millions of
pixels per second through DrawPixels and hundreds of millions of
texels per second through TexSubImage.
However, the same restrictions that limited vertex throughput also
limit pixel throughput.
By the time that any pixel operation that reads data from user memory
returns, OpenGL requires that it must be safe for the application to
start using that memory for a different purpose. This coherency
model prevents asynchronous DMA transfers directly out of the user's
There are also no restrictions on the pointer provided to pixel
operations or on the size of the data. To facilitate DMA
implementations, the driver needs to know in advance what region of
the address space to lock down.
Vertex arrays faced both of these restrictions already, but pixel
operations have one additional complicating factor -- they are
bidirectional. Vertex array data is always being transfered from the
application to the driver and the HW, whereas pixel operations
sometimes transfer data to the application from the driver and HW.
Note that the types of memory that are suitable for DMA for reading
and writing purposes are often different. For example, on many PC
platforms, DMA pulling is best accomplished with write-combined
(uncached) AGP memory, while pushing data should use cached memory so
that the application can read the data efficiently once it has been
read back over the AGP bus.
This extension defines an API where an application can specify two
pixel data ranges, which are analogous to vertex array ranges, except
that one is for operations where the application is reading data
(e.g. glReadPixels) and one is for operations where the application
is writing data (e.g. glDrawPixels, glTexSubImage2D, etc.). Each
pixel data range has a pointer to its start and a length in bytes.
When the pixel data range is enabled, and if the pointer specified
as the argument to a pixel operation is inside the corresponding
pixel data range, the implementation may choose to asynchronously
pull data from the pixel data range or push data to the pixel data
range. Data pulled from outside the pixel data range is undefined,
while pushing data to outside the pixel data range produces undefined
The application may synchronize with the hardware in one of two ways:
by flushing the pixel data range (or causing an implicit flush) or by
using the NV_fence extension to insert fences in the command stream.
The official definition of this extension is available here:
|EXTENSION_NAME = 'GL_NV_pixel_data_range'|
GL_READ_PIXEL_DATA_RANGE_LENGTH_NV = GL_READ_PIXEL_DATA_RANGE_LENGTH_NV
GL_READ_PIXEL_DATA_RANGE_NV = GL_READ_PIXEL_DATA_RANGE_NV
GL_READ_PIXEL_DATA_RANGE_POINTER_NV = GL_READ_PIXEL_DATA_RANGE_POINTER_NV
GL_WRITE_PIXEL_DATA_RANGE_LENGTH_NV = GL_WRITE_PIXEL_DATA_RANGE_LENGTH_NV
GL_WRITE_PIXEL_DATA_RANGE_NV = GL_WRITE_PIXEL_DATA_RANGE_NV
GL_WRITE_PIXEL_DATA_RANGE_POINTER_NV = GL_WRITE_PIXEL_DATA_RANGE_POINTER_NV
__file__ = '/home/mcfletch/pylive/OpenGL/GL/NV/pixel_data_range.pyc'
__name__ = 'OpenGL.GL.NV.pixel_data_range'
__package__ = 'OpenGL.GL.NV'
glFlushPixelDataRangeNV = <OpenGL.platform.baseplatform.glFlushPixelDataRangeNV object at 0x7976e10>
glPixelDataRangeNV = <OpenGL.platform.baseplatform.glPixelDataRangeNV object at 0x7976d50>