OpenGL extension APPLE.vertex_array_range
This module customises the behaviour of the
OpenGL.raw.GL.APPLE.vertex_array_range to provide a more
Overview (from the spec)
This extension is designed to allow very high vertex processing rates which
are facilitated both by relieving the CPU of as much processing burden as
possible and by allowing graphics hardware to directly access vertex data.
Because this extension is implemented as an addition to the vertex array
specification provided by OpenGL 1.1, applications can continue to use
existing vertex submission logic while taking advantage of vertex array
ranges to more efficiently process those arrays.
The vertex array coherency model provided by OpenGL 1.1 requires that
vertex data specified in vertex arrays be transferred from system memory
each time Begin, DrawArrays, or DrawElements is called. Further, OpenGL
1.1 requires that the transfer of data be completed by the time End,
DrawArrays, or DrawElements returns. Both of these requirements are
relaxed by the vertex array range extension. Vertex data may be cached
by the GL so there is no guarantee that changes to the vertex data will
be reflected in following drawing commands unless it is flushed with
FlushVertexArrayRangeAPPLE. The reading of vertex data may be deferred
by the GL so there is no guarantee that the GL will be finished reading
the data until completion is forced by the use of Finish or the APPLE_fence
EnableClientState must be used with the VERTEX_ARRAY_RANGE_APPLE param to
enable vertex array range. Once this is done use of vertex array range
requires the definition of a specific memory range for vertex data through
VertexArrayRangeAPPLE. It is recommended this data be page aligned (4096
byte boundaries) and a multiple of page size in length for maximum
efficiency in data handling and internal flushing, but this is not a
requirement and any location and length of data can be defined as a vertex
array. This extension provides no memory allocators as any convenient
memory allocator can be used.
Once a data set is established, using VertexArrayRangeAPPLE, it can be can
be drawn using standard OpenGL vertex array commands, as one would do
without this extension. Note, if any the data for any enabled array for a
given array element index falls outside of the vertex array range, an
undefined vertex is generated. One should also understand removing or
replacing all calls to vertex array range functions with no-ops or disabling
the vertex array range by disabling the VERTEX_ARRAY_RANGE_APPLE client
state should not change the results of an application's OpenGL drawing.
For static data no addition coherency nor synchronization must be done and
the client is free to draw with the specified draw as it sees fit.
If data is dynamic, thus to be modified, FlushVertexArrayRangeAPPLE should
be used. The command is issued when data has been modified since the last
call to VertexArrayRangeAPPLE or FlushVertexArrayRangeAPPLE and prior to
drawing with such data. FlushVertexArrayRangeAPPLE only provides memory
coherency prior to drawing (such as ensuring CPU caches are flushed or VRAM
cached copies are updated) and does not provide any synchronization with
previously issued drawing commands. The range flushed can be the specific
range modified and does not have to be the entire vertex array range.
Additionally, data maybe read immediately after a flush without need for
further synchronization, thus overlapping areas of data maybe read, modified
and written between two successive flushings and the data will be
To synchronize data modification after drawing two methods can be used. A
Finish command can be issued which will not return until all previously
issued commands are complete, forcing completely synchronous operation.
While this guarantees all drawing is complete it may not be the optimal
solution for clients which just need to ensure drawing with the vertex array
range or a specific range with the array is compete. The APPLE_fence
extension can be used when dynamic data modifications need to be
synchronized with drawing commands. Specifically, if data is to be modified,
a fence can be set immediately after drawing with the data. Once it comes
time to modify the data, the application must test (or finish) this fence to
ensure the drawing command has completed. Failure to do this could result in
new data being used by the previously issued drawing commands. It should be
noted that providing the maximum time between the drawing set fence and the
modification test/finish fence allows the most asynchronous behavior and
will result in the least stalling waiting for drawing completion. Techniques
such as double buffering vertex data can be used to help further prevent
stalls based on fence completion but are beyond the scope of this extension.
Once an application is finished with a specific vertex array range or at
latest prior to exit, and prior to freeing the memory associated with this
vertex array, the client should call VertexArrayRangeAPPLE with a data
location and length of 0 to allow the internal memory managers to complete
any commitments for the array range. In this case once
VertexArrayRangeAPPLE returns it is safe to de-allocate the memory.
An additional option is presented for vertex array ranges, the ability of
the client to hint to OpenGL the type of access and/or use the vertex array
memory is expected to have, by using VertexArrayParameteriAPPLE with either
of the STORAGE_CACHED_APPLE or STORAGE_SHARED_APPLE parameter. By default,
all vertex array ranges are considered to be "shared". These options allow
the tuning of memory handling by OpenGL. "Shared" memory is normally used
for dynamic data that is expected to be modified and is likely mapped to AGP
space for access by the graphics hardware and client. "Cached" memory is
designed to support static data and could be cached into to VRAM to provide
the maximum access bandwidth for the vertex array. Note, these hints can
affect how array flushes are handled and the overhead associated with
flushing the array, it is recommended that data be left as "shared" unless
it really is static and there are no plans to modify it.
To summarize the vertex array range extension provides relaxed
synchronization rules for handling vertex array data allowing high bandwidth
asynchronous data transfer from client memory to graphics hardware.
Different flushing and synchronization rules are required to ensure data
coherency when modifying data. Lastly, memory handling hints are provided
to allow the tunning of memory storage and access for maximum efficiency.
The official definition of this extension is available here:
|EXTENSION_NAME = 'GL_APPLE_vertex_array_range'|
GL_STORAGE_CACHED_APPLE = GL_STORAGE_CACHED_APPLE
GL_STORAGE_SHARED_APPLE = GL_STORAGE_SHARED_APPLE
GL_VERTEX_ARRAY_RANGE_APPLE = GL_VERTEX_ARRAY_RANGE_APPLE
GL_VERTEX_ARRAY_RANGE_LENGTH_APPLE = GL_VERTEX_ARRAY_RANGE_LENGTH_APPLE
GL_VERTEX_ARRAY_RANGE_POINTER_APPLE = GL_VERTEX_ARRAY_RANGE_POINTER_APPLE
GL_VERTEX_ARRAY_STORAGE_HINT_APPLE = GL_VERTEX_ARRAY_STORAGE_HINT_APPLE
__file__ = '/home/mcfletch/pylive/OpenGL/GL/APPLE/vertex_array_range.pyc'
__name__ = 'OpenGL.GL.APPLE.vertex_array_range'
__package__ = 'OpenGL.GL.APPLE'
glFlushVertexArrayRangeAPPLE = <OpenGL.platform.baseplatform.glFlushVertexArrayRangeAPPLE object at 0x77d18d0>
glVertexArrayParameteriAPPLE = <OpenGL.platform.baseplatform.glVertexArrayParameteriAPPLE object at 0x77d1950>
glVertexArrayRangeAPPLE = <OpenGL.platform.baseplatform.glVertexArrayRangeAPPLE object at 0x77d1890>