OpenGL.GL.APPLE.fence

OpenGL extension APPLE.fence

This module customises the behaviour of the OpenGL.raw.GL.APPLE.fence to provide a more Python-friendly API

Overview (from the spec)

This extension is provided a finer granularity of synchronizing GL command completion than offered by standard OpenGL, which currently offers only two mechanisms for synchronization: Flush and Finish. Since Flush merely assures the user that the commands complete in a finite (though undetermined) amount of time, it is, thus, of only modest utility. Finish, on the other hand, stalls CPU execution until all pending GL commands have completed forcing completely synchronous operation, which most often not the desired result. This extension offers a middle ground - the ability to "finish" a subset of the command stream, and the ability to determine whether a given command has completed or not.

This extension introduces the concept of a "fence" to the OpenGL command stream with SetFenceAPPLE. Once the fence is inserted into the command stream, it can be tested for its completion with TestFenceAPPLE. Moreover, the application may also request a partial Finish up to a particular "fence" using the FinishFenceAPPLE command -- that is, all commands prior to the fence will be forced to complete until control is returned to the calling process. These new mechanisms allow for synchronization between the host CPU and the GPU, which may be accessing the same resources (typically memory).

Fences are created and deleted, as are other objects in OpenGL, specifically with GenFencesAPPLE and DeleteFencesAPPLE. The former returns a list of unused fence names and the later deletes the provided list of fence names.

In addition to being able to test or finish a fence this extension allows testing for other types of completion, including texture objects, vertex array objects, and draw pixels. This allows the client to use TestObjectAPPLE or FinishObjectAPPLE with FENCE_APPLE, TEXTURE, VERTEX_ARRAY, or DRAW_PIXELS_APPLE with the same type of results as TestFenceAPPLE and FinishFenceAPPLE. Specifically, using the FENCE_APPLE type is equivalent to calling TestFenceAPPLE or FinishFenceAPPLE with the particular fence name. Using TEXTURE as the object type tests or waits for completion of a specific texture, meaning when there are no pending rendering commands which use that texture object. Using the VERTEX_ARRAY type will test or wait for drawing commands using that particular vertex array object name. Finally, DRAW_PIXELS_APPLE will wait or test for completion of all pending DrawPixels commands. These tests and finishes operate with the same limitations and results as test and finish fence.

One use of this extension is in conjunction with APPLE_vertex_array_range to determine when graphics hardware has completed accessing vertex data from a vertex array range. Once a fence has been tested TRUE or finished, all vertex indices issued before the fence must have completed being accessed. This ensures that the vertex data memory corresponding to the issued vertex indices can be safely modified (assuming no other outstanding vertex indices are issued subsequent to the fence).

The official definition of this extension is available here: http://www.opengl.org/registry/specs/APPLE/fence.txt