OpenGL.WGL.NV.vertex_array_range

OpenGL extension NV.vertex_array_range

This module customises the behaviour of the OpenGL.raw.WGL.NV.vertex_array_range to provide a more Python-friendly API

Overview (from the spec)

The goal of this extension is to permit extremely high vertex processing rates via OpenGL vertex arrays even when the CPU lacks the necessary data movement bandwidth to keep up with the rate at which the vertex engine can consume vertices. CPUs can keep up if they can just pass vertex indices to the hardware and let the hardware "pull" the actual vertex data via Direct Memory Access (DMA). Unfortunately, the current OpenGL 1.1 vertex array functionality has semantic constraints that make such an approach hard. Hence, the vertex array range extension.

This extension provides a mechanism for deferring the pulling of vertex array elements to facilitate DMAed pulling of vertices for fast, efficient vertex array transfers. The OpenGL client need only pass vertex indices to the hardware which can DMA the actual index's vertex data directly out of the client address space.

The OpenGL 1.1 vertex array functionality specifies a fairly strict coherency model for when OpenGL extracts vertex data from a vertex array and when the application can update the in memory vertex array data. The OpenGL 1.1 specification says "Changes made to array data between the execution of Begin and the corresponding execution of End may affect calls to ArrayElement that are made within the same Begin/End period in non-sequential ways. That is, a call to ArrayElement that precedes a change to array data may access the changed data, and a call that follows a change to array data may access the original data."

This means that by the time End returns (and DrawArrays and DrawElements return since they have implicit Ends), the actual vertex array data must be transferred to OpenGL. This strict coherency model prevents us from simply passing vertex element indices to the hardware and having the hardware "pull" the vertex data out (which is often long after the End for the primitive has returned to the application).

Relaxing this coherency model and bounding the range from which vertex array data can be pulled is key to making OpenGL vertex array transfers faster and more efficient.

The first task of the vertex array range extension is to relax the coherency model so that hardware can indeed "pull" vertex data from the OpenGL client's address space long after the application has completed sending the geometry primitives requiring the vertex data.

The second problem with the OpenGL 1.1 vertex array functionality is the lack of any guidance from the API about what region of memory vertices can be pulled from. There is no size limit for OpenGL 1.1 vertex arrays. Any vertex index that points to valid data in all enabled arrays is fair game. This makes it hard for a vertex DMA engine to pull vertices since they can be potentially pulled from anywhere in the OpenGL client address space.

The vertex array range extension specifies a range of the OpenGL client's address space where vertices can be pulled. Vertex indices that access any array elements outside the vertex array range are specified to be undefined. This permits hardware to DMA from finite regions of OpenGL client address space, making DMA engine implementation tractable.

The extension is specified such that an (error free) OpenGL client using the vertex array range functionality could no-op its vertex array range commands and operate equivalently to using (if slower than) the vertex array range functionality.

Because different memory types (local graphics memory, AGP memory) have different DMA bandwidths and caching behavior, this extension includes a window system dependent memory allocator to allocate cleanly the most appropriate memory for constructing a vertex array range. The memory allocator provided allows the application to tradeoff the desired CPU read frequency, CPU write frequency, and memory priority while still leaving it up to OpenGL implementation the exact memory type to be allocated.

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