OpenGL.GL.ARB.draw_elements_base_vertex

OpenGL extension ARB.draw_elements_base_vertex

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

Overview (from the spec)

This extension provides a method to specify a "base vertex offset" value which is effectively added to every vertex index that is transferred through DrawElements.

This mechanism can be used to decouple a set of indices from the actual vertex array that it is referencing. This is useful if an application stores multiple indexed models in a single vertex array. The same index array can be used to draw the model no matter where it ends up in a larger vertex array simply by changing the base vertex value. Without this functionality, it would be necessary to rebind all the vertex attributes every time geometry is switched and this can have larger performance penalty.

For example consider the (very contrived and simple) example of drawing two triangles to form a quad. In the typical example you have the following setup:

vertices indices ---------- ----- 0 | (-1, 1) | 0 | 0 | 1 | (-1, -1) | 1 | 1 | 2 | ( 1, -1) | 2 | 2 | 3 | ( 1, 1) | 3 | 3 | ---------- 4 | 0 | 5 | 2 | ----- which is normally rendered with the call

DrawElements(TRIANGLES, 6, UNSIGNED_BYTE, &indices).

Now consider the case where the vertices you want to draw are not at the start of a vertex array but are instead located at offset 100 into a larger array:

vertices2 indices2 ---------- ----- .... 0 | 100 | 100 | (-1, 1) | 1 | 101 | 101 | (-1, -1) | 2 | 102 | 102 | ( 1, -1) | 3 | 103 | 103 | ( 1, 1) | 4 | 100 | .... 5 | 102 | ---------- -----

The typical choices for rendering this are to rebind your vertex attributes with an additional offset of 100*stride, or to create an new array of indices (as indices2 in the example). However both rebinding vertex attributes and rebuilding index arrays can be quite costly activities.

With the new drawing commands introduced by this extension you can instead draw using vertices2 and the new draw call:

DrawElementsBaseVertex(TRIANGLES, 6, UNSIGNED_BYTE, &indices, 100)

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