Viewing file:  draw_elements_base_vertex.py (2.79 KB) -rw-r--r--
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'''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
'''
from OpenGL import platform, constants, constant, arrays
from OpenGL import extensions, wrapper
from OpenGL.GL import glget
import ctypes
from OpenGL.raw.GL.ARB.draw_elements_base_vertex import *
### END AUTOGENERATED SECTION
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