All kernels take offset/length/stride to apply subdivision partially in each vertex elements.
Also the offset can be used for client-based VBO aggregation, without modifying index buffers.
This is useful for topology sharing, in conjunction with glDrawElementsBaseVertex etc.
However, gregory patch shader fetches vertex buffer via texture buffer, which index should also
be offsetted too. Although gl_BaseVertexARB extension should be able to do that job, it's a
relatively new extension. So we use OsdBaseVertex() call to mitigate the compatibility
issue as clients can provide it in their way at least for the time being.
Important notice: all client shader code must have following functions and compose them to osd intrinsic shaders (vertex/tessEval/tessControl)
mat4 OsdModelViewMatrix()
mat4 OsdProjectionMatrix()
mat4 OsdModelViewProjectionMatrix()
float OsdTessLevel()
int OsdGreogryQuadOffsetBase()
int OsdPrimitiveIdBase()
We probably should write a utility class for basic binding of them, to make client code simpler.
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