primitive variables

Simon Bunker <si...@...>

Just wondering if you have thought about binding shader parameters to
geometry like you can with primitive variables in RenderMan.
Specifically - if you are going to do something like this - it would
be good to have a better approach than RenderMan. The two areas that
always cause issues for me:

- working out if it exists at all. In RenderMan eg for a rest position
variable (eg Pref) you have to have the variable available to be
filled in, but it may not be on a primitive. It is quite hard to
determine if it is the default value, or if it has been assigned by
the primitive. I think Houdini VEX does have a shadeop to tell you

- primitive variable space. RenderMan tries to be too clever by doing
an implicit object-> current transformation. Often this is not what
you want. This should either have a flag on the geometry definition to
say what space it is in, or leave it alone and leave it up to the
shader writer.

I think it is really important to have this functionality in place to
make shader writers lives easier!



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