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• There is no support for custom C++ plugins. Patterns written in OSL are supported
• Dynamic String construction in your own patterns is not supported
• It is possible in some cases that strings within OSL patterns are not properly constant folded, resulting in errors at runtime
• Many "builtin" variables that exist in RIS do not exist in XPU. 
• PxrSeExpr is not supported
• PxrDirt & PxrCurvature require the trace() OSL call, which is not yet supported. Support for a single level of recursion will be added in a future release.
• Point clouds are not supported
• PxrBakeTexture and PxrBakePointCloud are not supported
• XPU samples textures at one MIP level coarser than RIS so that it may store more textures in limited GPU memory.  This results in slightly blurrier renders from XPU than RIS.

• We are currently in the process of updating the texture cache for the GPU to give better performance if it isn't large enough to hold the textures required for your render.

OSL

• In XPU, the first argument to the OSL getbuiltin() shadeop is ignored. The distinction between "primvar", "builtin", and "attribute" that exists in RIS does not exist in XPU.
• Many builtins that are supported in RIS are not supported in XPU.
• trace() is not currently supported
• Dynamic path construction in your own patterns is not supported, but please note that we do support the following path tokens <UDIM>, <u>, <v>, <U>, <V> and <primstr:varname> that will result in a "dynamic" path to your textures.  <primstr:varname> can be used to reference the value in a constant primvar or user attribute to build a dynamic path.  Also note that <primstr:varname> is not in beta1 for XPU, but will be in a later beta.
• XPU samples textures at one MIP level coarser than RIS so that it may store more textures in limited GPU memory.  This results in slightly blurrier renders from XPU than RIS.
• We are currently in the process of updating the texture cache for the GPU to give better performance if it isn't large enough to hold the textures required for your render.

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Lighting

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