Below is a list of features supported and unsupported in this phase one release of XPU

General

• Behavior of XPU when it runs out of memory on your GPU:  XPU will crash if it runs out of memory on your GPU.  If you are running XPU inside of Maya, Houdini, or Blender, this will bring down your application as well.  Be sure to save often.  We are going to work to mitigate the two biggest causes of this condition.
• Releasing a license:  It is possible to get XPU into a situation where it doesn't release the license back to the RenderMan license server.  If this happens and you end up using all of your licenses, just restart your application (Maya, Houdini, Katana, or Blender) and you will be able to continue your work.

DCCS

• Warnings on the console when rendering to XPU from Katana.  There are some warnings you will see on the console about a bad DSO being found for the socket display driver.  You can ignore these.
• Picking from "it" does not relay the selection back to RenderMan for Maya
• RenderMan for Maya does not yet support batch renderinng to XPU

Shading

• Custom Bxdf or Displacement plugins are unsupported.
• Lama support will be implemented in a future release.
• In XPU, we support the following SSS models:
  • Burley Normalized
  • Exponential Path Traced
  • Non-Exponential Path Traced
• subsurfaceSubset is not yet supported.

Patterns & Textures

• There is no support for custom C++ plugins. Patterns written in OSL are supported.
• PxrSeExpr is not supported.
• PxrDirt & PxrCurvature require the trace() OSL call, which is not yet supported.
  
• 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. You may continue to specify a first argument in order for your shader to remain compatible with RIS, but the distinction between "primvar", "builtin", and "attribute" that exists in RIS does not exist in XPU.
• getattribute() calls asking for geometry primvars (in RIS, getattribute("primvar")) are fully supported.

• getattribute() calls targeting "builtins" (renderer computed quantities that are not directly geometry primvars) are partially supported. Some builtins known to RIS are not currently available in XPU. The most important changes are summarized in the table below. If an alternative is suggested, it means that OSL patterns should be rewritten to use the alternative to be as forward looking as possible.

  Builtin	Supported in XPU	Alternative (if possible)
  P	✅	global P may be preferred
  PRadius	❌
  Po	✅
  Nn	❌	use global N
  Non	✅	supported, but only if displacement took place
  Ngn	❌	use global Ng
  Naon	❌	point Po; getattribute("primvar", "P", Po); normalize(cross(Dx(Po), Dy(Po)));
  Tn	✅
  Vn, VLen	❌
  curvature	❌
  dPdu, dPdv	❌	use globals dPdu and dPdv
  u, v	✅	globals u and v may be preferred
  st	✅
  du, dv	❌	use 0.5 * Dx(u), 0.5 * Dy(v)
  dPdtime	❌
  time	❌
  id, id2	✅

• The only getattribute("attribute", ...) lookups that are supported in XPU are "user" and "identifier" attributes (e.g. getattribute("attribute", "user:foo", foo))
• trace() is not currently supported. Support for a single level of recursion will be added in a future release.
• Dynamic string construction is not supported.
• It is possible in some cases that strings within OSL patterns are not properly constant folded, resulting in errors at runtime.
• 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.

Lighting

• Light Filters & Light Linking are currently not supported.
• "shot" based lighting features will be available in future releases.  Examples include:
  
  • Emission Focus
  • Light groups
  • Manifold Next Event Estimation
• Thin shadows are currently not implemented.
•  XPU does not support color temperature on lights yet. This means the correct color space won’t be applied when a temperature is converted to an RGB value.

Geometry

• Loop Subdivision is not supported. Catmull-Clark for both quads and triangles is supported.
• Point falloff is not yet supported.
• Object attributes such as maxdiffusedepth and maxspeculardepth have not yet been implemented.
• Deformation motion blur is not yet supported.

• Shading of nested instancing has artifacts.  In our tests we have seen shading issues with leaves that are rendered as nested instances.

• Inverted normals are possible when doing displacement, depending on the input file and DCC.  We have seen this with Alembic being imported into Houdini.

Integrators

• numBxdfSamples, numLightSamples, numIndirectSamples (and their manually set counterparts) are not supported.
• clampDepth and clampLuminance (used to suppress fireflies) have not been implemented.
• Russian Roulette has not been implemented.
• allowCaustics is always 1.

Rendering

• XPU currently only outputs EXR or TIFF frames.
• Adaptive sampling is yet to be implemented.
• Motion blur has not been fully implemented. Transformation motion blur is fully functional, but deformation motion blur of geometry is not supported.
• Trace Subsets have not been implemented.  This impacts the subsurfaceSubset feature of PxrSurface.
• Only some geometric built-in AOVs are available under the same name as they are available for RIS.
• Deep Output. Coming soon.
• Multi-camera, multi-frame is not yet supported.
• The built-in curvature calculation different is different to RIS, which results in differences in specular mollification.
• The "weighted" option for the pixelfiltermode parameter to the Hider is not yet supported.
• Baking - either at the pattern level via PxrBakeTexture or at the renderer level to bake global illumination is not yet supported.