The first phase of XPU delivers a complete feature set for look development. However, some features are not supported yet but will be as XPU matures to replace RIS.  Here is a high-level summary of those features supported currently in XPU

...

General

• XPU currently requires a CUDA-compatible GPU to be installed, even if the intention is to run only in CPU mode.
• The 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.
• If you have more than one GPU, we allow you to specify which GPU to use with XPU but do not yet support the ability to use more than one GPU at the same time.
• XPU is not yet supported on the Mac.
• XPU does not yet support bokeh and aperture control.

DCCS

• XPU does not currently support the redirection of errors away from the console, which means DCCs will not be able to report XPU errors in their UI.
• 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 supports batch rendering to XPU (PxrPathTracer integrator)
  
  
  

...

• 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.
• Textures with a non square pixel aspect ratio are not supported.If you stop interactive rendering while textures are still being made, sometimes you will get a crash.

Volumes

• In XPU, the expected workflow for heterogeneous volumes is via aggregate volumes. XPU has limited support for non-aggregate volumes: only homogeneous volumes are supported.
• XPU fully supports volume primitives of type "box" and "blobbydso" (ImplicitField plugins); however, VDB volumes are the optimized preferred choice, and non-VDB volumes will incur a cost for upfront internal conversion to VDB.
• Interior volume aggregates will be supported in a future 25.X point releaseare not currently supported.
• Single scattering support in PxrSurface is not complete in XPU.
• Deformation motion blur via velocity is supported, but currently requires specification of the velocity grid directly to impl_openvdb.so as the third string parameter. We will also only support velocity that comes directly from a grid, and has not been computed or altered by arbitrary shading. We anticipate lifting some of these restrictions in a future point release.
• Colored extinction is not correctly supported; we anticipate that this will be fixed in a future 25.X point release.
• Convergence of near-homogeneous volumes may not be on par with RIS; this will be addressed in a future 25.X point release.
• Joint sampling is not yet supported, so the convergence of volumes with complicated lighting that requires joint sampling will not be as performant as RIS on a per sample basis.
• Instancing of volumes is not fully supported.
• Certain editing operations on aggregate volumes are not currently supported; for example, editing the density of a volume material might not correctly update the rendered result in all cases. Restarting the IPR session should fix this.

...

• 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, particularly if a OSL global exists.

  
  

  P		global P may be preferred
  PRadius		use filterwidth() (scaled to radius if needed)
  Po
  Nn		use global N
  Non		supported, but only if displacement took place
  Ngn		use global Ng
  Naon		point Po; getattribute("primvar", "P", Po); Naon = normalize(cross(Dx(Po), Dy(Po)));
  Tn
  Vn, VLen		derive from global I: Vn = -normalize(I); VLen = length(I);
  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	Image Added
  Image Removed	use global dPdtime, but currently hardcoded to (0, 0, 0) (This will be fixed in 25.1)	time		use global time
  id, id2

  
  

  • The only getattribute("attribute", ...) lookups that are supported in XPU are "user" attributes (e.g. getattribute("attribute", "user:foo", foo)) and "user" options.
  • getattribute("context") and getattribute("rendererInfo") queries are not supported in XPU.
  • 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

  
  

...

Post Processing

...

Pipeline

	RIS	XPU	Notes
AOVs and LPEs			Motion blur outputs not supported until 25.1
EXR, TIFF
Deep Output
Holdouts
Advanced DOF controls (bokeh, etc)

...

• XPU currently only outputs EXR or TIFF frames.
• Adaptive sampling is yet to be implemented.
• Only some geometric built-in AOVs are available under the same name as they are available for RIS.The motionFore and motionBack AOVs are not currently supported in 25.0. This means that the denoiser will not be able to take full advantage of cross-frame denoising for XPU rendered images. This will be addressed in the 25.1 release.
• Multi-camera, multi-frame is not yet supported.
• The "weighted" option for the pixelfiltermode parameter to the Hider will not be supported.
• Baking - either at the pattern level via PxrBakeTexture or at the renderer level to bake global illumination is not yet supported.
• Cached presence or opacity is not supported in XPU. XPU renders of transparent surfaces may be sharper as a result (they will not be biased by the micropolygon size).

...