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• Analytic Lights — RenderMan XPU now supports most lighting features, including IES profiles and light temperature, giving artists a key and shot lighting toolset of production complexity. Mesh lights will be coming in a future release.
  
• Light Linking — A lighter’s bread and butter! Light and shadow linking is now fully supported in RenderMan XPU, allowing artists to isolate lighting to particular geometry sets for maximum creative control.
  
• Light Filters — XPU now supports all light filters, including gobos and cookies, maximizing artist control for shot lighting. You can light link them too!
  
• Camera Controls — XPU has greatly expanded support for Pixar Camera, including tilt shift, lens aberrations, vignetting, split diopter, shutter controls, and many other features which have made Pixar’s camera system a comprehensive cinematography tool.
• Interactivity — Progressive Pixels, a feature which displays fractional iterations for improved interactivity in XPU, can now be dialed in, allowing artists to find the sweet spot between speed and fidelity for their given project, be it a single asset, tens of volumes, or a citiscape.
• Adaptive Sampling — XPU can now render images to an acceptable variance metric instead of fully converging to a given sample count, greatly speeding up render times.
• Light Selection — Scenes with many hundreds of lights can now be efficiently rendered in XPU, significantly expanding interactivity for layout, as well as key and shot lighting.

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• XPU Guide - Overview of XPU in RenderMan.
• Shader & Look Development with XPU - Details on lookdev with XPU.
• Features & Limitations - Understand the differences between XPU and RIS and known limitations.
  
  
  

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• Adaptive sampling has been implemented in XPU.  The workflow and controls are the same as in RIS: PixelVariance, "darkfalloff", "exposurebracket", "adaptall", and per-channel "relativepixelvariance". There are two "adaptivemetric"s: "contrast" and "variance", with the latter being the default. RMAN-20888 

  Warning

  When rendering with adaptive sampling enabled using the GPU, there are known stability issues that cause crashes. On machines configured with virtualized Nvidia GPUs using GRID, these crashes can trigger a hypervisor driver crash that requires a hypervisor restart. This does not happen when rendering on the CPU only or with adaptive sampling disabled. These issues have already been addressed for the next dot release.  You can work around this issue by turning off adaptive sampling by setting minsamples and maxsamples to the same value, or by setting PixelVariance to 0.0.  RMAN-22085

• An issue where using Render region in Solaris in XPU mode could lead to a crash has been addressed. RMAN-20929
• An issue where using Render region in Solaris in XPU mode could lead to a crash has been addressed. RMAN-20929
• XPU will fall back to CPU only mode if the variant is "xpu" and no capable GPU hardware is detected. XPU will fall back to CPU only mode if the variant is "xpu" and no capable GPU hardware is detected. RMAN-21526
• Multiple crash issues surrounding switching between RIS and XPU in Solaris have been addressed.
• The amount of memory required to compile PTX shaders has been reduced, lowering GPU VRAM requirements significantly in some cases.

• Eliminated "IMAGE_REL_AMD64_ADDR32NB relocation requires an ordered section layout." errors that could lead to crashes. RMAN-21782

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• macOS/Apple Silicon: The denoiser is not supported on Apple Silicon, even with Rosetta.  RMAN-20408
• If your image has NaNs from either RIS or XPU, the denoiser will fail on macOS and Windows. 
• The interactive denoiser currently is not supported in XPU.

XPU

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• supported in XPU.

XPU

• The GPU portion of XPU can render artifacts with trace:reflectexcludesubset.  RMAN-20393
• Support for homogeneous single scattering in PxrSurface is incomplete. Renders will be noticeably incorrect after the first bounce off the interior of a closed object. RMAN-16374
• Lights inside objects with homogeneous single scattering in PxrSurface will not compute the correct shadows. RMAN-16367
• Due to differences in how XPU computes derivatives for normals, XPU may compute different results from RIS for bump mapped subdivision surfaces in areas of highly concave curvature, combined with a high bump scale. This happens especially when the bump mapping cannot reasonably approximate the displacement. Note that this difference does not occur with displacement. The issue can be ameliorated by reducing the bump scale. RMAN-21977 
• Please refer to the XPU section of the documentation for a more detailed current list of limitations.

RIS and XPU

• Color Management: Although we have upgraded to the VFX Reference Platform 2023, there is an outstanding issue with the Image Tool described below.

• Color Management: Although we have upgraded to the VFX Reference Platform 2023, there are still color management limitations:
• MaterialX: Material layering is not supported.  RMAN-20365

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• Shading
  
  • PxrUnified integrator does not yet support all the standard rendering features.
  • <primstr:nameofvalue> substitution of data from a constant primvar or user attribute on an object for dynamic filename substitution is not yet working for the gettextureinfo() OSL call.
  • Using the ' . ' character in the handle for an OSL shader could cause unpredictable results during re-rendering.
  • The RixTexture::TextureDerivatives calls always returns RixTexture::InvalidFile for non-texture-atlas files. This will be restored in 26.1.
• General rendering
  • Load on demand procedurals are not supported anymore, all procedurals are now loaded immediately.
  • RenderMan does not read point data from OpenVDB files.
  • Per-Instance baking is not supported, only the reference instance.
  • PxrBakePointCloud cannot directly render ptex.
  • Sample + Display filter plug-ins do not have access to lighting services for light dependent effects, e.g. lens flare.
  • Adding new mesh light on existing geometry during IPR results in double geometry.
  • Motion blurred polygons do not motion blur normals when deformed. Use Subdivision meshes instead.
  • When attempting to access an array primvar, you must first check the size of the array primvar and allocate the appropriate space. Not doing so may lead to a crash.
  • Points and curves cannot be used as geometric lights.
  • Analytical lights placed inside non-aggregate volumes may yield artifacts when made visible to the camera. As a work around, the light camera visibility should be turned off, and a geometry with a similar shape should be used (visible to camera, invisible to transmission and indirect rays), with the proper emissive bxdf.
  • Indirect Path Guiding in the PxrUnified integrator causes a crash.
• Bridge Products:
  • RfH: Soloing MaterialX Lama nodes in complex shading networks can give an incorrect result.

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