USD Viewer (Kit 108): Make building “X-ray” semi-transparent via Session Layer (displayOpacity not affecting RTX)

Omniverse Core Platform
1

Hello,

I’m developing a custom extension on Omniverse USD Viewer (Kit 108.x) and trying to implement an “X-ray mode” for a building in a USD stage.

Goal

  • When X-ray is ON:

    • The building becomes semi-transparent (e.g., opacity 0.1–0.2) and optionally tinted (cyan)

    • This must be done non-destructively using the Session Layer (so the original USD is untouched)

  • When X-ray is OFF:

    • All overrides are removed and the stage returns to the exact original look (materials, colors, opacity)

  • X-ray image

Current Stage Structure (example)

  • /root/World/Building/inside

  • /root/World/Building/outside

  • (sometimes I toggle /root/World/Building/wired visibility too)

What I tried

  1. Session Layer override using primvars:displayColor / primvars:displayOpacity

  • I set UsdGeom.PrimvarsAPI(prim).CreatePrimvar("displayOpacity", ...) (or imageable.CreateDisplayOpacityAttr()) in the Session Layer.

  • Result:

    • displayColor affects the appearance slightly

    • displayOpacity does NOT make it transparent (under RTX viewport in USD Viewer)

  1. Removing material bindings in Session Layer

  • Tried removing material:binding / material:binding:preview properties in Session Layer for the target prim(s).

  • Still could not reliably get semi-transparency.

Environment

  • App: USD Viewer (Kit 108.x)

  • Renderer: RTX

  • Platform: Windows 11

  • GPU: RTX 3090

Questions

  1. In USD Viewer with RTX, is displayOpacity expected to affect actual transparency?
    If not, what is the recommended approach?

  2. What is the best practice to implement an X-ray mode non-destructively?

    • Should I create a dedicated X-ray material (e.g., UsdPreviewSurface/MDL) and bind it with bindingStrength=strongerThanDescendants to override all child materials?

    • If so, what is the most reliable way to:

      • Override all descendants (including GeomSubsets)

      • Remove the override cleanly on OFF (restore original bindings)

  3. Are there known settings/flags in USD Viewer / Kit 108 that prevent transparency from showing (e.g., alpha handling, sorting, post-processing)?

Any references or example code (Kit 108 / Python) would be appreciated.

Thanks!

2

In USD Viewer / Kit 108 with RTX, displayOpacity is not used by the RTX material pipeline, so setting primvars:displayOpacity in the Session Layer will not make geometry visually transparent in the RTX viewport. For an X‑ray mode you need to override the actual material (UsdPreviewSurface/MDL) in a non‑destructive layer rather than relying on display attributes.developer.nvidia+1​

Why displayOpacity does not work

  • displayColor / displayOpacity are viewport hint attributes primarily respected by simple preview renderers (e.g., Storm). RTX in Omniverse shades from bound materials (MDL, UsdPreviewSurface, etc.) and ignores displayOpacity for real transparency.sidefx+1​
  • As long as a material with its own opacity settings is bound, RTX will use that shading network and not the displayOpacity primvar, so the building remains opaque.omniverse.nvidia+1​

Recommended X‑ray workflow (non‑destructive)

The usual pattern is:

  • Create a dedicated X‑ray material (either MDL or UsdPreviewSurface) with:
    • Base color = cyan (or your tint).
    • opacity in the shader set to ~0.1–0.2 and opacityThreshold set appropriately so transparent blending works.sidefx+1​
  • In the Session Layer, bind this material to the building root with stronger strength so it overrides all descendants:
    • Set material:binding (and, if needed, material:binding:preview) on /root/World/Building with bindingStrength = "strongerThanDescendants".omniverse.nvidia+1​
    • Because of USD’s material binding rules, this one opinion will override child bindings and GeomSubset bindings for visual shading.omniverse.nvidia​
  • When X‑ray is turned OFF:
    • Remove those binding opinions from the Session Layer (clear the material:binding* attributes or deactivate the Session Layer), and the composed stage falls back to the original bindings and appearance.omniverse.nvidia+1​

This gives:

  • Fully non‑destructive behavior (all edits live only in the Session Layer).omniverse.nvidia​
  • A single, predictable material controlling color and opacity for the entire building, independent of how many materials or GeomSubsets exist underneath.developer.nvidia+1​

Implementation tips (Kit 108 / Python)

High‑level steps in your extension:

  • On X‑ray ON:
    • Ensure Session Layer is the edit target.
    • Create or reference the X‑ray material prim (e.g., under /Looks/XRay_Mat).
    • On /root/World/Building, author:
      • material:binding = </Looks/XRay_Mat>
      • material:binding:preview = </Looks/XRay_Mat> (if you use preview)
      • material:binding:collection:bindingStrength = "strongerThanDescendants" (or equivalent attribute, depending on API helper).developer.nvidia+1​
  • On X‑ray OFF:
    • In the Session Layer, clear these binding properties on /root/World/Building. Once removed, the original bindings from weaker layers resume control.omniverse.nvidia​

There are no special RTX flags in USD Viewer 108 you need to flip beyond having a correctly transparent material; just make sure the material’s own opacity is below 1, and that you are not using debug settings like “Hide Geometry That Uses Opacity” in the RTX renderer options.omniverse.nvidia​

  1. USD Viewer (Kit 108): Make building “X-ray” semi-transparent via Session Layer (displayOpacity not affecting RTX)
  2. RTX - Common — Omniverse Materials and Rendering
  3. UsdPreviewSurface use alpha of UsdUVTexture as opacity input | Forums | SideFX
  4. material-binding — Requirement Documentation
  5. Layers — Omniverse Extensions
  6. Lights — Omniverse Materials and Rendering
  7. https://ppl-ai-file-upload.s3.amazonaws.com/web/direct-files/attachments/images/95700674/0df6635b-7f07-4596-89c1-e64039ff3ffd/image.jpg
  8. Kit 108.0 Release Notes — Omniverse Developer Guide
  9. https://www.facebook.com/groups/OctaneRender/posts/2570991036411292/
  10. Core Platform - NVIDIA Developer Forums
  11. https://www.youtube.com/watch%3Fv=cjGawB63cq4
  12. https://www.youtube.com/watch%3Fv=JX8c6xmra1g
  13. Kit 108.1 Release Notes — Omniverse Developer Guide
  14. https://www.facebook.com/groups/OctaneRender/posts/3014542538722804/
  15. https://www.youtube.com/watch?v=mUsfSD241Ck
  16. USD Connecting non-existent opacity texture | Forums | SideFX