Proposal: DisplayXR for SideFX Houdini (HDK adapter on the shared host core, dedicated 3D window; Leia SR tested)

[joehillthunder]

Houdini has no OpenXR mode, so it can't run as a hosted legacy app. It runs as a thin in-process HDK adapter in the dedicated-3D-window class, on the same app-neutral host core as the MotionBuilder (#2423) and Nuke adapters.

Code: joehillthunder/displayxr-houdini @ 553494d (private, Apache-2.0 + NOTICE; internals in src/core/coupling.h). Tested on Houdini 22.0.466 (Apprentice), Windows 11, Leia SR as the tested display.

What it does

• Start 3D opens the core's own window over the Scene Viewer viewport, bound with XR_DXR_win32_window_binding. It weaves there next to other 3D apps by construction; F11 gives the full panel. Status text reads "3D active" / "3D off".
• Following the viewport: the eyes follow the active Scene Viewer camera, with the glass at the view pivot.
• Depth:
  • The true-geometry rig takes panel size and nominal viewing distance from XR_DXR_display_info.
  • The comfort budget (0.14 % front / 0.42 % behind of panel width) is a named constant until display info publishes it (Display info: publish the comfort budget (front/behind disparity limits), e.g. layout.comfort {front_frac, behind_frac} #2388).
  • The depth range fitted to the budget comes from the renderer's depth buffer.
  • Depth runs 0–200 %, clamped to comfort unless the user opts in. The shared depth slider works through DXR_HostedDepthRel.
• Omniverse: "Send to Omniverse (USD)" writes Kit's layout: Y-up, cm, /World defaultPrim, remapped paths, relative textures. It passes 28/28 tests and usdchecker. Under the Apprentice licence, Houdini's USD ROP writes .usdnc, so the tool refuses with a clear message rather than working around the licence.
• Generated content: an Image → 3D shelf tool references generated assets (see Proposal: generated 3D content in DisplayXR apps (image→3D asset bridge with comfort-budget placement) #2391).

Measured

• Panel, runtime v2.55.0: "3D active" held for 30 s while a second DisplayXR app wove at the same time (510–651 frames), followed by a clean stop.
• sim-display atlas:
  • front −0.137 %, behind +0.070 %, glass −0.002 %, matching the budget for the visible content;
  • 50 % depth halves the disparity (0.204 → 0.112 % behind on a deeper scene);
  • 200 % clamps to the budget.

Renderer caveat

The eyes are rendered with Karma (XPU when the GPU is free, otherwise CPU), because Houdini VK's HUSD_Imaging::render() fails offscreen. So the 3D updates only about 1–2 times per second while the camera moves. It's fine for review, but not yet viewport speed.

What the shared host library needs (from this adapter)

1. A frame hand-off decoupled from the host's render rate: an eye pair plus the eyes it was rendered from, which the core already does with a serial.
2. Texture mode for a Vulkan host. Houdini 22's viewport is Vulkan-only, so in-viewport 3D needs the woven texture back in a Vulkan context, e.g. importing the shared handle with VK_KHR_external_memory_win32. An offscreen Vulkan render path in Houdini would remove the Karma limit.
3. The comfort budget in display info (Display info: publish the comfort budget (front/behind disparity limits), e.g. layout.comfort {front_frac, behind_frac} #2388).

Platforms

• Windows: yes.
• Linux: blocked. Houdini's viewport there is also Vulkan, but the adapter's host core is Windows/D3D11, and there's no Linux display path for it yet.
• macOS: blocked; there's no display plug-in.

Pending

• Content: a production-quality demo scene. The current test content is placeholder; spheres render faceted in Karma XPU.
• Omniverse: opening the export in USD Composer, which needs a non-Apprentice licence.
• Speed: a viewport-speed eye path.

🤖 Generated with Claude Code

[joehillthunder]

Readiness on the released runtime: Pass (2026-10-28)

Plain houdini.exe with the installed package, on runtime v2.57.1 from Program Files with the Leia SR plug-in 2.19.1.

Check	Result	Conditions
Runtime and plug-in	Pass	runtime from Program Files; weaving with the leia-sr D3D11 weaver
"3D active"	Pass	30 s on Karma XPU beside the core self test, 613 frames
Depth control, live	Pass	50 % halves the behind depth, 0.42 % → 0.21 % of panel width
Clean stop and quit	Pass	—
Seated check at the tested display (Leia SR)	Pass	resolution and latency below target: the eyes render below window resolution, and Karma refines progressively. A fix is in progress.

Not tested: Send to Omniverse (needs an Indie or commercial licence; Apprentice writes .usdnc).