Proposal: a stereo camera contract for offline / path-traced renderers (Leia SR tested)

[joehillthunder]

Problem. Film path tracers have no OpenXR mode and render offline; Karma, RenderMan, Arnold and Cycles are examples. They can still make stereo frames for a DisplayXR display, but their depth comfort is fixed when the frames are rendered, and nothing tells the renderer the display's geometry or budget. Hand-tuned stereo rigs either look flat or go past comfort, and the result depends on the window the frames end up in.

Code: private repo, commit 4f3c2dc (Apache-2.0 + NOTICE; internals in python/dxr_stereo_rig/coupling.py and native/core/coupling.h). Access on request.

Target class: dedicated 3D window

• A small player binds its own window with XR_DXR_win32_window_binding and weaves alongside other 3D apps. Full panel is via F11 only.
• It uses no overlay embed and no DXR_LEGACY_CAMERA_RIG.
• When the texture-mode contract lands, the same pair can be drawn into a host's viewer, such as a DCC's render view.

Proposal

1. A renderer-neutral rig, dxr_stereo_rig/1.
   • Parallel, off-axis eye cameras, written as USD /DXR_Rig/Cam_L|R with horizontalApertureOffset, plus the same rig as JSON.
   • The rig is solved for the depth the camera sees, using the true-geometry model p(z) = K·(1 − g/z).
   • The glass goes where the front and behind limits meet, g* = (f+b)/(b/near + f/far). K is filled to the budget, and the camera is never moved.
   • Panel width, pixels and viewing distance come from display info (displayxr-cli status --json: layout.width_m, nominal_viewer_m).
2. The view width is part of the contract. Frames are rendered for the window width they'll be shown at, and the player never shows them wider. The budget is physical, so frames sized for a window would exceed it at full panel.
3. Controls after rendering.
   • Focus is a horizontal image shift, clamped to comfort.
   • Focus follows the shared depth slider (DXR_HostedDepthRel) and reports the value in effect in DXR_HostedDepthRelNow.
   • Depth needs a re-render. An interactive preview does that automatically: set_eye_cameras / progressive_pass / cancel.
4. Status text reads "3D active" / "3D paused".

What the shared host library needs to provide

• One windowed 3D-window session that accepts a pre-rendered stereo pair (RGBA, any rate) and keeps weaving the latest pair at display rate. Today each in-process adapter carries its own copy.
• The window's physical size and per-eye render size, for letterboxing to the rendered view width.
• The shared-slider plumbing (strength and focus in, values in effect out), exposed so a player can map focus to an image shift.
• The comfort budget from display info (Display info: publish the comfort budget (front/behind disparity limits), e.g. layout.comfort {front_frac, behind_frac} #2388). Today every adapter hard-codes 0.14 % / 0.42 % of panel width. Here they sit in one function, so a published budget is a one-line swap.

Evidence

Cycles is the stand-in renderer, and Leia SR is the tested display.

• Rendered pairs, block-matched against the model: slope 0.96–0.99, median error ≤ 0.005 % of panel width. An asset measured −0.126 / +0.414 %, against a budget of −0.14 / +0.42 %.
• On the display: a 2268 px window measured −0.112 / +0.375 %, against −0.130 / +0.390 % expected at the width shown.
• USD cameras: they reproduce the model through Gf.Camera frusta, so other USD renderers read the convention the same way.
• Shared slider: a focus beyond comfort was held at the limit and reported back.
• Concurrent weaving: the player's window and another app's 3D window wove at the same time (runtime logs).

Notes

• Python-hosted apps all share the exe name python, so the per-exe atlas trigger collides between them. This adds to Proposal: per-instance atlas capture trigger (displayxr_atlas_trigger.<exe>.<pid>) — two copies of one app collide under windowed weaving #2351.
• An app that submits eye images at window size gets VIEW SIZE MISMATCH every frame when the display's view scale is below 1. It still weaves, and resampling to the recommended size clears the warning. It's worth a line in the docs for pre-rendered-pair apps.

Pending: a seated check on the display.

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