physics: rigid-body visual polish — bodies fall but render swallows them in default scenes

[havokentity]

Context

User feedback during integration smoke test: "btw rigidbodies, nothing is falling. is this another 'oh i created rigidbodies but theres no real bodies' thing?"

Investigation (Mac, integration HEAD)

Ran A/B on tests/goldens/scenes/phys_rb_smoke.cfg:

Test	Result
phys_enabled 0 vs phys_enabled 1 @ frame 60	11591 pixels differ (3.77%), max delta 407
Isolation render (pt_smoke_skip_csg_seed 1 + pt_smoke_skip_prim_seed 1)	Bodies clearly visible & falling
Frame 4 → 30 → 90 progression	Center body falls from y≈20 → 70 → 110 in screen-space (gravity integrating)

Physics is fully working. Bodies translate under gravity, collisions hit ground, spawns are continuous.

What the user sees in the default render

• 5 rigid bodies (radius 0.3–0.4m) spawned at heights y=4 to y=7
• Render as faint white spheres against bright procedural sky
• Visually swallowed by the cornell_csg seed cluster which renders front-and-center in every default scene
• At first glance "nothing is falling" because the cluster dominates

NOT the same as smoke MVP

Smoke MVP (#180) was a genuinely incomplete optical model (in-scatter only, no Beer-Lambert until opt-in). Rigid bodies are fully integrated — they just lack visual distinction in the default render.

Wave-2 fixes (proposed)

• Colored materials per body type (red for spheres, blue for boxes) so they pop visually
• OR: dedicated phys_rb_demo.cfg fixture without cornell_csg cluster, used as the user-facing rigid-body demo
• OR: emissive tag on bodies so they self-illuminate above the cluster
• Velocity-color overlay (r_phys_show_velocity 1) for debug — color bodies by speed
• Trails / motion blur via shutter-jitter (renderer: motion blur via shutter-time jitter (PT-native) #85) so falling is unmistakable in a single frame

Acceptance

Single-frame screenshot of default phys_rb_smoke.cfg should make it obvious bodies are falling — without needing A/B against phys_enabled 0.

Related

• [Roadmap] Physics engine — Verlet/XPBD on analytic primitives #131 physics roadmap
• physics: rigid body (orientation, multi-contact, box stacking) (Phase 2 of physics roadmap #131) #138 rigid body Phase 2 (this issue's implementation)
• renderer: motion blur via shutter-time jitter (PT-native) #85 motion blur (potential Wave-2 enabler)