粒子隧道穿行

<!-- motion-registry-context:{"format":"1.0.0","recipe_id":"background.particle-tunnel-flythrough","recipe_version":"0.2.0","recipe_source":"recipes/background/particle-tunnel-flythrough/recipe.yaml"} -->
# Motion Registry Context

<!-- motion-registry-section:recipe-identity:start -->
## Recipe identity
- Recipe ID: background.particle-tunnel-flythrough
- Schema version: 0.2.0
- Recipe source: recipes/background/particle-tunnel-flythrough/recipe.yaml
<!-- motion-registry-section:recipe-identity:end -->

<!-- motion-registry-section:motion-intent:start -->
## Motion intent and scope
- Purpose (zh): 相机沿有限深度穿过确定性粒子隧道，最后停在可读的空间构图。
- Purpose (en): A camera flies through a finite deterministic particle tunnel and settles on a readable spatial composition.
- Scope: scene
- Complexity: advanced
- Subjects: background
- Intents: reveal, transition, emphasize
- Use cases: knowledge-video, product-demo, social-video, presentation
- Recipe capabilities: depth-particle-tunnel, camera-flythrough, finite-motion, seeded-layout
<!-- motion-registry-section:motion-intent:end -->

<!-- motion-registry-section:inputs:start -->
## Inputs
- label: string (required; default=ENTER THE SIGNAL) — Short label anchored near the tunnel vanishing point.
- particle_count: integer (optional; default=42) — Deterministic count of particles distributed through depth.
- depth_span: number (optional; default=12) — Finite depth of the tunnel in scene units.
<!-- motion-registry-section:inputs:end -->

<!-- motion-registry-section:normalized-timeline:start -->
## Normalized motion timeline (0 to 1)
- 0.04–0.26 · tunnel.reveal · opacity.fade → tunnel · parameters={"from":0,"to":1}
- 0.06–0.68 · particles.stagger · collection.stagger → particles · parameters={"order":"source","fraction":0.12}
- 0.08–0.70 · camera.flythrough · camera.push → tunnel · parameters={"from":0,"to":1,"curve":"ease-in-out"}
- 0.42–0.66 · caption.settle · transform.translate → caption · parameters={"axis":"y","from":0.05,"to":0,"unit":"relative-canvas"}
- 0.76–1.00 · tunnel.hold · state.hold → tunnel · parameters={}
<!-- motion-registry-section:normalized-timeline:end -->

<!-- motion-registry-section:output-envelope:start -->
## Output envelope
- Aspect ratios: 16:9, 9:16, 1:1
- Duration: min 2.8s; preferred 4.2s; max 6s
<!-- motion-registry-section:output-envelope:end -->

<!-- motion-registry-section:acceptance:start -->
## Acceptance constraints
- finite-motion: The camera and every particle complete their travel within the active interval without looping.
- deterministic: The same inputs produce identical particle positions, timing, and final camera state.
- depth-readability: The vanishing point, near/mid/far particle layers, and label remain distinguishable in the final hold.
- no-obstruction: The tunnel does not cover the caption safe area or make the label unreadable.
<!-- motion-registry-section:acceptance:end -->

<!-- motion-registry-section:handoff-invariants:start -->
## Handoff invariants
- Preserve the subject, input meaning, event order, overlaps, final state, and approximate normalized timing.
- Keep the implementation deterministic and retain a readable, stable final hold.
- Treat the Recipe document as the source of truth; do not infer engine APIs in this shared core.
<!-- motion-registry-section:handoff-invariants:end -->