spec(design): scrub-driven altitude transitions (knob position drives morph + audio)

Approved design for the next increment: continuous knob position scrubs the morph
video + crossfades audio (hold-partway, reversible, re-roll-on-fresh-approach,
wheel auto-scrubs one altitude). Includes morph all-intra re-bake for smooth
seeking. To be planned + built in a separate session.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/superpowers/specs/2026-06-27-scrub-driven-altitude-transitions-design.md

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+# Scrub-driven Altitude Transitions — Design
+
+**Status:** Approved (brainstormed 2026-06-27, session 0024). Ready for writing-plans.
+
+## Goal
+
+Make the altitude transition **driven by the knob position** rather than auto-played.
+Turning the physical knob (or, in the simulator, dragging the Altitude dial) at any
+speed scrubs the morph video and crossfades the audio in lockstep with the knob. The
+knob position *is* the scene: turn slow → the morph eases slowly; turn fast → it
+races; stop halfway → it holds in a mid-morph blend; turn back → it reverses exactly.
+
+This supersedes the timed dial-needle sweep (the dial-vs-video sync problem dissolves
+because the knob and the transition become the same thing).
+
+## Core model
+
+A continuous **altitude position** `pos` (float): integers are altitudes/detents,
+fractions are mid-morph blend. `pos = 2.4` ⇒ 40 % from coast (index 2) toward reef
+(index 3). The knob/needle maps directly to `pos`; the morph video's `currentTime`
+and the audio crossfade are scrubbed by `frac(pos)`.
+
+**Decisions (locked):**
+- **In-between state:** hold the blend wherever the knob stops — no auto-complete
+  (continuous encoder model). *(Q1 → a)*
+- **Turn-back:** scrubs the same morph in reverse; lands on exactly the previous
+  altitude's clip you started from (fully reversible).
+- **Destination randomness:** the next altitude's clip is a random pick from its pool,
+  **fixed during a single continuous gesture**, **re-rolled on each fresh approach**.
+  *(Q → a)*
+- **Scroll wheel:** auto-scrub one altitude over ~0.6 s, then lock. *(Q2 → a)*
+- **Lock-per-altitude** still holds: a resting altitude's clip stays locked until you
+  commit into a different one.
+
+## Components
+
+### 1. Scrub controller (frontend — `simulator/static/app.js`)
+Replaces the discrete `advance(delta)` "commit then play" for drag/knob input.
+- Drives `pos` from the dial pointer angle: `pos = restPos + accum / dialStep`. The
+  needle follows continuously (the needle *is* the knob — live follow is correct here).
+- For the active segment `[floor, floor+1]` in the travel direction, sets
+  `vid.currentTime = frac(pos) × vid.duration`, **throttled to one seek per
+  `requestAnimationFrame`** (avoid seek thrash).
+- Crossing an integer **commits**: the crossed altitude's chosen clip becomes the
+  locked `activeClipId`, `ringIndex` updates, and the next segment begins (re-roll).
+- Reversible: `pos` decreasing scrubs the morph backward.
+- Keep the pure, testable bits separable (position→segment, `frac`→currentTime,
+  integer-crossing commit, re-roll-on-fresh-approach) from the DOM/event glue.
+
+### 2. Clip selection / lock
+Entering a segment toward neighbor `k±1`, pick a random clip from that scale's pool
+(client-side) and resolve the directed morph via the existing
+`morphByPair["<fromClip>→<toClip>"]` lookup (built from `ring.transitions`). Fixed for
+the gesture; re-rolled on a fresh approach. *(This moves the random pick client-side
+for responsiveness vs the current server pick in `/api/ring/advance`; acceptable for
+the simulator — the Pi player can mirror the same client-side pick. `pick_clip_id` in
+`player/ring.py` stays the canonical pure helper.)*
+
+### 3. Audio crossfade (frontend)
+During a segment, crossfade the two adjacent scale soundtracks by `frac(pos)` (gains
+`1-frac` / `frac`); at rest only the current scale plays. Hook into the existing audio
+layer (`applyAudio` / the per-scale `audio` field in the ring).
+
+### 4. Morph media re-bake (pipeline — `simulator/build_pool_manifest.py`)
+Smooth scrubbing = seeking to arbitrary frames, which needs **dense keyframes**. The
+current morphs use a sparse GOP, so they scrub "steppy." Re-bake the 154 morphs
+**all-intra** (e.g. x264 `-g 1` / `keyint=1`, or `-intra`) so every frame is seekable.
+Files grow (~176 MB → ~0.5–0.9 GB) but each clip stays small (front-end upload limit is
+a non-issue; that was the 68 MB *base*, unrelated). Media is committed via **git-LFS**
+(see `.gitattributes`); re-push after re-baking.
+
+**Phasing:** build the interaction first against the *current* morphs to validate the
+feel (will scrub somewhat steppy), then re-bake all-intra for smoothness. Two plan
+phases.
+
+### 5. Wheel + tap
+- **Wheel:** auto-scrub `pos` by ±1 over ~0.6 s (drives the same scrub path), then lock.
+- **Tap a dial label:** auto-scrub to that altitude the shortest way around.
+
+## Testing
+- **Unit (pytest or JS):** the pure controller logic — position→segment mapping,
+  `frac`→`currentTime`, commit+lock on integer crossing, re-roll on fresh approach,
+  reverse direction.
+- **E2E (Playwright, `simulator/e2e`):** drag the dial partway → assert the morph
+  `currentTime` and the two audio gains track the angle; drag back → values reverse;
+  a full turn → commits and locks the destination clip; the wheel → auto-scrubs one
+  altitude and locks. (Extends the existing `altitude-lock.spec.ts` tier.)
+
+## Out of scope
+- Physical hardware / encoder firmware (the simulator drag is the stand-in; the model
+  is designed to map 1:1 to an encoder position later — see the networked control-surface
+  spec).
+- Changing the clip-pair morph *set* or naming (`<src>__<dst>.mp4` + `.rev`) — reused as-is.
+
+## Key existing context (for the implementer)
+- Dial + transition logic: `simulator/static/app.js` (`advance`, `playTransition`,
+  `onDialDown/Move/Up`, `renderDial/setNeedle`, `morphByPair`, the preload helpers).
+- Ring/morph contract: `player/ring.py` (`Scale.pool`, `pick_clip_id`, `morph_for`,
+  `resolve_move`), `simulator/clips.py` (`ring_to_dict`, `resolved_move_to_dict`),
+  `/api/ring/advance` in `simulator/app.py`.
+- Morph manifest + baking: `simulator/build_pool_manifest.py` (`_make_transition`,
+  `_make_reverse`, `generate_media`, `build_manifest`); morphs at
+  `simulator/sample_media/transitions/<src>__<dst>.mp4` (+ `.rev`).
+- The clip-pair morph + lock feature this builds on shipped in session 0024 (plan:
+  `docs/superpowers/plans/2026-06-27-altitude-clip-pair-morph-lock.md`).