Slice 12 — Beat Grid: detection, nudge, energy #13

Parent

#1

What to build

Beat Grid module: decode the soundtrack (Web Audio), estimate BPM and beat offsets; nudge(targetDuration, beatGrid, energy) snaps each slide's boundary to the nearest beat without hard-quantizing. Energy setting (calm/medium/punchy) maps to beats-held-per-slide bands. Beat sync is a cascade setting with a global off switch.

Type: AFK | Stories: 18, 19, 20, 22

User stories covered

• Story 18: slide transitions nudged to land on music beats
• Story 19: Energy setting controlling how many beats each slide holds (calm vs punchy)
• Story 20: beat sync uses target durations as baseline (nudged, not hard-quantized)
• Story 22: turn beat sync off entirely so durations are exactly what was set

Acceptance criteria

• Given a synthesized click track of known BPM, when analyzed, then detected BPM is within ±2% and offset within ±30ms.
• Given beat sync on, when planned, then every slide boundary lands on a beat and each duration deviates from its target by at most half a beat interval.
• Given beat sync off, when planned, then durations equal targets exactly.
• Given each energy level on the same timeline+track, when planned, then average slide duration differs in the expected direction.

Technical notes

Decode adapter thin; all math pure and heavily tested with synthetic fixtures. Detection results cached in slideshow.json so reopen doesn't re-analyze.

Blocked by

• Blocked by Slice 10 — Folder audio + soundtrack playback #11