Timing engine: per-note tempo lookup for ms->PPQ conversion

[wretcher207]

Summary

C4 from internal review. apply_timing sampled Master_GetTempo() once at function entry, derived a single ppq_per_ms ratio, and applied it to every note. Any project with tempo automation produced wrong PPQ offsets for notes after the first tempo change — by exactly the ratio of (new BPM / sampled BPM).

Independent of #1 and #2; all three branch from main and can land in any order.

The fix

Round-trip each note's ms offset through project time so Reaper applies the correct local tempo:

local function ms_offset_to_ppq(take, ppq_pos, offset_ms)
  local t_sec      = reaper.MIDI_GetProjTimeFromPPQPos(take, ppq_pos)
  local target_ppq = reaper.MIDI_GetPPQPosFromProjTime(take, t_sec + offset_ms * 0.001)
  return target_ppq - ppq_pos
end

Cost

2 API calls per note instead of 1 multiplication. APPLY is user-triggered, not per-frame. For a 500-note item that's 1000 API roundtrips at APPLY-time — sub-millisecond total in practice. No effect on render-loop performance.

Preserved behavior

• Note duration is still preserved in PPQ via n.new_endppq = new_start + (n.endppq - n.ppq). Inside a tempo change that crosses a note, this keeps musical-grid duration stable; only the note start gets the per-note ms-to-PPQ adjustment. This matches the conservative choice — a humanizer should not silently retime note ends as a side effect of nudging the starts.
• get_tempo_info() is unchanged. It powers the timing-slider tooltip preview ("Lean center: X ms (Y PPQ)"), which is a rough display estimate — the user isn't pointing at a specific note when looking at the slider tooltip.

Test plan

• Static tempo (e.g. 120 BPM throughout): APPLY TIMING with lean=+10ms. Verify offsets match pre-fix behavior numerically — should be the same since no tempo change is in play.
• Tempo ramp (120 → 180 over the item): APPLY TIMING with lean=+10ms. Notes near the end should land roughly 10ms ahead in time, not 10ms ahead at the 120-BPM PPQ ratio. Listen on playback; the rush should feel uniform across the ramp instead of compressing toward the start.
• Tempo jump mid-item (sudden 120 → 240): APPLY TIMING on notes spanning the boundary. Pre-fix would push post-jump notes too far in PPQ; post-fix should be musically consistent on both sides.
• Tooltip display still reads sensible values — it's still BPM-sampled-at-entry, that's intentional.
• Undo round-trip: APPLY, Ctrl+Z, verify notes restore exactly to original PPQ.
• luac -p already verified on both file copies.

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 09a2c35d-1c9a-4391-a1cd-c8a8a0516f45

📥 Commits

Reviewing files that changed from the base of the PR and between 0f8aa62 and 3a6cc67.

📒 Files selected for processing (2)

• Scripts/dehumanizer-pro.lua
• dehumanizer-pro.lua

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/timing-engine-per-note-tempo

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