Add optional octave-snap: fold isolated single-note octave spikes onto the melody

[MrDix]

What

A toggleable post-pass (--octave_snap, GUI "Octave Spike Snap", off by default) that folds isolated single-note octave jumps back onto the melody: the occasional lone note a pitch tracker lifts or drops by an octave while its neighbours stay put — jarring to sing and read.

Why conservative — validated, not guessed

I prototyped this against the 8 professional reference songs and measured note range, median octave offset vs the reference, and how many notes moved. Findings that shaped the design:

• A naive per-note snap (fold anything far from a windowed median) tightens ranges but destroys legitimately wide-range songs (one reference spans 42 semitones; the naive version folded it to 23) and can worsen others.
• The shipped version only folds a note that is (a) a local extremum above/below both immediate voiced neighbours, (b) in a stable context (the two neighbours agree within a few semitones), and (c) about an octave away, landing within a small residual of the local line. On the 8 songs this regressed zero while removing the isolated spikes on the songs that had them.

Because octave is scoring-irrelevant (the ptAKF scorer folds octaves), this never changes the game score — it is purely a display/singability polish.

Scope / honesty

This fixes isolated spikes. It does not fix a whole passage that is consistently an octave off (2 of the 8 songs have that) — that is a deeper pitch-tracking problem (octave continuity on the raw f0), out of scope here and called out in the docs.

Surfaces (kept in sync)

CLI --octave_snap, GUI toggle in Post-Processing, config default, runner arg, CLI help, README, and tooltip — all documenting that it is conservative, score-neutral, and does not fix whole-section shifts.

Tests

7 unit tests (isolated up/down spike folded; genuine leap, small interval, unstable context, wide-range all untouched; pitch class preserved; short-input no-op). Full suite 858 passed / 4 skipped.

Default: off (opt-in) for now — happy to revisit the default per your call.

[MrDix]

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 2fb93b65-98a1-453a-b6f0-f6f53943517d

📥 Commits

Reviewing files that changed from the base of the PR and between fa97aec and 6a77b56.

📒 Files selected for processing (9)

• README.md
• pytest/modules/Midi/test_octave_snap.py
• src/Settings.py
• src/UltraSinger.py
• src/gui/config.py
• src/gui/settings_tab.py
• src/gui/ultrasinger_runner.py
• src/modules/Midi/midi_creator.py
• src/modules/common_print.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/octave-snap

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