feat: improve spectrogram pitch mapping and vertical smoothing

src/util/spectrogramUtil.test.ts

@@ -1,7 +1,10 @@
 import { describe, expect, it } from 'vitest';
 import {
+  frequencyToMidiPitch,
+  getSpectrogramAnalysisResolution,
   getSpectrogramPitchBinCount,
   getSpectrogramVisibleBinRange,
+  mapFrequencyToSpectrogramPosition,
   mapMidiPitchToSpectrogramPosition,
   normalizeSpectrogramHeightResolution,
 } from './spectrogramUtil';
@@ -21,6 +24,12 @@ describe('spectrogramUtil', () => {
     expect(getSpectrogramPitchBinCount(5)).toBe(640);
   });
 
+  it('doubles the internal analysis resolution for each user-facing option', () => {
+    expect(getSpectrogramAnalysisResolution(1)).toBe(2);
+    expect(getSpectrogramAnalysisResolution(3)).toBe(6);
+    expect(getSpectrogramAnalysisResolution(5)).toBe(10);
+  });
+
   it('maps MIDI pitches into full-range spectrogram positions', () => {
     expect(mapMidiPitchToSpectrogramPosition(0, 1)).toBe(0);
     expect(mapMidiPitchToSpectrogramPosition(12.5, 3)).toBe(38.5);
@@ -29,6 +38,12 @@ describe('spectrogramUtil', () => {
     expect(mapMidiPitchToSpectrogramPosition(128, 3)).toBeNull();
   });
 
+  it('converts frequencies into MIDI pitch space and spectrogram positions', () => {
+    expect(frequencyToMidiPitch(440)).toBeCloseTo(69, 6);
+    expect(mapFrequencyToSpectrogramPosition(440, 5)).toBeCloseTo(347, 6);
+    expect(frequencyToMidiPitch(0)).toBeNull();
+  });
+
   it('derives the visible C0-B7 subrange inside the full-resolution buffer', () => {
     expect(getSpectrogramVisibleBinRange(1)).toEqual({ start: 12, end: 108 });
     expect(getSpectrogramVisibleBinRange(3)).toEqual({ start: 36, end: 324 });

src/util/spectrogramUtil.ts

@@ -6,6 +6,7 @@ export const SPECTROGRAM_FULL_SEMITONES =
   SPECTROGRAM_FULL_MAX_MIDI_PITCH - SPECTROGRAM_FULL_MIN_MIDI_PITCH + 1;
 export const SPECTROGRAM_VISIBLE_SEMITONES =
   SPECTROGRAM_MAX_MIDI_PITCH - SPECTROGRAM_MIN_MIDI_PITCH + 1;
+export const SPECTROGRAM_ANALYSIS_RESOLUTION_MULTIPLIER = 2;
 
 export type SpectrogramHeightResolution = 1 | 3 | 5;
 
@@ -19,13 +20,13 @@ export function normalizeSpectrogramHeightResolution(value: unknown): Spectrogra
 }
 
 export function getSpectrogramPitchBinCount(
-  resolution: SpectrogramHeightResolution,
+  resolution: number,
 ): number {
   return SPECTROGRAM_FULL_SEMITONES * resolution;
 }
 
 export function getSpectrogramVisibleBinRange(
-  resolution: SpectrogramHeightResolution,
+  resolution: number,
 ): { start: number; end: number } {
   return {
     start: (SPECTROGRAM_MIN_MIDI_PITCH - SPECTROGRAM_FULL_MIN_MIDI_PITCH) * resolution,
@@ -33,9 +34,15 @@ export function getSpectrogramVisibleBinRange(
   };
 }
 
+export function getSpectrogramAnalysisResolution(
+  resolution: SpectrogramHeightResolution,
+): number {
+  return resolution * SPECTROGRAM_ANALYSIS_RESOLUTION_MULTIPLIER;
+}
+
 export function mapMidiPitchToSpectrogramPosition(
   midiPitch: number,
-  resolution: SpectrogramHeightResolution,
+  resolution: number,
 ): number | null {
   const pitchOffset = midiPitch - SPECTROGRAM_FULL_MIN_MIDI_PITCH;
   if (pitchOffset < 0 || pitchOffset > SPECTROGRAM_FULL_MAX_MIDI_PITCH) {
@@ -48,3 +55,21 @@ export function mapMidiPitchToSpectrogramPosition(
   const maxBin = getSpectrogramPitchBinCount(resolution) - 1;
   return Math.max(0, Math.min(maxBin, scaled));
 }
+
+export function frequencyToMidiPitch(frequency: number): number | null {
+  if (!Number.isFinite(frequency) || frequency <= 0) {
+    return null;
+  }
+  return 69 + 12 * Math.log2(frequency / 440);
+}
+
+export function mapFrequencyToSpectrogramPosition(
+  frequency: number,
+  resolution: number,
+): number | null {
+  const midiPitch = frequencyToMidiPitch(frequency);
+  if (midiPitch === null) {
+    return null;
+  }
+  return mapMidiPitchToSpectrogramPosition(midiPitch, resolution);
+}