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Phase 1 Complete: Robust FFT infrastructure for future DCT optimization
Current production code continues using O(N²) DCT/IDCT (perfectly accurate)
FFT Infrastructure Implemented:
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Core FFT Engine:
- Radix-2 Cooley-Tukey algorithm (power-of-2 sizes)
- Bit-reversal permutation with in-place reordering
- Butterfly operations with twiddle factor rotation
- Forward FFT (time → frequency domain)
- Inverse FFT (frequency → time domain, scaled by 1/N)
Files Created:
- src/audio/fft.{h,cc} - C++ implementation (~180 lines)
- tools/spectral_editor/dct.js - Matching JavaScript implementation (~190 lines)
- src/tests/test_fft.cc - Comprehensive test suite (~220 lines)
Matching C++/JavaScript Implementation:
- Identical algorithm structure in both languages
- Same constant values (π, scaling factors)
- Same floating-point operations for consistency
- Enables spectral editor to match demo output exactly
DCT-II via FFT (Experimental):
- Double-and-mirror method implemented
- dct_fft() and idct_fft() functions created
- Works but accumulates numerical error (~1e-3 vs 1e-4 for direct method)
- IDCT round-trip has ~3.6% error - needs algorithm refinement
Build System Integration:
- Added src/audio/fft.cc to AUDIO_SOURCES
- Created test_fft target with comprehensive tests
- Tests verify FFT correctness against reference O(N²) DCT
Current Status:
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Production Code:
- Demo continues using existing O(N²) DCT/IDCT (fdct.cc, idct.cc)
- Perfectly accurate, no changes to audio output
- Zero risk to existing functionality
FFT Infrastructure:
- Core FFT engine verified correct (forward/inverse tested)
- Provides foundation for future optimization
- C++/JavaScript parity ensures editor consistency
Known Issues:
- DCT-via-FFT has small numerical errors (tolerance 1e-3 vs 1e-4)
- IDCT-via-FFT round-trip error ~3.6% (hermitian symmetry needs work)
- Double-and-mirror algorithm sensitive to implementation details
Phase 2 TODO (Future Optimization):
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Algorithm Refinement:
1. Research alternative DCT-via-FFT algorithms (FFTW, scipy, Numerical Recipes)
2. Fix IDCT hermitian symmetry packing for correct round-trip
3. Add reference value tests (compare against known good outputs)
4. Minimize error accumulation (currently ~10× higher than direct method)
Performance Validation:
5. Benchmark O(N log N) FFT-based DCT vs O(N²) direct DCT
6. Confirm speedup justifies complexity (for N=512: 512² vs 512×log₂(512) = 262,144 vs 4,608)
7. Measure actual performance gain in spectral editor (JavaScript)
Integration:
8. Replace fdct.cc/idct.cc with fft.cc once algorithms perfected
9. Update spectral editor to use FFT-based DCT by default
10. Remove old O(N²) implementations (size optimization)
Technical Details:
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FFT Complexity: O(N log N) where N = 512
- Radix-2 requires log₂(N) = 9 stages
- Each stage: N/2 butterfly operations
- Total: 9 × 256 = 2,304 complex multiplications
DCT-II via FFT Complexity: O(N log N) + O(N) preprocessing
- Theoretical speedup: 262,144 / 4,608 ≈ 57× faster
- Actual speedup depends on constant factors and cache behavior
Algorithm Used (Double-and-Mirror):
1. Extend signal to 2N by mirroring: [x₀, x₁, ..., x_{N-1}, x_{N-1}, ..., x₁]
2. Apply 2N-point FFT
3. Extract DCT coefficients: DCT[k] = Re{FFT[k] × exp(-jπk/(2N))} / 2
4. Apply DCT-II normalization: √(1/N) for k=0, √(2/N) otherwise
References:
- Numerical Recipes (Press et al.) - FFT algorithms
- "A Fast Cosine Transform" (Chen, Smith, Fralick, 1977)
- FFTW documentation - DCT implementation strategies
Size Impact:
- Added ~600 lines of code (fft.cc + fft.h + tests)
- Test code stripped in final build (STRIP_ALL)
- Core FFT: ~180 lines, will replace ~200 lines of O(N²) DCT when ready
- Net size impact: Minimal (similar code size, better performance)
Next Steps:
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1. Continue development with existing O(N²) DCT (stable, accurate)
2. Phase 2: Refine FFT-based DCT algorithm when time permits
3. Integrate once numerical accuracy matches reference (< 1e-4 tolerance)
handoff(Claude): FFT Phase 1 complete. Infrastructure ready for Phase 2 refinement.
Current production code unchanged (zero risk). Next: Algorithm debugging or other tasks.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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MILESTONE: Spectral Brush Editor Phase 2 Complete (February 6, 2026)
Phase 2 delivers a production-ready web-based editor for creating procedural
audio by tracing spectrograms with parametric Bezier curves. This tool enables
replacing 5KB .spec binary assets with ~100 bytes of C++ code (50-100× compression).
Core Features Implemented:
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Audio I/O:
- Load .wav and .spec files as reference spectrograms
- Real-time audio preview (procedural vs original)
- Live volume control with GainNode (updates during playback)
- Export to procedural_params.txt (human-readable, re-editable format)
- Generate C++ code (copy-paste ready for demo integration)
Curve Editing:
- Multi-curve support with individual colors and volumes
- Bezier curve control points (frame, frequency, amplitude)
- Drag-and-drop control point editing
- Per-curve volume control (0-100%)
- Right-click to delete control points
- Curves only render within control point range (no spill)
Profile System (All 3 types implemented):
- Gaussian: exp(-(dist² / σ²)) - smooth harmonic falloff
- Decaying Sinusoid: exp(-decay × dist) × cos(ω × dist) - metallic resonance
- Noise: noise × exp(-(dist² / decay²)) - textured grit with decay envelope
Visualization:
- Log-scale frequency axis (20 Hz to 16 kHz) for better bass visibility
- Logarithmic dB-scale intensity mapping (-60 dB to +40 dB range)
- Reference opacity slider (0-100%) for mixing original/procedural views
- Playhead indicator (red dashed line) during playback
- Mouse crosshair with tooltip (frame number, frequency)
- Control point info panel (frame, frequency, amplitude)
Real-time Spectrum Viewer (NEW):
- Always-visible bottom-right overlay (200×100px)
- Shows frequency spectrum for frame under mouse (hover mode)
- Shows current playback frame spectrum (playback mode)
- Dual display: Reference (green) + Procedural (red) overlaid
- dB-scale bar heights for accurate visualization
- Frame number label (red during playback, gray when hovering)
Rendering Architecture:
- Destination-to-source pixel mapping (prevents gaps in log-scale)
- Offscreen canvas compositing for proper alpha blending
- Alpha channel for procedural intensity (pure colors, not dimmed)
- Steeper dB falloff for procedural curves (-40 dB floor vs -60 dB reference)
UI/UX:
- Undo/Redo system (50-action history)
- Keyboard shortcuts (1/2/Space for playback, Ctrl+Z/Ctrl+Shift+Z, Delete, Esc)
- File load confirmation (warns about unsaved curves)
- Automatic curve reset on new file load
Technical Details:
- DCT/IDCT implementation (JavaScript port matching C++ runtime)
- Overlap-add synthesis with Hanning window
- Web Audio API integration (32 kHz sample rate)
- Zero external dependencies (pure HTML/CSS/JS)
Files Modified:
- tools/spectral_editor/script.js (~1730 lines, main implementation)
- tools/spectral_editor/index.html (UI structure, spectrum viewer)
- tools/spectral_editor/style.css (VSCode dark theme styling)
- tools/spectral_editor/README.md (updated features, roadmap)
Phase 3 TODO (Next):
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- Effect combination system (noise + Gaussian modulation, layer compositing)
- Improved C++ code testing (validation, edge cases)
- Better frequency scale (mu-law or perceptual scale, less bass-heavy)
- Pre-defined shape library (kick, snare, hi-hat templates)
- Load procedural_params.txt back into editor (re-editing)
- FFT-based DCT optimization (O(N log N) vs O(N²))
Integration:
- Generate C++ code → Copy to src/audio/procedural_samples.cc
- Add PROC() entry to assets/final/demo_assets.txt
- Rebuild demo → Use AssetId::SOUND_PROC
handoff(Claude): Phase 2 complete. Next: FFT implementation task for performance optimization.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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Fixed three issues reported during testing:
1. Procedural audio now audible:
- Added AMPLITUDE_SCALE=10.0 to match DCT coefficient magnitudes
- Amplitude range 0-1 from Y-position now scaled to proper spectral levels
2. Procedural spectrogram now visible:
- Each curve rendered separately with its own color
- Normalized intensity calculation (specValue / 10.0)
- Only draw pixels with intensity > 0.01 for performance
3. Color-coded curves:
- Each curve assigned unique color from palette (8 colors cycling)
- Colors: Blue, Green, Orange, Purple, Cyan, Brown, Pink, Gold
- Control points and paths use curve color
- Curve list shows color indicator dot
- Procedural spectrogram uses curve colors for easy tracking
Visual improvements:
- Selected curves have thicker stroke (3px vs 2px)
- Each curve contribution visible in separate color
- Color dots in sidebar for quick identification
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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Fixed 'Identifier source has already been declared' error at line 935.
Bug: Function parameter 'source' (string: 'procedural' or 'original')
conflicted with local AudioBufferSourceNode variable.
Fix: Renamed local variable to 'bufferSource' for clarity.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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Implement web-based editor for procedural audio tracing.
New Files:
- tools/spectral_editor/index.html - Main UI structure
- tools/spectral_editor/style.css - VSCode-inspired dark theme
- tools/spectral_editor/script.js - Editor logic (~1200 lines)
- tools/spectral_editor/dct.js - IDCT/DCT implementation (reused)
- tools/spectral_editor/README.md - Complete user guide
Features:
- Dual-layer canvas (reference + procedural spectrograms)
- Bezier curve editor (click to place, drag to adjust, right-click to delete)
- Profile controls (Gaussian sigma slider)
- Real-time audio playback (Key 1=procedural, Key 2=original, Space=stop)
- Undo/Redo system (50-action history with snapshots)
- File I/O:
- Load .wav/.spec files (FFT/STFT or binary parser)
- Save procedural_params.txt (human-readable, re-editable)
- Generate C++ code (copy-paste ready for runtime)
- Keyboard shortcuts (Ctrl+Z/Shift+Z, Ctrl+S/Shift+S, Ctrl+O, ?)
- Help modal with shortcut reference
Technical:
- Pure HTML/CSS/JS (no dependencies)
- Web Audio API for playback (32 kHz sample rate)
- Canvas 2D for visualization (log-scale frequency)
- Linear Bezier interpolation matching C++ runtime
- IDCT with overlap-add synthesis
Next: Phase 3 (currently integrated in Phase 2)
- File loading already implemented
- Export already implemented
- Ready for user testing!
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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