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diff --git a/src/audio/spectral_brush.h b/src/audio/spectral_brush.h
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+// This file is part of the 64k demo project.
+// It implements the "Spectral Brush" primitive for procedural audio generation.
+// Spectral brushes trace Bezier curves through spectrograms with vertical profiles.
+
+#pragma once
+
+#include <cstdint>
+
+// Profile types for vertical distribution around central Bezier curve
+enum ProfileType {
+  PROFILE_GAUSSIAN = 0,            // Smooth harmonic falloff
+  PROFILE_DECAYING_SINUSOID = 1,   // Resonant/metallic texture
+  PROFILE_NOISE = 2                // Random texture/grit
+};
+
+// Evaluate linear Bezier interpolation at given frame
+// control_frames: Array of frame positions for control points
+// control_values: Array of values at control points (freq_hz or amplitude)
+// n_points: Number of control points
+// frame: Frame number to evaluate at
+// Returns: Interpolated value at frame (linearly interpolated between control points)
+float evaluate_bezier_linear(const float* control_frames,
+                              const float* control_values,
+                              int n_points,
+                              float frame);
+
+// Draw a spectral brush stroke onto a spectrogram
+// Traces a Bezier curve through time-frequency space with a vertical profile
+// spectrogram: Output buffer (dct_size × num_frames), modified in-place
+// dct_size: Number of frequency bins (e.g., 512)
+// num_frames: Number of time frames
+// control_frames: Frame positions of Bezier control points
+// control_freqs_hz: Frequency values (Hz) at control points
+// control_amps: Amplitude values at control points (0.0-1.0 typical)
+// n_control_points: Number of control points (minimum 2 for a curve)
+// profile_type: Type of vertical profile to apply
+// profile_param1: First parameter (sigma for Gaussian, decay for sinusoid, amplitude for noise)
+// profile_param2: Second parameter (unused for Gaussian, frequency for sinusoid, seed for noise)
+void draw_bezier_curve(float* spectrogram,
+                       int dct_size,
+                       int num_frames,
+                       const float* control_frames,
+                       const float* control_freqs_hz,
+                       const float* control_amps,
+                       int n_control_points,
+                       ProfileType profile_type,
+                       float profile_param1,
+                       float profile_param2 = 0.0f);
+
+// Additive variant of draw_bezier_curve (adds to existing spectrogram content)
+// Use for compositing multiple profiles (e.g., Gaussian + Noise)
+// Parameters same as draw_bezier_curve()
+void draw_bezier_curve_add(float* spectrogram,
+                            int dct_size,
+                            int num_frames,
+                            const float* control_frames,
+                            const float* control_freqs_hz,
+                            const float* control_amps,
+                            int n_control_points,
+                            ProfileType profile_type,
+                            float profile_param1,
+                            float profile_param2 = 0.0f);
+
+// Evaluate vertical profile at given distance from central curve
+// type: Profile type (Gaussian, Decaying Sinusoid, Noise)
+// distance: Distance in frequency bins from curve center
+// param1: First profile parameter
+// param2: Second profile parameter
+// Returns: Profile amplitude at given distance (0.0-1.0 range typically)
+float evaluate_profile(ProfileType type,
+                       float distance,
+                       float param1,
+                       float param2);
+
+// Home-brew deterministic RNG for noise profile
+// Simple linear congruential generator (LCG) for small code size
+// seed: Input seed value
+// Returns: Pseudo-random uint32_t value
+uint32_t spectral_brush_rand(uint32_t seed);
+