Fix synchrosqueezing: correct complex division sign and add noise threshold

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

src/spectrogram.c

@@ -473,13 +473,19 @@ static void GenerateSpectrogramTexture(StftResult* stft, Image* image, Texture2D
         // Accumulation buffer for reassigned energy
         float* accumBuffer = (float*)calloc(width * height, sizeof(float));
         
+        // Noise threshold: only reassign bins with significant energy
+        // This prevents noise from being reassigned to random frequencies
+        float noiseThreshold = maxAmplitude * 0.01f;  // 1% of max amplitude
+        
         for (int seg = 0; seg < width; seg++) {
             for (int bin = 0; bin < height; bin++) {
                 FrequencyData* V_f = &stft->segments[seg].spectrum[bin];
                 FrequencyData* V_fd = &stft->segments[seg].derivativeSpectrum[bin];
                 
                 float amplitude = V_f->amplitude;
-                if (amplitude < 0.0001f) continue;
+                
+                // Skip noise bins
+                if (amplitude < noiseThreshold) continue;
                 
                 // Compute instantaneous frequency using synchrosqueezing formula:
                 // ω̂ = bin_freq + Re[V_fd / (i * V_f)]
@@ -496,8 +502,9 @@ static void GenerateSpectrogramTexture(StftResult* stft, Image* image, Texture2D
                 float trueFreq = V_f->frequency;  // Default to bin frequency
                 
                 if (denom > 1e-10f) {
-                    // Re[V_fd / (i * V_f)] = (V_fd_real * V_f_imag + V_fd_imag * V_f_real) / denom
-                    float correction = (V_fd_real * V_f_imag + V_fd_imag * V_f_real) / denom;
+                    // Re[V_fd / (i * V_f)] = (-V_fd_real * V_f_imag + V_fd_imag * V_f_real) / denom
+                    // Note the MINUS sign on the first term
+                    float correction = (-V_fd_real * V_f_imag + V_fd_imag * V_f_real) / denom;
                     trueFreq = V_f->frequency + correction;
                 }