spectrogram decimation

src/ui/Application.cpp

@@ -702,6 +702,7 @@ void Application::loadConfig() {
     cursors_.snapToPeaks          = config_.getBool("snap_to_peaks", cursors_.snapToPeaks);
     measurements_.traceMinFreq    = config_.getFloat("trace_min_freq", measurements_.traceMinFreq);
     measurements_.traceMaxFreq    = config_.getFloat("trace_max_freq", measurements_.traceMaxFreq);
+    ui_.specMinPixPerBin          = config_.getInt("spec_min_pix_per_bin", ui_.specMinPixPerBin);
 
     // Clamp
     controlPanel_.fftSizeIdx   = std::clamp(controlPanel_.fftSizeIdx, 0, ControlPanel::kNumFFTSizes - 1);
@@ -766,6 +767,7 @@ void Application::saveConfig() const {
     cfg.setBool("snap_to_peaks", cursors_.snapToPeaks);
     cfg.setFloat("trace_min_freq", measurements_.traceMinFreq);
     cfg.setFloat("trace_max_freq", measurements_.traceMaxFreq);
+    cfg.setInt("spec_min_pix_per_bin", ui_.specMinPixPerBin);
 
     int devIdx = audio_.deviceIdx();
     if (devIdx >= 0 && devIdx < static_cast<int>(devices.size()))

src/ui/ControlPanel.cpp

@@ -178,6 +178,15 @@ void ControlPanel::render(AudioEngine& audio, UIState& ui,
             }
             if (ImGui::IsItemHovered()) ImGui::SetTooltip("Reset to 2x zoom");
         }
+
+        {
+            ImGui::AlignTextToFramePadding();
+            ImGui::Text("Decim:");
+            ImGui::SameLine();
+            ImGui::SetNextItemWidth(ImGui::GetContentRegionAvail().x);
+            ImGui::SliderInt("##decimation", &ui.specMinPixPerBin, 1, 8, "%d px/bin");
+            if (ImGui::IsItemHovered()) ImGui::SetTooltip("Spectrum frequency decimation (peak-detect)");
+        }
     }
 
     // ── Channels ──

src/ui/DisplayPanel.cpp

@@ -86,7 +86,7 @@ void DisplayPanel::renderSpectrum(AudioEngine& audio, UIState& ui,
     specDisplay.draw(allSpectraScratch_, stylesScratch_, ui.minDB, ui.maxDB,
                      settings.sampleRate, settings.isIQ, ui.freqScale,
                      specPosX, specPosY, specSizeX, specSizeY,
-                     ui.viewLo, ui.viewHi);
+                     ui.viewLo, ui.viewHi, ui.specMinPixPerBin);
 
     cursors.draw(specDisplay, specPosX, specPosY, specSizeX, specSizeY,
                  settings.sampleRate, settings.isIQ, ui.freqScale, ui.minDB, ui.maxDB,

src/ui/SpectrumDisplay.cpp

@@ -60,9 +60,11 @@ static void buildPolyline(const std::vector<float>& spectrumDB,
                            bool isIQ, FreqScale freqScale,
                            float posX, float posY, float sizeX, float sizeY,
                            float viewLo, float viewHi,
-                           std::vector<ImVec2>& outPoints) {
+                           std::vector<ImVec2>& outPoints,
+                           int minPixPerBin = 1) {
     int bins = static_cast<int>(spectrumDB.size());
-    int displayPts = std::min(bins, static_cast<int>(sizeX));
+    int maxPts = std::max(1, static_cast<int>(sizeX) / std::max(1, minPixPerBin));
+    int displayPts = std::min(bins, maxPts);
     if (displayPts < 2) displayPts = 2;
 
     outPoints.resize(displayPts);
@@ -108,7 +110,8 @@ void SpectrumDisplay::draw(const std::vector<std::vector<float>>& spectra,
                            FreqScale freqScale,
                            float posX, float posY,
                            float sizeX, float sizeY,
-                           float viewLo, float viewHi) const {
+                           float viewLo, float viewHi,
+                           int minPixPerBin) const {
     if (spectra.empty() || spectra[0].empty() || sizeX <= 0 || sizeY <= 0) return;
 
     ImDrawList* dl = ImGui::GetWindowDrawList();
@@ -178,7 +181,7 @@ void SpectrumDisplay::draw(const std::vector<std::vector<float>>& spectra,
         if (spectra[ch].empty()) continue;
         buildPolyline(spectra[ch], minDB, maxDB,
                       isIQ, freqScale, posX, posY, sizeX, sizeY,
-                      viewLo, viewHi, allPoints[ch]);
+                      viewLo, viewHi, allPoints[ch], minPixPerBin);
     }
 
     // Draw spectra: all fills first, then all lines on top.
@@ -237,7 +240,7 @@ void SpectrumDisplay::draw(const std::vector<std::vector<float>>& spectra,
             ImU32 col = (st.lineColor & 0x00FFFFFF) | 0x90000000;
             buildPolyline(peakHold_[ch], minDB, maxDB,
                           isIQ, freqScale, posX, posY, sizeX, sizeY,
-                          viewLo, viewHi, phPoints);
+                          viewLo, viewHi, phPoints, minPixPerBin);
             if (phPoints.size() >= 2)
                 dl->AddPolyline(phPoints.data(), static_cast<int>(phPoints.size()),
                                 col, ImDrawFlags_None, 1.0f);

src/ui/SpectrumDisplay.h

@@ -22,7 +22,8 @@ public:
               double sampleRate, bool isIQ,
               FreqScale freqScale,
               float posX, float posY, float sizeX, float sizeY,
-              float viewLo = 0.0f, float viewHi = 1.0f) const;
+              float viewLo = 0.0f, float viewHi = 1.0f,
+              int minPixPerBin = 1) const;
 
     // Convenience: single-channel draw (backward compat).
     void draw(const std::vector<float>& spectrumDB,

src/ui/UIState.h

@@ -17,6 +17,7 @@ struct UIState {
 
     int  waterfallChannel = 0;
     bool waterfallMultiCh = true;
+    int  specMinPixPerBin = 1;   // min screen pixels per spectrum bin (1 = no decimation)
     std::array<bool, kMaxChannels> channelEnabled = {true,true,true,true,true,true,true,true};
     // Complementary pairs: colors at indices 0+1, 2+3, 4+5, 6+7 sum to white.
     std::array<ImVec4, kMaxChannels> channelColors = {{