Calculates afterglow from a value 0.0 to 1.0

which is the factor the screen will be after 1 second (provided it draws at the set FPS rate)

mainwindow.cpp

@@ -124,10 +124,11 @@ MainWindow::MainWindow(QWidget *parent) :
         auto widget = new QWidget;
         auto layout = new QFormLayout(widget);
         {
-            auto input = new QSpinBox;
-            input->setRange(0, 255);
+            auto input = new QDoubleSpinBox;
+            input->setRange(0, 1.0);
+            input->setSingleStep(0.1);
             input->setValue(m_ui->widget->afterglow());
-            connect(input, qOverload<int>(&QSpinBox::valueChanged), m_ui->widget, &OsciWidget::setAfterglow);
+            connect(input, qOverload<double>(&QDoubleSpinBox::valueChanged), m_ui->widget, &OsciWidget::setAfterglow);
             layout->addRow(tr("Afterglow:"), input);
         }
         {

osciwidget.cpp

@@ -30,6 +30,13 @@ void OsciWidget::setFps(int fps)
     m_redrawTimerId = startTimer(1000/m_fps);
 }
 
+void OsciWidget::setAfterglow(float afterglow){
+    m_afterglow = afterglow;
+    // percentage of the image that should be visible after one second
+    // i.e. factor^fps=afterglow -> factor = afterglow^(1/fps)
+    m_afterglowColor = 255 * pow(afterglow, 1.0/m_fps);
+}
+
 void OsciWidget::setLightspeed(int lightspeed) {
     const auto temp = (float(lightspeed)/20.f);
     m_lightspeed = temp*temp*temp;
@@ -73,7 +80,7 @@ void OsciWidget::updateFrameBuffer()
     // darkening last frame
     painter.setCompositionMode(QPainter::CompositionMode_Multiply);
     painter.setPen({});
-    painter.setBrush(QColor(m_afterglow, m_afterglow, m_afterglow));
+    painter.setBrush(QColor(m_afterglowColor, m_afterglowColor, m_afterglowColor));
     painter.drawRect(m_pixmap.rect());
 
     // drawing new lines ontop

osciwidget.h

@@ -21,7 +21,7 @@ public:
 
     float factor() const { return m_factor; }
     int fps() const { return m_fps; }
-    int afterglow() const { return m_afterglow; }
+    float afterglow() const { return m_afterglow; }
     int lightspeed() const;
 
 signals:
@@ -30,7 +30,7 @@ signals:
 public slots:
     void setFactor(float factor) { m_factor = factor; }
     void setFps(int fps);
-    void setAfterglow(int afterglow) { m_afterglow = afterglow; }
+    void setAfterglow(float afterglow);
     void setLightspeed(int lightspeed);
 
     void renderSamples(const SamplePair *begin, const SamplePair *end);
@@ -44,7 +44,8 @@ private:
 
 private:
     float m_factor{2.f};
-    int m_fps{30}, m_afterglow{175};
+    int m_fps{30}, m_afterglowColor{175};
+    float m_afterglow{0.2};
     float m_lightspeed{35.f};
 
     std::vector<SamplePair> m_buffer;