Merge pull request #1067 from gqrx-sdr/meter-siglevel-float

Improve accuracy and precision of S-meter display

resources/news.txt

@@ -7,6 +7,7 @@
   IMPROVED: FFT and S-meter performance.
   IMPROVED: Reduced FFT memory usage.
   IMPROVED: Increased S-meter resolution to 0.1 dB.
+  IMPROVED: Accuracy of S-meter display.
 
 
     2.15.2: Released January 8, 2022

src/qtgui/meter.cpp

@@ -119,7 +119,7 @@ void CMeter::setLevel(float dbfs)
     float level = m_dBFS;
     float alpha  = dbfs < level ? ALPHA_DECAY : ALPHA_RISE;
     m_dBFS -= alpha * (level - dbfs);
-    m_Siglevel = (int)((level - MIN_DB) * m_pixperdb);
+    m_Siglevel = (level - MIN_DB) * m_pixperdb;
 
     draw();
 }
@@ -169,12 +169,17 @@ void CMeter::draw()
     qreal ht = (qreal) h * CTRL_NEEDLE_TOP;
     qreal x = marg + m_Siglevel;
 
-    painter.setBrush(QBrush(QColor(0, 190, 0, 255)));
-    painter.setOpacity(1.0);
-
-    // Qt 4.8+ has a 1-pixel error (or they fixed line drawing)
-    // see http://stackoverflow.com/questions/16990326
-    painter.drawRect(marg - 1, ht + 1, x - marg, 6);
+    if (m_Siglevel > 0.0f)
+    {
+        QColor color(0, 190, 0, 255);
+        QPen pen(color);
+        pen.setJoinStyle(Qt::MiterJoin);
+        painter.setPen(pen);
+        painter.setBrush(QBrush(color));
+        painter.setOpacity(1.0);
+
+        painter.drawRect(QRectF(marg, ht + 2, x - marg, 4));
+    }
 
     if (m_SqlLevel > 0.0f)
     {

src/qtgui/meter.h

@@ -72,7 +72,7 @@ public slots:
     QSize   m_Size;
     QString m_Str;
     qreal   m_pixperdb;     // pixels / dB
-    int     m_Siglevel;
+    qreal   m_Siglevel;
     float   m_dBFS;
     qreal   m_Sql;
     qreal   m_SqlLevel;