Fix WKnob / WDisplay active pixmap calculation to bias towards the lower of the two middle pixmaps when there are an even number.

1 files changed, 37 insertions, 1 deletions

diff --git a/src/widget/wdisplay.cpp b/src/widget/wdisplay.cpp
index 7c325e598a..a77ba70ba0 100644
--- a/src/widget/wdisplay.cpp
+++ b/src/widget/wdisplay.cpp
@@ -108,7 +108,43 @@ void WDisplay::setPixmap(QVector<PaintablePointer>* pPixmaps, int iPos,
 }
 
 int WDisplay::getActivePixmapIndex() const {
-    return static_cast<int>(m_value * m_pixmaps.size());
+    // When there are an even number of pixmaps by convention we want a value of
+    // 0.5 to align to the lower of the two middle pixmaps. In Mixxx < 1.12.0 we
+    // accomplished this by the below formula:
+    // index = (m_value - 64.0/127.0) * (numPixmaps() - 1) + numPixmaps() / 2.0;
+
+    // But it's just as good to use m_value * numPixmaps() - epsilon. Using
+    // numPixmaps() instead of numPixmaps() - 1 ensures that every pixmap shares
+    // an equal slice of the value. Using m_value * (numPixmaps() - 1) gives an
+    // unequal slice of the value to the last pixmaps.
+
+    // Example:
+    // 3 pixmaps
+    // m_value * numPixmaps()
+    // idx: 0       1       2       3
+    // val: 0.0 ... 0.3 ... 0.6 ... 1.0
+    // Even distribution of value range, value 1 is out of bounds (3).
+
+    // m_value * (numPixmaps() - 1)
+    // idx: 0       1       2
+    // val: 0.0 ... 0.5 ... 1.0
+    // Pixmap 2 is only shown at value 1.
+
+    // floor(m_value * (numPixmaps() - 1) + 0.5)
+    // idx: 0       1        2
+    // val: 0.0 ... 0.25 ... 0.75 ... 1.0
+    // Pixmap 0 and Pixmap 2 only shown for 0.25 of value range
+
+    // 4 pixmaps
+    // m_value * numPixmaps()
+    // idx: 0       1        2       3        4
+    // val: 0.0 ... 0.25 ... 0.5 ... 0.75 ... 1.0
+    // Even distribution of value range, value 1 is out of bounds (4).
+
+    // Subtracting an epsilon prevents out of bound values at the end of the
+    // range and biases the middle value towards the lower of the 2 center
+    // pixmaps when there are an even number of pixmaps.
+    return static_cast<int>(m_value * numPixmaps() - 0.00001);
 }
 
 void WDisplay::paintEvent(QPaintEvent* ) {