Use contrasting text color wherever custom background colors are used

CMakeLists.txt

@@ -101,10 +101,12 @@ set(SACNVIEW_HEADERS
     src/widgets/checkableheader.h
     src/models/sacnlistenermodel.h
     src/delegates/resettablecounterdelegate.h
+    src/color_helpers.h
 )
 
 set(SACNVIEW_SOURCES
     sacnview.natvis
+    src/color_helpers.cpp
     src/commandline.cpp
     src/firewallcheck.cpp
     src/ipc.cpp

src/color_helpers.cpp

@@ -0,0 +1,26 @@
+#include "color_helpers.h"
+#include <QApplication>
+#include <QPalette>
+
+double calcLuminance(const QColor & color)
+{
+    // https://w3c.github.io/wcag/guidelines/22/#dfn-relative-luminance
+    return 0.2126 * color.redF() + 0.7152 * color.greenF() + 0.0722 * color.blueF();
+}
+
+double calcContrastRatio(const QColor & color1, const QColor & color2)
+{
+    const double luminance1 = calcLuminance(color1);
+    const double luminance2 = calcLuminance(color2);
+    const double lighter = std::max(luminance1, luminance2);
+    const double darker = std::min(luminance1, luminance2);
+    // https://w3c.github.io/wcag/guidelines/22/#dfn-contrast-ratio
+    return (lighter + 0.05) / (darker + 0.05);
+}
+
+QColor findBestContrastingColor(const QColor & bgColor)
+{
+    const QPalette palette = qApp->palette();
+    const std::array fgColors = {palette.color(QPalette::Text), palette.color(QPalette::BrightText)};
+    return findBestContrastingColor(bgColor, fgColors.cbegin(), fgColors.cend());
+}

src/color_helpers.h

@@ -0,0 +1,57 @@
+#ifndef COLOR_HELPERS_H
+#define COLOR_HELPERS_H
+
+#include <QColor>
+
+/**
+ * Calculate luminance in the sRGB color space.
+ *
+ * @see https://w3c.github.io/wcag/guidelines/22/#dfn-relative-luminance
+ */
+double calcLuminance(const QColor & color);
+
+/**
+ * Calculate contrast ratio between two colors.
+ *
+ * @see https://w3c.github.io/wcag/guidelines/22/#dfn-contrast-ratio
+ */
+double calcContrastRatio(const QColor & color1, const QColor & color2);
+
+/**
+ * Find the color in the iterator @p begin .. @p end that contrasts best with @p bgColor.
+ *
+ * @param bgColor Background color
+ * @param begin Start QColor iterator of possible foreground colors.
+ * @param end End QColor iterator.
+ */
+template<class FgColorIt>
+QColor findBestContrastingColor(const QColor & bgColor, FgColorIt begin, FgColorIt end)
+{
+    using ColorContrastRatio = std::tuple<QColor, double>;
+    std::vector<ColorContrastRatio> contrastRatios;
+
+    // Calculate ratios.
+    for (FgColorIt it = begin; it != end; ++it)
+    {
+        contrastRatios.emplace_back(*it, calcContrastRatio(bgColor, *it));
+    }
+
+    // Find the highest contrast ratio.
+    ColorContrastRatio best = contrastRatios.front();
+    for (const auto & contrastRatio : contrastRatios)
+    {
+        if (std::get<1>(best) < std::get<1>(contrastRatio))
+        {
+            best = contrastRatio;
+        }
+    }
+    return std::get<0>(best);
+}
+
+/**
+ * Find the which of the application palette's text colors best contrasts with @p bgColor.
+ * @param bgColor Background color
+ */
+QColor findBestContrastingColor(const QColor & bgColor);
+
+#endif //COLOR_HELPERS_H

src/models/sacnsourcetablemodel.cpp

@@ -14,13 +14,18 @@
 
 #include "sacnsourcetablemodel.h"
 
+#include <QApplication>
+
 #include "preferences.h"
 
 #include "sacn/sacnlistener.h"
 
+#include "color_helpers.h"
+
 SACNSourceTableModel::SACNSourceTableModel(QObject * parent)
     : QAbstractTableModel(parent)
-{}
+{
+}
 
 SACNSourceTableModel::~SACNSourceTableModel() {}
 
@@ -68,6 +73,8 @@ QVariant SACNSourceTableModel::data(const QModelIndex & index, int role) const
 
         case Qt::BackgroundRole: return getBackgroundData(rowData, index.column());
 
+        case Qt::ForegroundRole: return getForegroundData(rowData, index.column());
+
         case Qt::TextAlignmentRole:
             switch (index.column())
             {
@@ -218,6 +225,23 @@ QVariant SACNSourceTableModel::getBackgroundData(const RowData & rowData, int co
     return QVariant();
 }
 
+QVariant SACNSourceTableModel::getForegroundData(const RowData & rowData, int column) const
+{
+    const QVariant bgData = getBackgroundData(rowData, column);
+    if (!bgData.isValid())
+    {
+        return QVariant();
+    }
+
+    const QColor bgColor = bgData.value<QColor>();
+    if (bgColor.isValid())
+    {
+        return findBestContrastingColor(bgColor);
+    }
+
+    return QVariant();
+}
+
 QVariant SACNSourceTableModel::getTimingSummary(const RowData & rowData, int column) const
 {
     const FpsCounter::Histogram & histogram = rowData.histogram;

src/models/sacnsourcetablemodel.h

@@ -176,6 +176,7 @@ private Q_SLOTS:
     // Data
     QVariant getDisplayData(const RowData & rowData, int column) const;
     QVariant getBackgroundData(const RowData & rowData, int column) const;
+    QVariant getForegroundData(const RowData & rowData, int column) const;
     QVariant getTimingSummary(const RowData & rowData, int column) const;
 
     void RefreshAllTimingData();

src/widgets/glscopewidget.cpp

@@ -22,6 +22,8 @@
 
 #include <QJsonArray>
 
+#include "color_helpers.h"
+
 static constexpr qreal AXIS_LABEL_WIDTH = 45.0;
 static constexpr qreal AXIS_LABEL_HEIGHT = 20.0;
 static constexpr qreal TOP_GAP = 10.0;
@@ -716,6 +718,8 @@ QVariant ScopeModel::data(const QModelIndex & index, int role) const
                 case COL_COLOUR:
                     if (role == Qt::BackgroundRole || role == Qt::DisplayRole || role == Qt::EditRole)
                         return trace->color();
+                    if (role == Qt::ForegroundRole)
+                        return findBestContrastingColor(trace->color());
                     if (role == DataSortRole) return static_cast<uint32_t>(trace->color().rgba());
                     if (role == Qt::ToolTipRole) return tr("Trace color (#RRGGBB)");
                     break;

src/widgets/glscopewidget.h

@@ -684,4 +684,4 @@ class GlScopeWidget : public QOpenGLWidget, protected QOpenGLFunctions
     std::vector<QVector2D> makeTriggerLine(ScopeModel::Trigger type);
 
     void updateCursor(const QPoint & widgetPos);
-};
+};

src/widgets/gridwidget.cpp

@@ -14,6 +14,8 @@
 // limitations under the License.
 
 #include "gridwidget.h"
+#include "color_helpers.h"
+
 #include <QApplication>
 #include <QMouseEvent>
 #include <QPainter>
@@ -29,7 +31,11 @@
 #define CELL_COUNT 512
 
 GridWidget::GridWidget(QWidget * parent)
-    : QWidget(parent), m_colors(CELL_COUNT, this->palette().color(QPalette::Base)), m_cellHeight(CELL_HEIGHT)
+    : QWidget(parent)
+    , m_bgColors(CELL_COUNT, this->palette().color(QPalette::Base))
+    , m_fgColors(CELL_COUNT, this->palette().color(QPalette::Text))
+    , m_possibleFgColors({this->palette().color(QPalette::Text), this->palette().color(QPalette::BrightText)})
+    , m_cellHeight(CELL_HEIGHT)
 {
     for (int i = 0; i < CELL_COUNT; i++)
     {
@@ -117,10 +123,11 @@ void GridWidget::paintEvent(QPaintEvent * event)
 
             if (!value.isEmpty())
             {
-                QColor fillColor = m_colors[address];
+                QColor fillColor = m_bgColors[address];
 
                 QString rowLabel = value;
                 painter.fillRect(textRect, fillColor);
+                painter.setPen(m_fgColors[address]);
                 painter.drawText(textRect, Qt::AlignHCenter | Qt::AlignVCenter, rowLabel);
             }
         }
@@ -333,17 +340,29 @@ void GridWidget::mouseDoubleClickEvent(QMouseEvent * event)
 void GridWidget::setAllCellColor(const QColor & color)
 {
     if (color.isValid())
-        m_colors.fill(color);
+    {
+        m_bgColors.fill(color);
+        const QColor fgColor = findBestContrastingColor(color, m_possibleFgColors.cbegin(), m_possibleFgColors.cend());
+        m_fgColors.fill(fgColor);
+    }
     else
+    {
         setAllCellColor(palette().color(QPalette::Base));
+    }
 }
 
 void GridWidget::setCellColor(int cell, const QColor & color)
 {
     if (color.isValid())
-        m_colors[cell] = color;
+    {
+        m_bgColors[cell] = color;
+        const QColor fgColor = findBestContrastingColor(color, m_possibleFgColors.cbegin(), m_possibleFgColors.cend());
+        m_fgColors[cell] = fgColor;
+    }
     else
+    {
         setCellColor(cell, palette().color(QPalette::Base));
+    }
 }
 
 void GridWidget::setCellValue(int cell, const QString & value)

src/widgets/gridwidget.h

@@ -76,7 +76,9 @@ class GridWidget : public QWidget
 private:
 
     QList<int> m_selectedAddresses;
-    QVector<QColor> m_colors;
+    QVector<QColor> m_bgColors;
+    QVector<QColor> m_fgColors;
+    std::array<QColor, 2> m_possibleFgColors;
     QStringList m_values;
     bool m_allowMultiSelect = false;
     QPoint m_lastClickPoint;