GAL: Make high-contrast meld into background

This mixes the de-emphasized layers into the background color rather
than merely a black color.  This also keeps some of the original layer
color.

Fixes: lp:1831316
* https://bugs.launchpad.net/kicad/+bug/1831316

common/painter.cpp

@@ -37,7 +37,7 @@ RENDER_SETTINGS::RENDER_SETTINGS()
     m_layerOpacity          = 0.8;
     m_highlightEnabled      = false;
     m_hiContrastEnabled     = false;
-    m_hiContrastFactor      = 0.2;
+    m_hiContrastFactor      = 0.2; //TODO: Make this user-configurable
     m_highlightNetcode      = -1;
     m_outlineWidth          = 1;
     m_worksheetLineWidth    = 100000;
@@ -52,12 +52,11 @@ RENDER_SETTINGS::~RENDER_SETTINGS()
 
 void RENDER_SETTINGS::update()
 {
-    m_hiContrastColor = COLOR4D( m_hiContrastFactor, m_hiContrastFactor, m_hiContrastFactor,
-                                 m_layerOpacity );
-
     // Calculate darkened/highlighted variants of layer colors
     for( int i = 0; i < LAYER_ID_COUNT; i++ )
     {
+        m_hiContrastColor[i] = m_layerColors[i].Mix( m_layerColors[LAYER_PCB_BACKGROUND],
+                m_hiContrastFactor );
         m_layerColorsHi[i]   = m_layerColors[i].Brightened( m_highlightFactor );
         m_layerColorsDark[i] = m_layerColors[i].Darkened( 1.0 - m_highlightFactor );
         m_layerColorsSel[i]  = m_layerColors[i].Brightened( m_selectFactor );

gerbview/gerbview_painter.cpp

@@ -133,7 +133,7 @@ const COLOR4D& GERBVIEW_RENDER_SETTINGS::GetColor( const VIEW_ITEM* aItem, int a
 
     // Return grayish color for non-highlighted layers in the high contrast mode
     if( m_hiContrastEnabled && m_activeLayers.count( aLayer ) == 0)
-        return m_hiContrastColor;
+        return m_hiContrastColor[aLayer];
 
     // Catch the case when highlight and high-contraste modes are enabled
     // and we are drawing a not highlighted track

include/gal/color4d.h

@@ -222,6 +222,22 @@ public:
                         a );
     }
 
+    /**
+     * Function Mix
+     * Returns a color that is mixed with the input by a factor
+     * @param aFactor Specifies how much of the original color to keep (valid values: 0.0 .. 1.0).
+     * @return COLOR4D Mixed color.
+     */
+    COLOR4D Mix( const COLOR4D& aColor, double aFactor ) const
+    {
+        assert( aFactor >= 0.0 && aFactor <= 1.0 );
+
+        return COLOR4D( aColor.r * ( 1.0 - aFactor ) + r * aFactor,
+                        aColor.g * ( 1.0 - aFactor ) + g * aFactor,
+                        aColor.b * ( 1.0 - aFactor ) + b * aFactor,
+                        a );
+    }
+
     /**
      * Function WithAlpha
      * Returns a colour with the same colour, but the given alpha

include/painter.h

@@ -271,9 +271,11 @@ protected:
     ///> Colors for all layers (darkened)
     COLOR4D m_layerColorsDark[LAYER_ID_COUNT];
 
+    ///< Colora used for high contrast display mode
+    COLOR4D m_hiContrastColor[LAYER_ID_COUNT];
+
     /// Parameters for display modes
     bool    m_hiContrastEnabled;    ///< High contrast display mode on/off
-    COLOR4D m_hiContrastColor;      ///< Color used for high contrast display mode
     float   m_hiContrastFactor;     ///< Factor used for computing high contrast color
 
     bool    m_highlightEnabled;     ///< Highlight display mode on/off

pcbnew/pcb_painter.cpp

@@ -254,7 +254,7 @@ const COLOR4D& PCB_RENDER_SETTINGS::GetColor( const VIEW_ITEM* aItem, int aLayer
 
     // Return grayish color for non-highlighted layers in the high contrast mode
     if( m_hiContrastEnabled && m_activeLayers.count( aLayer ) == 0 )
-        return m_hiContrastColor;
+        return m_hiContrastColor[aLayer];
 
     // Catch the case when highlight and high-contraste modes are enabled
     // and we are drawing a not highlighted track