From 079d4a603af92c007a2d3f9f00c558877919438e Mon Sep 17 00:00:00 2001
From: Jeff Young <jeff@rokeby.ie>
Date: Fri, 8 Apr 2022 11:35:52 +0100
Subject: [PATCH] Improve transparency handling while plotting.

Only SVG can actually handle transparency, but we should at least mimic
the color value of other transparent fills by blending with a (presumed)
white paper.

Fixes https://gitlab.com/kicad/code/kicad/issues/11304
---
 common/plotters/PDF_plotter.cpp    | 12 +++++++++++-
 common/plotters/PS_plotter.cpp     | 21 ++++++++++++++++-----
 common/plotters/SVG_plotter.cpp    | 16 +++++++++++-----
 include/plotters/plotters_pslike.h |  9 +++++----
 4 files changed, 43 insertions(+), 15 deletions(-)

diff --git a/common/plotters/PDF_plotter.cpp b/common/plotters/PDF_plotter.cpp
index fb19c53368..37092b59f0 100644
--- a/common/plotters/PDF_plotter.cpp
+++ b/common/plotters/PDF_plotter.cpp
@@ -159,9 +159,19 @@ void PDF_PLOTTER::SetCurrentLineWidth( int aWidth, void* aData )
 }
 
 
-void PDF_PLOTTER::emitSetRGBColor( double r, double g, double b )
+void PDF_PLOTTER::emitSetRGBColor( double r, double g, double b, double a )
 {
     wxASSERT( workFile );
+
+    // PDF treats all colors as opaque, so the best we can do with alpha is generate an
+    // appropriate blended color assuming white paper.
+    if( a < 1.0 )
+    {
+        r = ( r * a ) + ( 1 - a );
+        g = ( g * a ) + ( 1 - a );
+        b = ( b * a ) + ( 1 - a );
+    }
+
     fprintf( workFile, "%g %g %g rg %g %g %g RG\n", r, g, b, r, g, b );
 }
 
diff --git a/common/plotters/PS_plotter.cpp b/common/plotters/PS_plotter.cpp
index 8b0d18a908..d210123a3c 100644
--- a/common/plotters/PS_plotter.cpp
+++ b/common/plotters/PS_plotter.cpp
@@ -64,9 +64,9 @@ void PSLIKE_PLOTTER::SetColor( const COLOR4D& color )
     if( m_colorMode )
     {
         if( m_negativeMode )
-            emitSetRGBColor( 1 - color.r, 1 - color.g, 1 - color.b );
+            emitSetRGBColor( 1 - color.r, 1 - color.g, 1 - color.b, color.a );
         else
-            emitSetRGBColor( color.r, color.g, color.b );
+            emitSetRGBColor( color.r, color.g, color.b, color.a );
     }
     else
     {
@@ -80,9 +80,9 @@ void PSLIKE_PLOTTER::SetColor( const COLOR4D& color )
             k = 0;
 
         if( m_negativeMode )
-            emitSetRGBColor( 1 - k, 1 - k, 1 - k );
+            emitSetRGBColor( 1 - k, 1 - k, 1 - k, 1.0 );
         else
-            emitSetRGBColor( k, k, k );
+            emitSetRGBColor( k, k, k, 1.0 );
     }
 }
 
@@ -527,10 +527,21 @@ void PS_PLOTTER::SetCurrentLineWidth( int aWidth, void* aData )
 }
 
 
-void PS_PLOTTER::emitSetRGBColor( double r, double g, double b )
+void PS_PLOTTER::emitSetRGBColor( double r, double g, double b, double a )
 {
     wxASSERT( m_outputFile );
 
+    // Postscript treats all colors as opaque, so the best we can do with alpha is generate
+    // an appropriate blended color assuming white paper.  (It's possible that a halftone would
+    // work better on *some* drivers, but most drivers are known to still treat halftones as
+    // opaque and remove any colors underneath them.)
+    if( a < 1.0 )
+    {
+        r = ( r * a ) + ( 1 - a );
+        g = ( g * a ) + ( 1 - a );
+        b = ( b * a ) + ( 1 - a );
+    }
+
     // XXX why %.3g ? shouldn't %g suffice? who cares...
     fprintf( m_outputFile, "%.3g %.3g %.3g setrgbcolor\n", r, g, b );
 }
diff --git a/common/plotters/SVG_plotter.cpp b/common/plotters/SVG_plotter.cpp
index 791ba98447..c5cb4dbb21 100644
--- a/common/plotters/SVG_plotter.cpp
+++ b/common/plotters/SVG_plotter.cpp
@@ -168,6 +168,7 @@ SVG_PLOTTER::SVG_PLOTTER()
     m_fillMode        = FILL_T::NO_FILL; // or FILLED_SHAPE or FILLED_WITH_BG_BODYCOLOR
     m_pen_rgb_color   = 0;          // current color value (black)
     m_brush_rgb_color = 0;          // current color value (black)
+    m_brush_alpha     = 1.0;
     m_dashed          = PLOT_DASH_TYPE::SOLID;
     m_useInch         = false;      // millimeters are always the svg unit
     m_precision       = 4;          // default: 4 digits in mantissa.
@@ -233,10 +234,14 @@ void SVG_PLOTTER::setSVGPlotStyle( bool aIsGroup, const std::string& aExtraStyle
 
     switch( m_fillMode )
     {
-    case FILL_T::NO_FILL:                  fputs( "fill-opacity:0.0; ", m_outputFile ); break;
-    case FILL_T::FILLED_SHAPE:             fputs( "fill-opacity:1.0; ", m_outputFile ); break;
+    case FILL_T::NO_FILL:
+        fputs( "fill-opacity:0.0; ", m_outputFile );
+        break;
+    case FILL_T::FILLED_SHAPE:
     case FILL_T::FILLED_WITH_BG_BODYCOLOR:
-    case FILL_T::FILLED_WITH_COLOR:        fputs( "fill-opacity:0.6; ", m_outputFile ); break;
+    case FILL_T::FILLED_WITH_COLOR:
+        fprintf( m_outputFile, "fill-opacity:%.*f; ", m_precision, m_brush_alpha );
+        break;
     }
 
     double pen_w = userToDeviceSize( GetCurrentLineWidth() );
@@ -342,7 +347,7 @@ void SVG_PLOTTER::EndBlock( void* aData )
 }
 
 
-void SVG_PLOTTER::emitSetRGBColor( double r, double g, double b )
+void SVG_PLOTTER::emitSetRGBColor( double r, double g, double b, double a )
 {
     int red     = (int) ( 255.0 * r );
     int green   = (int) ( 255.0 * g );
@@ -356,6 +361,7 @@ void SVG_PLOTTER::emitSetRGBColor( double r, double g, double b )
 
         // Currently, use the same color for brush and pen (i.e. to draw and fill a contour).
         m_brush_rgb_color = rgb_color;
+        m_brush_alpha = a;
     }
 }
 
@@ -746,7 +752,7 @@ bool SVG_PLOTTER::StartPlot()
     double opacity = 1.0;      // 0.0 (transparent to 1.0 (solid)
     fprintf( m_outputFile,
              "<g style=\"fill:#%6.6lX; fill-opacity:%.*f;stroke:#%6.6lX; stroke-opacity:%.*f;\n",
-             m_brush_rgb_color, m_precision, opacity, m_pen_rgb_color, m_precision, opacity );
+             m_brush_rgb_color, m_precision, m_brush_alpha, m_pen_rgb_color, m_precision, opacity );
 
     // output the pen cap and line joint
     fputs( "stroke-linecap:round; stroke-linejoin:round;\"\n", m_outputFile );
diff --git a/include/plotters/plotters_pslike.h b/include/plotters/plotters_pslike.h
index 026be4d583..5424e48321 100644
--- a/include/plotters/plotters_pslike.h
+++ b/include/plotters/plotters_pslike.h
@@ -131,7 +131,7 @@ protected:
     virtual std::string encodeStringForPlotter( const wxString& aUnicode );
 
     /// Virtual primitive for emitting the setrgbcolor operator
-    virtual void emitSetRGBColor( double r, double g, double b ) = 0;
+    virtual void emitSetRGBColor( double r, double g, double b, double a ) = 0;
 
     /// Height of the postscript font (from the AFM)
     static const double postscriptTextAscent; // = 0.718;
@@ -232,7 +232,7 @@ public:
                        void*                       aData = nullptr ) override;
 
 protected:
-    virtual void emitSetRGBColor( double r, double g, double b ) override;
+    virtual void emitSetRGBColor( double r, double g, double b, double a ) override;
 };
 
 
@@ -373,7 +373,7 @@ protected:
      * engines. Also arcs are filled as pies but only the arc is stroked so
      * it would be difficult to handle anyway.
      */
-    virtual void emitSetRGBColor( double r, double g, double b ) override;
+    virtual void emitSetRGBColor( double r, double g, double b, double a ) override;
 
     /**
      * Allocate a new handle in the table of the PDF object. The
@@ -521,7 +521,7 @@ protected:
      * Initialize m_pen_rgb_color from reduced values r, g ,b
      * ( reduced values are 0.0 to 1.0 )
      */
-    virtual void emitSetRGBColor( double r, double g, double b ) override;
+    virtual void emitSetRGBColor( double r, double g, double b, double a ) override;
 
     /**
      * Output the string which define pen and brush color, shape, transparency
@@ -544,6 +544,7 @@ protected:
                                         // (written in hex to svg files)
     long           m_brush_rgb_color;   // same as m_pen_rgb_color, used to fill
                                         // some contours.
+    double         m_brush_alpha;
     bool           m_graphics_changed;  // true if a pen/brush parameter is modified
                                         // color, pen size, fill mode ...
                                         // the new SVG stype must be output on file