Put background color behind transparent bitmaps in eeschema printing.

Most virtual printers don't support alpha blending via AlphaBlend (MSW)
properly, nor masks.

common/bitmap_base.cpp

@@ -281,7 +281,8 @@ const BOX2I BITMAP_BASE::GetBoundingBox() const
 }
 
 
-void BITMAP_BASE::DrawBitmap( wxDC* aDC, const VECTOR2I& aPos )
+void BITMAP_BASE::DrawBitmap( wxDC* aDC, const VECTOR2I& aPos,
+                              const KIGFX::COLOR4D& aBackgroundColor )
 {
     if( m_bitmap == nullptr )
         return;
@@ -358,7 +359,26 @@ void BITMAP_BASE::DrawBitmap( wxDC* aDC, const VECTOR2I& aPos )
     aDC->SetClippingRegion( clipAreaPos, wxSize( size.x, size.y ) );
     #endif
 
-    if( GetGRForceBlackPenState() )
+    if( aBackgroundColor != COLOR4D::UNSPECIFIED && m_bitmap->HasAlpha() )
+    {
+        // Most printers don't support transparent images properly,
+        // so blend the image with background color.
+
+        int w = m_bitmap->GetWidth();
+        int h = m_bitmap->GetHeight();
+
+        wxImage  image( w, h );
+        wxColour bgColor = aBackgroundColor.ToColour();
+
+        image.SetRGB( wxRect( 0, 0, w, h ), bgColor.Red(), bgColor.Green(), bgColor.Blue() );
+        image.Paste( m_bitmap->ConvertToImage(), 0, 0, wxIMAGE_ALPHA_BLEND_COMPOSE );
+
+        if( GetGRForceBlackPenState() )
+            image = image.ConvertToGreyscale();
+
+        aDC->DrawBitmap( wxBitmap( image ), pos.x, pos.y, true );
+    }
+    else if( GetGRForceBlackPenState() )
     {
         wxBitmap result( m_bitmap->ConvertToImage().ConvertToGreyscale() );
         aDC->DrawBitmap( result, pos.x, pos.y, true );

common/drawing_sheet/ds_draw_item.cpp

@@ -463,7 +463,8 @@ void DS_DRAW_ITEM_BITMAP::PrintWsItem( const RENDER_SETTINGS* aSettings, const V
     if( !bitmap->m_ImageBitmap )
         return;
 
-    bitmap->m_ImageBitmap->DrawBitmap( aSettings->GetPrintDC(), m_pos + aOffset );
+    bitmap->m_ImageBitmap->DrawBitmap( aSettings->GetPrintDC(), m_pos + aOffset,
+                                       aSettings->GetBackgroundColor() );
 }
 
 

eeschema/sch_bitmap.cpp

@@ -131,7 +131,7 @@ void SCH_BITMAP::Print( const RENDER_SETTINGS* aSettings, const VECTOR2I& aOffse
 {
     VECTOR2I pos = m_pos + aOffset;
 
-    m_bitmapBase->DrawBitmap( aSettings->GetPrintDC(), pos );
+    m_bitmapBase->DrawBitmap( aSettings->GetPrintDC(), pos, aSettings->GetBackgroundColor() );
 }
 
 

include/bitmap_base.h

@@ -2,7 +2,7 @@
  * This program source code file is part of KiCad, a free EDA CAD application.
  *
  * Copyright (C) 2018 jean-pierre.charras jp.charras at wanadoo.fr
- * Copyright (C) 2013-2020 KiCad Developers, see AUTHORS.txt for contributors.
+ * Copyright (C) 2013-2023 KiCad Developers, see AUTHORS.txt for contributors.
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License
@@ -29,11 +29,7 @@
 #include <wx/image.h>
 #include <kiid.h>
 #include <math/box2.h>
-
-namespace KIGFX
-{
-class COLOR4D;
-}
+#include <gal/color4d.h>
 
 class LINE_READER;
 class PLOTTER;
@@ -134,7 +130,8 @@ public:
      */
     const BOX2I GetBoundingBox() const;
 
-    void DrawBitmap( wxDC* aDC, const VECTOR2I& aPos );
+    void DrawBitmap( wxDC* aDC, const VECTOR2I& aPos,
+                     const KIGFX::COLOR4D& aBackgroundColor = KIGFX::COLOR4D::UNSPECIFIED );
 
     /**
      * Reads and stores in memory an image file.