Adding a native implementation for clearing bitmap/window contexts

Filling a bitmap surface is filled with ARGB 0,0,0,0. This way eg buffered transparent layers can be properly cleared.

include/wx/graphics.h

@@ -653,6 +653,9 @@ public:
     // draws a path by first filling and then stroking
     virtual void DrawPath( const wxGraphicsPath& path, wxPolygonFillMode fillStyle = wxODDEVEN_RULE );
 
+    // paints a transparent rectangle (only useful for bitmaps or windows)
+    virtual void ClearRectangle(wxDouble x, wxDouble y, wxDouble w, wxDouble h);
+
     //
     // text
     //

src/common/dcgraph.cpp

@@ -1249,12 +1249,15 @@ wxCoord wxGCDCImpl::GetCharHeight(void) const
 void wxGCDCImpl::Clear(void)
 {
     wxCHECK_RET( IsOk(), wxT("wxGCDC(cg)::Clear - invalid DC") );
+    
+    if ( m_backgroundBrush.IsOk() )
+    {
         m_graphicContext->SetBrush( m_backgroundBrush );
         wxPen p = *wxTRANSPARENT_PEN;
         m_graphicContext->SetPen( p );
         wxCompositionMode formerMode = m_graphicContext->GetCompositionMode();
         m_graphicContext->SetCompositionMode(wxCOMPOSITION_SOURCE);
-#ifdef __WXOSX__
+    #ifdef __WXOSX__
         // This is a legacy implementation which doesn't take advantage
         // of clipping region bounds retrieved by wxMacCoreGraphicsContext::GetClipBox
         // because this function is not yet verified.
@@ -1268,14 +1271,27 @@ void wxGCDCImpl::Clear(void)
         DoDrawRectangle(
             DeviceToLogicalX(0), DeviceToLogicalY(0),
             DeviceToLogicalXRel(0x800000 - 64), DeviceToLogicalYRel(0x800000 - 64));
-#else
+    #else
         double x, y, w, h;
         m_graphicContext->GetClipBox(&x, &y, &w, &h);
         m_graphicContext->DrawRectangle(x, y, w, h);
-#endif // __WXOSX__ / !__WXOSX__
+    #endif // __WXOSX__ / !__WXOSX__
         m_graphicContext->SetCompositionMode(formerMode);
         m_graphicContext->SetPen( m_pen );
         m_graphicContext->SetBrush( m_brush );
+    }
+    else
+    {
+#ifdef __WXOSX__
+        // same comment as above applies
+        m_graphicContext->ClearRectangle(DeviceToLogicalX(0), DeviceToLogicalY(0),
+                                         DeviceToLogicalXRel(0x8000 - 64), DeviceToLogicalYRel(0x8000 - 64));
+#else
+        double x, y, w, h;
+        m_graphicContext->GetClipBox(&x, &y, &w, &h);
+        m_graphicContext->ClearRectangle(x, y, w, h);
+#endif
+    }
 }
 
 void wxGCDCImpl::DoGetSize(int *width, int *height) const

src/common/graphcmn.cpp

@@ -759,6 +759,11 @@ void wxGraphicsContext::DrawRectangle( wxDouble x, wxDouble y, wxDouble w, wxDou
     DrawPath( path );
 }
 
+void wxGraphicsContext::ClearRectangle( wxDouble WXUNUSED(x), wxDouble WXUNUSED(y), wxDouble WXUNUSED(w), wxDouble WXUNUSED(h))
+{
+    
+}
+
 void wxGraphicsContext::DrawEllipse( wxDouble x, wxDouble y, wxDouble w, wxDouble h)
 {
     wxGraphicsPath path = CreatePath();

src/generic/graphicc.cpp

@@ -98,7 +98,6 @@ using namespace std;
 #ifdef __WXMAC__
 #include "wx/osx/private.h"
 #include <cairo-quartz.h>
-#include <cairo-atsui.h>
 #endif
 
 // Helper functions for dealing with alpha pre-multiplication.
@@ -476,6 +475,7 @@ public:
 
     virtual void StrokePath( const wxGraphicsPath& p ) wxOVERRIDE;
     virtual void FillPath( const wxGraphicsPath& p , wxPolygonFillMode fillStyle = wxWINDING_RULE ) wxOVERRIDE;
+    virtual void ClearRectangle( wxDouble x, wxDouble y, wxDouble w, wxDouble h ) wxOVERRIDE;
     
     virtual void Translate( wxDouble dx , wxDouble dy ) wxOVERRIDE;
     virtual void Scale( wxDouble xScale , wxDouble yScale ) wxOVERRIDE;
@@ -2440,6 +2440,15 @@ void wxCairoContext::FillPath( const wxGraphicsPath& path , wxPolygonFillMode fi
     }
 }
 
+void wxCairoContext::ClearRectangle( wxDouble x, wxDouble y, wxDouble w, wxDouble h )
+{
+    cairo_save(m_context);
+    cairo_set_operator(m_context, CAIRO_OPERATOR_CLEAR);
+    cairo_rectangle (m_context, x, y, w, h);
+    cairo_fill (m_context);
+    cairo_restore(m_context);
+}
+
 void wxCairoContext::Rotate( wxDouble angle )
 {
     cairo_rotate(m_context,angle);

src/osx/carbon/graphics.cpp

@@ -1338,6 +1338,9 @@ public:
     // draws a path by first filling and then stroking
     virtual void DrawPath( const wxGraphicsPath &path, wxPolygonFillMode fillStyle = wxODDEVEN_RULE ) wxOVERRIDE;
     
+    // paints a transparent rectangle (only useful for bitmaps or windows)
+    virtual void ClearRectangle(wxDouble x, wxDouble y, wxDouble w, wxDouble h) wxOVERRIDE;
+    
     virtual bool ShouldOffset() const wxOVERRIDE
     {
         if ( !m_enableOffset )
@@ -2348,6 +2351,14 @@ void * wxMacCoreGraphicsContext::GetNativeContext()
     return m_cgContext;
 }
 
+void wxMacCoreGraphicsContext::ClearRectangle( wxDouble x, wxDouble y, wxDouble w, wxDouble h )
+{
+    if (!EnsureIsValid())
+        return;
+    
+    CGRect rect = CGRectMake( (CGFloat) x , (CGFloat) y , (CGFloat) w , (CGFloat) h );
+    CGContextClearRect(m_cgContext, rect);
+}
 
 void wxMacCoreGraphicsContext::DrawRectangle( wxDouble x, wxDouble y, wxDouble w, wxDouble h )
 {