Fix alpha channel values when using wxGCDC with wxMemoryDC in wxMSW.

Ensure that 32bpp bitmaps selected in wxMemoryDC use DIB for their internal
representation as GDI+ functions don't seem to work correctly with DDBs with
alpha channel.

Closes #13328.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75648 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/dcgraph.cpp

@@ -18,6 +18,7 @@
 #if wxUSE_GRAPHICS_CONTEXT
 
 #include "wx/dcgraph.h"
+#include "wx/rawbmp.h"
 
 #ifndef WX_PRECOMP
     #include "wx/icon.h"
@@ -171,6 +172,61 @@ wxGCDCImpl::wxGCDCImpl( wxDC *owner, const wxWindowDC& dc ) :
 wxGCDCImpl::wxGCDCImpl( wxDC *owner, const wxMemoryDC& dc ) :
    wxDCImpl( owner )
 {
+#ifndef NEVER_USE_DIB
+    // It seems that GDI+ sets invalid values for alpha channel when used with
+    // a compatible bitmap (DDB). So we need to convert the currently selected
+    // bitmap to a DIB before using it with any GDI+ functions to ensure that
+    // we get the correct alpha channel values in it at the end.
+
+    wxBitmap bmp = dc.GetSelectedBitmap();
+    wxASSERT_MSG( bmp.IsOk(), "Should select a bitmap before creating wxGCDC" );
+
+    // We don't need to convert it if it can't have alpha at all (any depth but
+    // 32) or is already a DIB with alpha.
+    if ( bmp.GetDepth() == 32 && (!bmp.IsDIB() || !bmp.HasAlpha()) )
+    {
+        // We need to temporarily deselect this bitmap from the memory DC
+        // before modifying it.
+        const_cast<wxMemoryDC&>(dc).SelectObject(wxNullBitmap);
+
+        bmp.ConvertToDIB(); // Does nothing if already a DIB.
+
+        if( !bmp.HasAlpha() )
+        {
+            // Initialize alpha channel, even if we don't have any alpha yet,
+            // we should have correct (opaque) alpha values in it for GDI+
+            // functions to work correctly.
+            {
+                wxAlphaPixelData data(bmp);
+                if ( data )
+                {
+                    wxAlphaPixelData::Iterator p(data);
+                    for ( int y = 0; y < data.GetHeight(); y++ )
+                    {
+                        wxAlphaPixelData::Iterator rowStart = p;
+
+                        for ( int x = 0; x < data.GetWidth(); x++ )
+                        {
+                            p.Alpha() = wxALPHA_OPAQUE;
+                            ++p;
+                        }
+
+                        p = rowStart;
+                        p.OffsetY(data, 1);
+                    }
+                }
+            } // End of block modifying the bitmap.
+
+            // Using wxAlphaPixelData sets the internal "has alpha" flag but we
+            // don't really have any alpha yet, so reset it back for now.
+            bmp.ResetAlpha();
+        }
+
+        // Undo SelectObject() at the beginning of this block.
+        const_cast<wxMemoryDC&>(dc).SelectObjectAsSource(bmp);
+    }
+#endif // !NEVER_USE_DIB
+
     Init(wxGraphicsContext::Create(dc));
 }