Always use DIBs in wxMSW wxGCDC.

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

Note: Code responsible for converting and fixing the bitmap has been moved
from wxGCDCImpl::wxGCDCImpl to wxGDIPlusRenderer::CreateContext because this
is the common entry point for creating context for GDI+.

Closes #17324.

src/common/dcgraph.cpp

@@ -18,7 +18,6 @@
 #if wxUSE_GRAPHICS_CONTEXT
 
 #include "wx/dcgraph.h"
-#include "wx/rawbmp.h"
 
 #ifndef WX_PRECOMP
     #include "wx/icon.h"
@@ -169,61 +168,6 @@ wxGCDCImpl::wxGCDCImpl( wxDC *owner, const wxWindowDC& dc ) :
 wxGCDCImpl::wxGCDCImpl( wxDC *owner, const wxMemoryDC& dc ) :
    wxDCImpl( owner )
 {
-#if defined(__WXMSW__) && wxUSE_WXDIB
-    // 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 // wxUSE_WXDIB
-
     Init(wxGraphicsContext::Create(dc));
 }
 

src/msw/graphics.cpp

@@ -46,6 +46,7 @@
     #include "wx/msw/enhmeta.h"
 #endif
 #include "wx/dcgraph.h"
+#include "wx/rawbmp.h"
 
 #include "wx/msw/private.h" // needs to be before #include <commdlg.h>
 
@@ -2107,6 +2108,61 @@ wxGraphicsContext * wxGDIPlusRenderer::CreateContext( const wxEnhMetaFileDC& dc)
 wxGraphicsContext * wxGDIPlusRenderer::CreateContext( const wxMemoryDC& dc)
 {
     ENSURE_LOADED_OR_RETURN(NULL);
+#if wxUSE_WXDIB
+    // 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 // wxUSE_WXDIB
+
     wxGDIPlusContext* context = new wxGDIPlusContext(this, dc);
     context->EnableOffset(true);
     return context;