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

include/wx/msw/bitmap.h

@@ -136,6 +136,8 @@ public:
 #if wxUSE_WXDIB
     // copies from a device independent bitmap
     bool CopyFromDIB(const wxDIB& dib);
+    bool IsDIB() const;
+    bool ConvertToDIB();
 #endif
 
     virtual bool Create(int width, int height, int depth = wxBITMAP_SCREEN_DEPTH);

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));
 }
 

src/msw/bitmap.cpp

@@ -68,8 +68,15 @@ public:
 
     virtual void Free();
 
+    // Creates a new bitmap (DDB or DIB) from the contents of the given DIB.
     void CopyFromDIB(const wxDIB& dib);
 
+#ifndef NEVER_USE_DIB
+    // Takes ownership of the given DIB.
+    bool AssignDIB(wxDIB& dib);
+#endif // !NEVER_USE_DIB
+
+
     // set the mask object to use as the mask, we take ownership of it
     void SetMask(wxMask *mask)
     {
@@ -118,6 +125,11 @@ public:
 private:
     void Init();
 
+    // Use the given bitmap handle and take the rest of the characteristics
+    // (size, depth, ...) from the given DIB.
+    void InitFromDIB(const wxDIB& dib, HBITMAP hbitmap);
+
+
     // optional mask for transparent drawing
     wxMask       *m_bitmapMask;
 
@@ -249,6 +261,22 @@ void wxBitmapRefData::Free()
     wxDELETE(m_bitmapMask);
 }
 
+void wxBitmapRefData::InitFromDIB(const wxDIB& dib, HBITMAP hbitmap)
+{
+    m_width = dib.GetWidth();
+    m_height = dib.GetHeight();
+    m_depth = dib.GetDepth();
+
+    m_hBitmap = (WXHBITMAP)hbitmap;
+
+#if wxUSE_PALETTE
+    wxPalette *palette = dib.CreatePalette();
+    if ( palette )
+        m_bitmapPalette = *palette;
+    delete palette;
+#endif // wxUSE_PALETTE
+}
+
 void wxBitmapRefData::CopyFromDIB(const wxDIB& dib)
 {
     wxCHECK_RET( !IsOk(), "bitmap already initialized" );
@@ -264,20 +292,26 @@ void wxBitmapRefData::CopyFromDIB(const wxDIB& dib)
     m_isDIB = true;
 #endif // SOMETIMES_USE_DIB/ALWAYS_USE_DIB
 
-    m_width = dib.GetWidth();
-    m_height = dib.GetHeight();
-    m_depth = dib.GetDepth();
-
-    m_hBitmap = (WXHBITMAP)hbitmap;
-
-#if wxUSE_PALETTE
-    wxPalette *palette = dib.CreatePalette();
-    if ( palette )
-        m_bitmapPalette = *palette;
-    delete palette;
-#endif // wxUSE_PALETTE
+    InitFromDIB(dib, hbitmap);
 }
 
+#ifndef NEVER_USE_DIB
+
+bool wxBitmapRefData::AssignDIB(wxDIB& dib)
+{
+    if ( !dib.IsOk() )
+        return false;
+
+    Free();
+
+    m_isDIB = true;
+    InitFromDIB(dib, dib.Detach());
+
+    return true;
+}
+
+#endif // !NEVER_USE_DIB
+
 // ----------------------------------------------------------------------------
 // wxBitmap creation
 // ----------------------------------------------------------------------------
@@ -457,6 +491,26 @@ bool wxBitmap::CopyFromDIB(const wxDIB& dib)
     return true;
 }
 
+bool wxBitmap::IsDIB() const
+{
+    return GetBitmapData() && GetBitmapData()->m_isDIB;
+}
+
+bool wxBitmap::ConvertToDIB()
+{
+    if ( IsDIB() )
+        return true;
+
+    wxDIB dib(*this);
+    if ( !dib.IsOk() )
+        return false;
+
+    // It is important to reuse the current GetBitmapData() instead of creating
+    // a new one, as our object identity shouldn't change just because our
+    // internal representation did, but IsSameAs() compares data pointers.
+    return GetBitmapData()->AssignDIB(dib);
+}
+
 #endif // NEVER_USE_DIB
 
 wxBitmap::~wxBitmap()