Fix conversion of 32 bit ARGB bitmaps to wxImage in wxMSW.

wxDIB::m_hasAlpha can't be trusted when the DIB was loaded from a file so
don't rely on it in wxDIB::ConvertToImage(). Instead, suppose that 32 bpp
bitmaps do have alpha channel and only get rid of it at the end of conversion
if it turns out that all alpha values were 0.

Closes #10133.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65898 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

diff --git a/src/msw/dib.cpp b/src/msw/dib.cpp
index be10fcf0470dc2cddcc0abff76a9520d6c6dff4f..3675d7e8a05ca50ebe8964515857785580bdc22f 100644 (file)
--- a/src/msw/dib.cpp
+++ b/src/msw/dib.cpp
@@ -748,20 +748,25 @@ wxImage wxDIB::ConvertToImage() const
         return wxNullImage;
     }
 
-    if ( m_hasAlpha )
+    const int bpp = GetDepth();
+    bool hasAlpha = false;
+    if ( bpp == 32 )
     {
+        // 32 bit bitmaps may be either 0RGB or ARGB and we don't know in
+        // advance which one do we have so suppose we have alpha of them and
+        // get rid of it later if it turns out we didn't (in particular, don't
+        // trust m_hasAlpha which is not set correctly when the image was
+        // loaded from file).
         image.SetAlpha();
     }
 
     // this is the same loop as in Create() just above but with copy direction
     // reversed
-    const int bpp = GetDepth();
     const int dstBytesPerLine = w * 3;
     const int srcBytesPerLine = GetLineSize(w, bpp);
     unsigned char *dst = image.GetData() + ((h - 1) * dstBytesPerLine);
     unsigned char *alpha = image.HasAlpha() ? image.GetAlpha() + (h - 1)*w
                                             : NULL;
-    const bool is32bit = bpp == 32;
     const unsigned char *srcLineStart = (unsigned char *)GetData();
     for ( int y = 0; y < h; y++ )
     {
@@ -773,20 +778,19 @@ wxImage wxDIB::ConvertToImage() const
             dst[1] = *src++;
             dst[0] = *src++;
 
-            if ( is32bit )
+            if ( bpp == 32 )
             {
-                if ( alpha )
+                // wxImage uses non premultiplied alpha so undo
+                // premultiplication done in Create() above
+                const unsigned char a = *src;
+                *alpha++ = a;
+                if ( a > 0 )
                 {
-                    // wxImage uses non premultiplied alpha so undo
-                    // premultiplication done in Create() above
-                    const unsigned char a = *src;
-                    *alpha++ = a;
-                    if ( a > 0 )
-                    {
-                        dst[0] = (dst[0] * 255) / a;
-                        dst[1] = (dst[1] * 255) / a;
-                        dst[2] = (dst[2] * 255) / a;
-                    }
+                    dst[0] = (dst[0] * 255) / a;
+                    dst[1] = (dst[1] * 255) / a;
+                    dst[2] = (dst[2] * 255) / a;
+
+                    hasAlpha = true;
                 }
 
                 src++;
@@ -804,6 +808,9 @@ wxImage wxDIB::ConvertToImage() const
         srcLineStart += srcBytesPerLine;
     }
 
+    if ( !hasAlpha && image.HasAlpha() )
+        image.ClearAlpha();
+
     return image;
 }