Much improved sensing if a machine will do Alpha blending. This may actually be the solution. If Alpha blending is not supported, we fall back to the old screen door transparency.

This does not take into account a multiple screen setup with different screen depths, but I assume that this is a rare enough constellation and would cause many other problems on top of this minor issue. As a fallback, we will *always* render the image, but worst case without any alpha channel.

git-svn-id: file:///fltk/svn/fltk/branches/branch-1.1@5437 ea41ed52-d2ee-0310-a9c1-e6b18d33e121

1 files changed, 34 insertions, 6 deletions

diff --git a/src/Fl_Double_Window.cxx b/src/Fl_Double_Window.cxx
index e648fa4ab..427c43807 100644
--- a/src/Fl_Double_Window.cxx
+++ b/src/Fl_Double_Window.cxx
@@ -80,6 +80,7 @@ typedef struct { BYTE a; BYTE b; BYTE c; BYTE d; } FL_BLENDFUNCTION;
 typedef BOOL (WINAPI* fl_alpha_blend_func)
     (HDC,int,int,int,int,HDC,int,int,int,int,FL_BLENDFUNCTION);
 static fl_alpha_blend_func fl_alpha_blend = NULL;
+static FL_BLENDFUNCTION blendfunc = { 0, 0, 255, 1};
 
 /*
  * This function checks if the version of MSWindows that we
@@ -89,13 +90,38 @@ static fl_alpha_blend_func fl_alpha_blend = NULL;
 char fl_can_do_alpha_blending() {
   static char been_here = 0;
   static char can_do = 0;
+  // do this test only once
   if (been_here) return can_do;
+  been_here = 1;
+  // load the library that implements alpha blending
   HMODULE hMod = LoadLibrary("MSIMG32.DLL");
+  // give up if that doesn't exist (Win95?)
   if (!hMod) return 0;
+  // now find the blending function inside that dll
   fl_alpha_blend = (fl_alpha_blend_func)GetProcAddress(hMod, "AlphaBlend");
+  // give up if we can't find it (Win95)
   if (!fl_alpha_blend) return 0;
-  can_do = 1;
-  return 1;
+  // we have the  call, but does our display support alpha blending?
+  HDC dc = 0L;//fl_gc;
+  // get the current or the desktop's device context
+  if (!dc) dc = GetDC(0L);
+  if (!dc) return 0;
+  // check the device capabilities flags. However GetDeviceCaps
+  // does not return anything useful, so we have to do it manually:
+
+  HBITMAP bm = CreateCompatibleBitmap(dc, 1, 1);
+  HDC new_gc = CreateCompatibleDC(dc);
+  int save = SaveDC(new_gc);
+  SelectObject(new_gc, bm);
+  COLORREF set = SetPixel(new_gc, 0, 0, 0x01010101);
+  BOOL alpha_ok = fl_alpha_blend(dc, 0, 0, 1, 1, new_gc, 0, 0, 1, 1, blendfunc);
+  RestoreDC(new_gc, save);
+  DeleteDC(new_gc);
+  DeleteObject(bm);
+
+  if (!fl_gc) ReleaseDC(0L, dc);
+  if (alpha_ok) can_do = 1;
+  return can_do;
 }
 
 HDC fl_makeDC(HBITMAP bitmap) {
@@ -119,13 +145,15 @@ void fl_copy_offscreen(int x,int y,int w,int h,HBITMAP bitmap,int srcx,int srcy)
 }
 
 void fl_copy_offscreen_with_alpha(int x,int y,int w,int h,HBITMAP bitmap,int srcx,int srcy) {
-  static FL_BLENDFUNCTION blendfunc = { 0, 0, 255, 1};
   HDC new_gc = CreateCompatibleDC(fl_gc);
   int save = SaveDC(new_gc);
   SelectObject(new_gc, bitmap);
-  if (fl_can_do_alpha_blending()) 
-    fl_alpha_blend(fl_gc, x, y, w, h, new_gc, srcx, srcy, w, h, blendfunc);
-  else
+  BOOL alpha_ok = 0;
+  // first try to alpha blend
+  if (fl_can_do_alpha_blending())
+    alpha_ok = fl_alpha_blend(fl_gc, x, y, w, h, new_gc, srcx, srcy, w, h, blendfunc);
+  // if that failed (it shouldn,t), still copy the bitmap over, but now alpha is 1
+  if (!alpha_ok)
     BitBlt(fl_gc, x, y, w, h, new_gc, srcx, srcy, SRCCOPY);
   RestoreDC(new_gc, save);
   DeleteDC(new_gc);