MacOS: fix capture of window titlebar under MacOS 10.13 "High Sierra"

git-svn-id: file:///fltk/svn/fltk/branches/branch-1.4@12547 ea41ed52-d2ee-0310-a9c1-e6b18d33e121

1 files changed, 16 insertions, 0 deletions

diff --git a/src/Fl_cocoa.mm b/src/Fl_cocoa.mm
index 85f8e8e4e..dfdbff8e1 100644
--- a/src/Fl_cocoa.mm
+++ b/src/Fl_cocoa.mm
@@ -4335,9 +4335,25 @@ void Fl_Cocoa_Window_Driver::capture_titlebar_and_borders(Fl_Shared_Image*& top,
   uchar *rgba = new uchar[4 * w() * htop * 4];
   CGContextRef auxgc = CGBitmapContextCreate(rgba, 2 * w(), 2 * htop, 8, 8 * w(), cspace, kCGImageAlphaPremultipliedLast);
   CGColorSpaceRelease(cspace);
+  memset(rgba, 0xff, 4 * w() * htop * 4); // initialize to opaque white
   CGContextScaleCTM(auxgc, 2, 2);
   if (layer) {
     Fl_Cocoa_Window_Driver::draw_layer_to_context(layer, auxgc, w(), htop);
+    if (fl_mac_os_version >= 101300) {
+      // drawn layer is left transparent and alpha-premultiplied: demultiply it and set it opaque.
+      uchar *p = rgba;
+      uchar *last = rgba + 4 * w() * htop * 4;
+      while (p < last) {
+        uchar q = *(p+3);
+        if (q) {
+          float m = 255./q;
+          *p++ *= m;
+          *p++ *= m;
+          *p++ *= m;
+          *p++ = 0xff;
+        } else p += 4;
+      }
+    }
   } else {
     CGImageRef img = CGImage_from_window_rect(0, -htop, w(), htop);
     CGContextSaveGState(auxgc);