Draw a translucent effect image

We may use windows GDI function AlphaBlend to draw translucent graphic. The following example extract a portion of desktop image and draw to TImage.canvas with translucent effect. The resulting graphic will looks like there is a green color transparent sheet cover on the image.

var hDesktop: HWND;
 DC: HDC;
 T: BLENDFUNCTION;
begin
hDesktop := GetDesktopWindow;
DC := GetDC(hDesktop);
try
 T.BlendOp := AC_SRC_OVER;
 T.BlendFlags := 0;
 T.SourceConstantAlpha := $80;
 T.AlphaFormat := 0;

 Image1.Canvas.Brush.Color := clGreen;
 Image1.Canvas.FillRect(Rect(0, 0, Image1.Width, Image1.Height));
 Windows.AlphaBlend(Image1.Canvas.Handle, 0, 0, Image1.Width, Image1.Height, DC, 0, 0, Image1.Width, Image1.Height, T);
finally
 ReleaseDC(hDesktop, DC);
end;
end;