Windows AlphaBlend函數

var
  BlendFunc: TBlendFunction;
begin
  BlendFunc.BlendOp := AC_SRC_OVER;
  BlendFunc.BlendFlags := 0;
  BlendFunc.SourceConstantAlpha := TrackBar1.Position;

  if bit.PixelFormat = pf32bit then
    BlendFunc.AlphaFormat := AC_SRC_ALPHA
  else
    BlendFunc.AlphaFormat := 0;


  Windows.AlphaBlend(Canvas.Handle,                 {目標 DC}
                     0, 0, bit.Width, bit.Height, {目標位置與大小}
                     bit.Canvas.Handle,             {源 DC}
                     0, 0, bit.Width, bit.Height,   {源位置與大小}
                     BlendFunc                      {透明相關的結構}
                     );
end;