技巧:以任意比例混合相同的颜色会产生相同的颜色。
因此,最简单的方法(也许也是最有效的方法)是首先将透明结果绘制到临时位图,然后在目标画布上对该位图进行 alphablend。
在绘图期间访问目标画布:
procedure TfrmMain.PaintBox1Paint(Sender: TObject);
const
BarSize = 30;
var
R: TRect;
Bmp: TBitmap;
BlendFunc: TBlendFunction;
begin
with PaintBox1 do
begin
R := ClientRect;
Canvas.Brush.Color := clBlue;
Canvas.FillRect(R);
Canvas.Brush.Color := clRed;
Canvas.Ellipse(R);
Bmp := TBitmap.Create;
try
Bmp.Width := Width;
Bmp.Height := Height;
BitBlt(Bmp.Canvas.Handle, 0, 0, Width, Height, Canvas.Handle, 0, 0,
SRCCOPY);
Bmp.Canvas.Brush.Color := clGreen;
R := Bounds(Width div 3, 0, Width div 3 + 1, BarSize + 1);
Bmp.Canvas.Rectangle(R);
BlendFunc.BlendOp := AC_SRC_OVER;
BlendFunc.BlendFlags := 0;
BlendFunc.SourceConstantAlpha := 80;
BlendFunc.AlphaFormat := 0;
Windows.AlphaBlend(Canvas.Handle, 0, 0, Width, Height, Bmp.Canvas.Handle,
0, 0, Width, Height, BlendFunc);
finally
Bmp.Free;
end;
end;
end;
并且在绘制过程中无法访问目标画布:
procedure GetRemoteBitmap(Bmp: TBitmap; Width, Height: Integer);
const
BarSize = 30;
var
R: TRect;
begin
Bmp.Canvas.Brush.Color := clFuchsia;
Bmp.Width := Width;
Bmp.Height := Height;
Bmp.TransparentColor := clFuchsia;
Bmp.Transparent := True;
Bmp.Canvas.Brush.Color := clGreen;
R := Bounds(Width div 3, 0, Width div 3 + 1, BarSize + 1);
Bmp.Canvas.Rectangle(R);
end;
procedure TfrmMain.PaintBox1Paint(Sender: TObject);
var
R: TRect;
Bmp: TBitmap;
Tmp: TBitmap;
BlendFunc: TBlendFunction;
begin
with PaintBox1 do
begin
R := ClientRect;
Canvas.Brush.Color := clBlue;
Canvas.FillRect(R);
Canvas.Brush.Color := clRed;
Canvas.Ellipse(R);
Bmp := TBitmap.Create;
Tmp := TBitmap.Create;
try
GetRemoteBitmap(Bmp, Width, Height);
Tmp.Width := Width;
Tmp.Height := Height;
BitBlt(Tmp.Canvas.Handle, 0, 0, Width, Height, Canvas.Handle, 0, 0,
SRCCOPY);
TransparentBlt(Tmp.Canvas.Handle, 0, 0, Width, Height, Bmp.Canvas.Handle,
0, 0, Width, Height, ColorToRGB(clFuchsia));
BlendFunc.BlendOp := AC_SRC_OVER;
BlendFunc.BlendFlags := 0;
BlendFunc.SourceConstantAlpha := 80;
BlendFunc.AlphaFormat := 0;
Windows.AlphaBlend(Canvas.Handle, 0, 0, Width, Height, Tmp.Canvas.Handle,
0, 0, Width, Height, BlendFunc);
finally
Tmp.Free;
Bmp.Free;
end;
end;
end;