在圖層中繪製一個透明的橢圓

Question

我在窗體上有一個TImgView32（名爲CityMap），並在其上加載圖像。現在，我創建了一個層（TBitmapLayer）和借鑑使用Canvas.Ellipse一個TBitmap32變量像下面這樣的循環：

procedure TfrmMain.Button1Click(Sender: TObject); 
var 
    tmpBmp: TBitmap32; 
    tmpBL: TBitmapLayer; 
begin 
    tmpBL:= TBitmapLayer.Create(CityMap.Layers); 

    tmpBmp:= TBitmap32.Create; 

    with tmpBmp do 
    begin 
    //Clear; 
    SetSize(50, 50); 
    Canvas.Brush.Color := clYellow; 
    Canvas.Brush.Style:= bsSolid; 
    Canvas.Pen.Color := clBlue; 
    Canvas.Pen.Width := 2; 
    Canvas.Ellipse(Rect(0, 0, 50, 50)); 
    end; 

    with tmpBL do 
    begin 
    Scaled:=true; 
    Bitmap.DrawMode:=dmBlend; 
    tmpBL.Bitmap:=(tmpBmp); 
    //tmpBmp.DrawTo(tmpBL.Bitmap, 0, 0); This line doesn't work! So using above line instead 
    end; 

    //... 

end; 

的結果是這樣的：

正如你看到的問題是那令人討厭的黑色矩形。如何創建這樣一個結果：

Answer 1

使用dmTransparent繪製模式爲您TBitmap32圖像的DrawMode屬性：

procedure TForm1.Button1Click(Sender: TObject); 
var 
    Bitmap: TBitmap32; 
    Layer: TBitmapLayer; 
begin 
    Layer := TBitmapLayer.Create(CityMap.Layers); 

    Bitmap := TBitmap32.Create; 
    Bitmap.DrawMode := dmTransparent; 
    Bitmap.SetSize(50, 50); 
    Bitmap.Canvas.Brush.Color := clYellow; 
    Bitmap.Canvas.Brush.Style:= bsSolid; 
    Bitmap.Canvas.Pen.Color := clBlue; 
    Bitmap.Canvas.Pen.Width := 2; 
    Bitmap.Canvas.Ellipse(Rect(0, 0, 50, 50)); 

    Layer.Scaled := True; 
    Layer.Bitmap := Bitmap; 
    ... 
end;