Canvas如何实现图像的居中、平铺效果

以Panel1的CustomDraw事件为例：

var
DeskTopImage: TGraphic;
vLeft, vTop: Integer;

//平铺
Panel1.Canvas.StretchDraw(Panel1.ClientRect,DeskTopImage);

//居中
vLeft:= (ARect.Left + ARect.Right - DeskTopImage.Width) div 2;
vTop:= (ARect.Top + ARect.Bottom - DeskTopImage.Height) div 2;
Panel1.Canvas.Draw(vLeft,vTop,DeskTopImage);