delphi的canvas问题！

Question

各位： 
image1.Canvas.LineTo(100,100);
image1.Canvas.Pen.Color:=clred;
image1.Canvas.LineTo(200,200);
为什么后来画的线不能覆盖前面画的呢？

Answer 1

image1.Canvas.LineTo(100,100);
image1.Canvas.Pen.Color:=clred;
image1.Canvas.MoveTo(0,0);
image1.Canvas.LineTo(100,100);

Answer 2

image1.Canvas.Pen.Color:=clred;
image1.Canvas.LineTo(100,100);//清除
image1.Canvas.Pen.Mode= pmXor;
image1.Canvas.LineTo(100,100);

Answer 3

image1.Canvas.MoveTo(0,0);
image1.Canvas.Pen.Color:=clred;
image1.Canvas.LineTo(100,100);//清除
image1.Canvas.MoveTo(0,0);
image1.Canvas.Pen.Mode= pmXor;
image1.Canvas.LineTo(100,100);