用SelectClipRgn进行区域屏蔽的.这是什么原因???(50分)

LeftDl:=1000;
mpoint[0] := Point(LeftDl, 0);
Mpoint[1]:=point(14363,0);
mpoint[2] := Point(mpoint[1].x, SCanvasH);
mpoint[3] := Point(mpoint[0].x, SCanvasH);



hand := CreatePolygonRgn(mpoint, 4, 2);
try
SelectClipRgn(Dcanvas.handle, hand);
try
Dcanvas.StretchDraw(rect(LeftDl - LeftSl, 0, scanvasw + LeftDl - LeftSl, scanvasH), Semf);
finally
SelectClipRgn(Dcanvas.handle, 0);
end;
finally
deleteobject(hand);
end;


当leftdl=1000时,,图可以画得出来..


为什么当我把leftdl改为2000时图就出不来了呢????