สอบถามเกี่ยวกับ canvas ค่ะ สอบถามเกี่ยวกับ canvas ค่ะ

var canvasDiv=document.getElementById("canvas");
var gr=new jsGraphics(canvasDiv);
setPen(true);

var num = 0;
	
var points=new Array(1);
var ie=false;
if(document.all)
	ie=true;
if (!ie)
{
 //canvasDiv.captureEvents(Event.MOUSEMOVE);
 //canvasDiv.captureEvents(Event.CLICK);
}
canvasDiv.onmousemove = getMouseXY;
canvasDiv.onclick = drawPoint;
var mouseX = 1;
var mouseY = 1;
//Get mouse position
function getMouseXY(e)
{
  if (ie) 
	{
    mouseX = event.clientX + document.body.parentElement.scrollLeft;
    mouseY = event.clientY + document.body.parentElement.scrollTop;
  } else { 
    mouseX = e.pageX;
    mouseY = e.pageY;
  }  
  if (mouseX < 1){mouseX = 1}
  if (mouseY < 1){mouseY = 1}  
  
  mouseX =mouseX - canvasDiv.offsetLeft;
  mouseY =mouseY - canvasDiv.offsetTop;
  return true;
}

function drawPoint()
{
	if(num > 4 )
	{
		alert('Limit 4 point');
	}
	else
	{
	    gr.fillRectangle(new jsColor("red"),new jsPoint(mouseX-4,mouseY-4),4,4);
		points[points.length]=new jsPoint(mouseX-3,mouseY-3);
		
	    showPoints();

	    num++;
	}
	
	//points=new Array();
}



function showPoints()
{	
	var txt=document.getElementById("txt");
	txt.innerHTML="";
		
	for(var i=1;i<points.length;i++)
	{
	txt.innerHTML=txt.innerHTML + "Point "+i+" : (" + points[i].x + "," + points[i].y + ")\n";
	}

	
}