[javascript] Real mouse position in canvas

You need to get the mouse position relative to the canvas

To do that you need to know the X/Y position of the canvas on the page.

This is called the canvas’s “offset”, and here’s how to get the offset. (I’m using jQuery in order to simplify cross-browser compatibility, but if you want to use raw javascript a quick Google will get that too).

    var canvasOffset=$("#canvas").offset();
    var offsetX=canvasOffset.left;
    var offsetY=canvasOffset.top;

Then in your mouse handler, you can get the mouse X/Y like this:

  function handleMouseDown(e){
      mouseX=parseInt(e.clientX-offsetX);
      mouseY=parseInt(e.clientY-offsetY);
}

Here is an illustrating code and fiddle that shows how to successfully track mouse events on the canvas:

http://jsfiddle.net/m1erickson/WB7Zu/

<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>

<style>
    body{ background-color: ivory; }
    canvas{border:1px solid red;}
</style>

<script>
$(function(){

    var canvas=document.getElementById("canvas");
    var ctx=canvas.getContext("2d");

    var canvasOffset=$("#canvas").offset();
    var offsetX=canvasOffset.left;
    var offsetY=canvasOffset.top;

    function handleMouseDown(e){
      mouseX=parseInt(e.clientX-offsetX);
      mouseY=parseInt(e.clientY-offsetY);
      $("#downlog").html("Down: "+ mouseX + " / " + mouseY);

      // Put your mousedown stuff here

    }

    function handleMouseUp(e){
      mouseX=parseInt(e.clientX-offsetX);
      mouseY=parseInt(e.clientY-offsetY);
      $("#uplog").html("Up: "+ mouseX + " / " + mouseY);

      // Put your mouseup stuff here
    }

    function handleMouseOut(e){
      mouseX=parseInt(e.clientX-offsetX);
      mouseY=parseInt(e.clientY-offsetY);
      $("#outlog").html("Out: "+ mouseX + " / " + mouseY);

      // Put your mouseOut stuff here
    }

    function handleMouseMove(e){
      mouseX=parseInt(e.clientX-offsetX);
      mouseY=parseInt(e.clientY-offsetY);
      $("#movelog").html("Move: "+ mouseX + " / " + mouseY);

      // Put your mousemove stuff here

    }

    $("#canvas").mousedown(function(e){handleMouseDown(e);});
    $("#canvas").mousemove(function(e){handleMouseMove(e);});
    $("#canvas").mouseup(function(e){handleMouseUp(e);});
    $("#canvas").mouseout(function(e){handleMouseOut(e);});

}); // end $(function(){});
</script>

</head>

<body>
    <p>Move, press and release the mouse</p>
    <p id="downlog">Down</p>
    <p id="movelog">Move</p>
    <p id="uplog">Up</p>
    <p id="outlog">Out</p>
    <canvas id="canvas" width=300 height=300></canvas>

</body>
</html>

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