Paper.js, gap between cursor and canvas

Question

I am trying to build a simple drawing app with Paper.js. I can draw with the mouse but I have a gap between the cursor and the line drawn on the canvas.

I have realised that the navigation menu on top of the webpage is pushing down the cursor (100px) I assume that the mouse coordinates are taken from top-left (0-0) of the window and the same coordinates are used on the canvas which measured these points from its own top-left corner (see the screenshot). I have tried setting the canvas position to absolute, which helps to get it to the top left corer, but I need it in the center of the window.

How could I fix this?

Thanks!

script:

...
    var doc = $(document),
            win = $(window),
            canvas = $('#paper'),
            ctx = canvas[0].getContext('2d')

    doc.on('mousemove',function(e){
            if($.now() - lastEmit > 30){
                socket.emit('mousemove',{
                    'x': e.pageX,
                    'y': e.pageY,
                    'drawing': drawing,
                    'id': id                
                });

                lastEmit = $.now();
            }

            // Draw a line for the current user's movement, as it is
            // not received in the socket.on('moving') event above
            // (because he only broadcats (not receiving))

            if(drawing){

                drawLine(prev.x, prev.y, e.pageX, e.pageY);

                prev.x = e.pageX;
                prev.y = e.pageY;
            }
        });




        function drawLine(fromx, fromy, tox, toy){
            console.log(fromy + ' ' + toy);

            ctx.moveTo(fromx, fromy);
            ctx.lineTo(tox, toy);
            ctx.stroke();
        }

Answer 1

I can repro your issue by using an early version of paperjs - notably this version is the one on jsfiddle. I see the lines offset the way you do with v 0.22 and then correctly rendered with version 0.9.25. Here's my code:

<!-- fails -->
<script type='text/javascript' src='http://cdnjs.cloudflare.com/ajax/libs/paper.js/0.22/paper.js'></script>

<!-- works --> <!--
<script type='text/javascript' src='https://cdnjs.cloudflare.com/ajax/libs/paper.js/0.9.25/paper-full.js'></script>
-->

<style type='text/css'>
    #canvas1{
        height: 400px;
        width: 400px;
        border: 1px solid black;
    }
</style>

<script type='text/javascript'>
    paper.install(window);

    window.onload = function() {
        paper.setup('canvas1');
        var path = new Path();
        var tool = new Tool();
        path.strokeColor = 'black';
        tool.onMouseMove = function(event) {
            path.add(event.point);
        }

    }
</script>