erasing with a circle on a canvas to see what is behind it on z-index

Question

I am trying to make a layered drawing application (a canvas on with something behind it) where I want to erase and see through the canvas on top of the z-index. But for some reason onmousemove I cant make it continue.

I think something is wrong with the way I deal with the canvas in the draw method but I am not sure.

The following code puts a canvas on top of another canvas where the upper one is blue and the one beneath is red, and the purpose is to erase with a circular shape on the upper blue canvas on mouse move, in order to wipe the canvas with the mouse, like with an eraser in Photoshop or GIMP.

<html>
<script type="text/javascript">
//inspired by http://stackoverflow.com/questions/2368784/draw-on-html5-canvas-using-a-mouse
var canvas, canvasUnder, ctx, ctxUnder, circle, drawing = false,
    prevX = 0,
    currX = 0,
    prevY = 0,
    currY = 0;

var x = "black",
    y = 2;

var w,h;

function init() {
    canvas = document.getElementById('can');
    ctx = canvas.getContext("2d");

    canvasUnder = document.getElementById('under');
    ctxUnder = canvasUnder.getContext("2d");

    w = canvas.width;
    h = canvas.height;
    canvas.addEventListener("mousemove", function (e) {
        findxy('move', e)
    }, false);
    canvas.addEventListener("mousedown", function (e) {
        findxy('down', e)
    }, false);
    canvas.addEventListener("mouseup", function (e) {
        findxy('up', e)
    }, false);
    canvas.addEventListener("mouseout", function (e) {
        findxy('out', e)
    }, false);

    ctx.rect(0,0,canvas.width,canvas.height);
    ctx.fillStyle="blue";
    ctx.fill();

    //fill the canvas under with solid color to start with
    ctxUnder.rect(0,0,canvasUnder.width,canvasUnder.height);
    ctxUnder.fillStyle="red";
    ctxUnder.fill();
}

function draw() {
    circle = new Path2D();
    circle.arc(currX,currY,16,0,2*Math.PI);
    ctx.clip(circle);
    ctx.clearRect(0, 0, w, h);
    ctx.restore();
}

function findxy(res, e) {
    if (res == 'down') {
        prevX = currX;
        prevY = currY;
        currX = e.clientX - canvas.offsetLeft;
        currY = e.clientY - canvas.offsetTop;

        drawing = true;
    }
    if (res == 'up' || res == "out") {
        drawing = false;
    }
    if (res == 'move') {
        if (drawing) {
            prevX = currX;
            prevY = currY;
            currX = e.clientX - canvas.offsetLeft;
            currY = e.clientY - canvas.offsetTop;
            draw();
        }
    }
}
</script>
<body onload="init()">
    <canvas id="under" width="400" height="400" style="position:absolute;top:10%;left:10%;border:2px solid;" z-index="1;"></canvas>
    <canvas id="can" width="400" height="400" style="position:absolute;top:10%;left:10%;border:2px solid;" z-index="2;"></canvas>
</body>
</html>

The question is: Why does it only work on first call of draw() and not after?

I set up a fiddle for this here: https://jsfiddle.net/avpv1ahr/1/

score 1 · Accepted Answer · answered Nov 19 '15 at 14:08

The .clip() method of a 2D rendering context only reduces the region (see also #14499). So when you try to erase the area of next "circle", the first circle is still in effect. Therefore, you don't actually see a change.

Since resetClip() isn't implemented in several current browsers, you have to save and restore the current context state:

function draw() {
    ctx.save(); // save the state

    circle = new Path2D();
    circle.arc(currX,currY,16,0,2*Math.PI);
    ctx.clip(circle);
    ctx.clearRect(0, 0, w, h);

    ctx.restore(); // restore the state
}

While your code already contained ctx.restore(), it was missing the ctx.save().

erasing with a circle on a canvas to see what is behind it on z-index

1 Answers1