increase performance on html canvas mousemove image mask

Question

I have a canvas that is drawing an image and clipping to create the effect that the image is being revealed. I have the code working properly I have tried using a debouce method and also rAF to increase the canvas rendering performance but I only saw small gains if any.

I suspect the way I am iterating through my array of x and y coordinates could be the issue. It seems to lag quite a bit when it is out putting the array in console about the same rate as the circle appear on the screen.

Here is the redraw function:

  function redraw(mouse) {
     m.push(mouse);
     m.forEach(function (a) {
        ctx.drawImage(img, 0, 0);
        ctx.beginPath();
        ctx.rect(0, 0, 500, 500);
        ctx.arc(a.x, a.y, 70, 0, Math.PI * 2, true);
        ctx.clip();
        ctx.fillRect(0, 0, 500, 500)
      })
    }

I guess what I am looking for is some advice to speed up my code so the rendering of the circles seems more like drawing.

Here is the working demo -> http://jsfiddle.net/naeluh/4h7GR/

GameAlchemist GameAlchemist · Accepted Answer · 2014-05-25T03:43:26

There are several issues here :
• Your mouse code is a nightmare, traversing the DOM on every move.
• You are redrawing everything on each move.

So i suggest a way more efficient solution :
• stack two canvases, the one below is your image, the one on top is the mask.
• Deal efficiently with the mouse.
• Only clear part of the mask canvas on mouse move : just one circle drawn on the mask canvas for each move.
(for that i used a globalCompositeOperation = 'destination-out' )

Result is perfectly smooth either on Firefox, Chrome, or Safari .

(tested on mac OS).

the fiddle : (you have to click to clear)

http://jsfiddle.net/gamealchemist/4h7GR/22/

html

<canvas   style='position: absolute; top: 0;left: 0;' id="canvas1" width="500" height="500"></canvas>
<canvas style='position: absolute;top: 0;left: 0;' id="canvas2" width="500" height="500"></canvas>

js

var can = document.getElementById("canvas1");
var ctx = can.getContext("2d");
var can2 = document.getElementById("canvas2");
var ctx2 = can2.getContext("2d");

var img = new Image();
img.onload = function () { ctx.drawImage(img,0,0); };
img.src = "http://placekitten.com/500/500";

ctx2.fillStyle='#000';
ctx2.fillRect(0,0,500,500);
ctx2.globalCompositeOperation = 'destination-out';

function clearThis(x,y) {
    console.log('toto');
    ctx2.fillStyle='#F00000';
    ctx2.beginPath();
    ctx2.arc(x, y, 70, 0, Math.PI * 2, true);
    ctx2.fill();
}

var mouse = {
    x: 0,
    y: 0,
    down: false
};

function setupMouse(canvas, onMouseMove, preventDefault) {
    var rectLeft, rectTop;
    var hook = canvas.addEventListener.bind(canvas);
    var mouseDown = updateMouseStatus.bind(null, true);
    var mouseUp = updateMouseStatus.bind(null, false);
    hook('mousedown', mouseDown);
    hook('mouseup', mouseUp);
    hook('mousemove', updateCoordinates);
    hook('scroll', updateRect);
    // var mouseOut = function() { mouse.down=false ; } ;
    // hook('mouseout', mouseOut); 
    function updateMouseStatus(b, e) {
        mouse.down = b;
        updateCoordinates(e);
        if (preventDefault) {
            e.stopPropagation();
            e.preventDefault();
        }
    }

    function updateCoordinates(e) {
        mouse.x = (e.clientX - rectLeft);
        mouse.y = (e.clientY - rectTop);
        onMouseMove(mouse.x, mouse.y);
    }

    function updateRect() {
        var rect = canvas.getBoundingClientRect();
        rectLeft = rect.left;
        rectTop = rect.top;
    }
    updateRect();
};    
setupMouse(can2, clearThis, true);

increase performance on html canvas mousemove image mask

2 Answers

UpdatedFiddle