Welcome to OStack Knowledge Sharing Community for programmer and developer-Open, Learning and Share
Welcome To Ask or Share your Answers For Others

Categories

0 votes
611 views
in Technique[技术] by (71.8m points)

html - HTML5 Canvas slows down with each stroke and clear

I've been playing around with the HTML5 Canvas and I've noticed something that I couldn't find a resolution for online. Here's the simple code I'm playing with

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <script src="//ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js" type="text/javascript"></script>
</head>
<body>
    <canvas id="canvas" style="border: 1px solid;" width="200" height="200"></canvas>
    <br />
    <button id="draw">draw</button>
    <button id="clear">clear</button>
</body>
</html>

<script type="text/javascript">
    (function () {
        var canvas = document.getElementById("canvas");
        var context = canvas.getContext("2d");
        $("#draw").bind("click", function () {
            for (var i = 0; i < 200; i++) {
                context.moveTo(0, i);
                context.lineTo(100, 100);
                context.stroke();
            }
        });
        $("#clear").bind("click", function () {
            context.clearRect(0, 0, 200, 200);
        });
    } ());
</script>

Each time that I press draw, the speed at which it completes seems to slow down exponentially. Could anyone know why this is happening?

It slows down the most through IE. Chrome seems to complete it faster with each draw click, but you can still definitely notice a speed reduction.

See Question&Answers more detail:os

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome To Ask or Share your Answers For Others

1 Answer

0 votes
by (71.8m points)

The <canvas> element keeps track of a current path (i.e., set of points, lines, and curves). canvas.moveTo, canvas.lineTo, and canvas.stroke all operate on the current path. Every time you call canvas.moveTo or canvas.lineTo you are adding to the current path. As the path gets more and more complex, drawing gets slower and slower.

You can clear the path by calling canvas.beginPath(). Doing this at the start of your draw function should get rid of the slowdown.


与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome to OStack Knowledge Sharing Community for programmer and developer-Open, Learning and Share
Click Here to Ask a Question

...