你必须知道的10个提高Canvas性能技巧
2011-11-29 17:07 【当耐特】 阅读(24250) 评论(24) 编辑 收藏 举报你还在抱怨自己写的canvas demo徘徊在10帧以下吗？你还在烦恼打开自己写的应用就听见CUP风扇转吗？你正在写一个javascript Canvas库吗？那么下面九点就是你必须知道的！
一.预渲染
错误代码：
var canvas = document.getElementById("myCanvas"); var context = this.canvas.getContext('2d'); var drawAsync = eval(Jscex.compile("async", function () { while (true) { drawMario(context); $await(Jscex.Async.sleep(1000)); } })) drawAsync().start();
正确代码：
var canvas = document.getElementById("myCanvas"); var context = this.canvas.getContext('2d'); var m_canvas = document.createElement('canvas'); m_canvas.width = 64; m_canvas.height = 64; var m_context = m_canvas.getContext('2d'); drawMario(m_context); var drawAsync = eval(Jscex.compile("async", function () { while (true) { context.drawImage(m_canvas, 0, 0); $await(Jscex.Async.sleep(1000)); } })) drawAsync().start();
这里m_canvas的宽度和高度控制得越小越好。
二.尽量少调用canvasAPI
错误代码：
for (var i = 0; i < points.length - 1; i++) {
var p1 = points[i];
var p2 = points[i + 1];
context.beginPath();
context.moveTo(p1.x, p1.y);
context.lineTo(p2.x, p2.y);
context.stroke();
}
正确代码：
context.beginPath(); for (var i = 0; i < points.length - 1; i++) { var p1 = points[i]; var p2 = points[i + 1]; context.moveTo(p1.x, p1.y); context.lineTo(p2.x, p2.y); } context.stroke();
三.尽量少改变CANVAS状态
错误代码：
for (var i = 0; i < STRIPES; i++) { context.fillStyle = (i % 2 ? COLOR1 : COLOR2); context.fillRect(i * GAP, 0, GAP, 480); }
正确代码：
context.fillStyle = COLOR1; for (var i = 0; i < STRIPES / 2; i++) { context.fillRect((i * 2) * GAP, 0, GAP, 480); } context.fillStyle = COLOR2; for (var i = 0; i < STRIPES / 2; i++) { context.fillRect((i * 2 + 1) * GAP, 0, GAP, 480); }
四.重新渲染的范围尽量小
错误代码：
context.fillRect(0, 0, canvas.width, canvas.height);
正确代码：
context.fillRect(20, 20, 100, 100);
五.复杂场景使用多层画布
<canvas width="600" height="400" style="position: absolute; z-index: 0"> </canvas> <canvas width="600" height="400" style="position: absolute; z-index: 1"> </canvas>
六.不要使用阴影
context.shadowOffsetX = 5; context.shadowOffsetY = 5; context.shadowBlur = 4; context.shadowColor = 'rgba(255, 0, 0, 0.5)'; context.fillRect(20, 20, 150, 100);
七.清除画布
详细性能差别：
http://simonsarris.com/blog/346-how-you-clear-your-canvas-matters
一般情况下：clearRect的性能优于fillRect优于canvas.width = canvas.width;
八.像素级别操作尽量用整数
几种取整数的方法：
rounded = (0.5 + somenum) | 0; rounded = ~ ~(0.5 + somenum); rounded = (0.5 + somenum) << 0;
九.使用requestAnimationFrame制作游戏或动画
(function () {
var lastTime = 0;
var vendors = ['ms', 'moz', 'webkit', 'o'];
for (var x = 0; x < vendors.length && !window.requestAnimationFrame; ++x) {
window.requestAnimationFrame = window[vendors[x] + 'RequestAnimationFrame'];
window.cancelAnimationFrame =
window[vendors[x] + 'CancelAnimationFrame'] || window[vendors[x] + 'CancelRequestAnimationFrame'];
}
if (!window.requestAnimationFrame)
window.requestAnimationFrame = function (callback, element) {
var currTime = new Date().getTime();
var timeToCall = Math.max(0, 16 - (currTime - lastTime));
var id = window.setTimeout(function () { callback(currTime + timeToCall); },
timeToCall);
lastTime = currTime + timeToCall;
return id;
};
if (!window.cancelAnimationFrame)
window.cancelAnimationFrame = function (id) {
clearTimeout(id);
};
} ());
十.其他
与渲染无关的计算交给worker
复杂的计算交给引擎（自己写，或者用开源的），比如3D、物理
缓存load好的图片，canvas上画canvas,而不是画image
同步
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