你必须知道的10个提高Canvas性能技巧

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