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

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：

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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