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 cont

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：

      <span>var </span><span>canvas = document.getElementById(</span><span>"myCanvas"</span><span>);
      </span><span>var </span><span>context = </span><span>this</span><span>.canvas.getContext(</span><span>'2d'</span><span>);
      </span><span>var </span><span>drawAsync = eval(Jscex.compile(</span><span>"async"</span><span>, </span><span>function </span><span>() {
          </span><span>while </span><span>(</span><span>true</span><span>) {
              drawMario(context);
              $await(Jscex.Async.sleep(</span><span>1000</span><span>));
          }
      }))
      drawAsync().start();
</span>

正确代码：

      <span>var </span><span>canvas = document.getElementById(</span><span>"myCanvas"</span><span>);
      </span><span>var </span><span>context = </span><span>this</span><span>.canvas.getContext(</span><span>'2d'</span><span>);
      </span><span>var </span><span>m_canvas = document.createElement(</span><span>'canvas'</span><span>);
</span><span>m_canvas.width = </span><span>64</span><span>;
      m_canvas.height = </span><span>64</span><span>;
      </span><span>var </span><span>m_context = m_canvas.getContext(</span><span>'2d'</span><span>);
      drawMario(m_context);
      </span><span>var </span><span>drawAsync = eval(Jscex.compile(</span><span>"async"</span><span>, </span><span>function </span><span>() {
          </span><span>while </span><span>(</span><span>true</span><span>) {
              context.drawImage(m_canvas, </span><span>0</span><span>, </span><span>0</span><span>);
              $await(Jscex.Async.sleep(</span><span>1000</span><span>));
          }
      }))
      drawAsync().start();
</span>

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：

   
    <span>for </span><span>(</span><span>var </span><span>i = </span><span>0</span><span>; i <span>1</span><span>; i++) {
          </span><span>var </span><span>p1 = points[i];
          </span><span>var </span><span>p2 = points[i + </span><span>1</span><span>];
          context.beginPath();
          context.moveTo(p1.x, p1.y);
          context.lineTo(p2.x, p2.y);
          context.stroke();
      } 
</span></span>

正确代码：

      <span>context.beginPath();
      </span><span>for </span><span>(</span><span>var </span><span>i = </span><span>0</span><span>; i <span>1</span><span>; i++) {
          </span><span>var </span><span>p1 = points[i];
          </span><span>var </span><span>p2 = points[i + </span><span>1</span><span>];
          context.moveTo(p1.x, p1.y);
          context.lineTo(p2.x, p2.y);
      }
      context.stroke();
</span></span>

三.尽量少改变CANVAS状态

错误代码：

      <span>for </span><span>(</span><span>var </span><span>i = </span><span>0</span><span>; i <span>2 </span><span>? COLOR1 : COLOR2);
          context.fillRect(i * GAP, </span><span>0</span><span>, GAP, </span><span>480</span><span>);
      } 
</span></span>

正确代码：

      <span>context.fillStyle = COLOR1;
      </span><span>for </span><span>(</span><span>var </span><span>i = </span><span>0</span><span>; i <span>2</span><span>; i++) {
          context.fillRect((i * </span><span>2</span><span>) * GAP, </span><span>0</span><span>, GAP, </span><span>480</span><span>);
      }
      context.fillStyle = COLOR2;
      </span><span>for </span><span>(</span><span>var </span><span>i = </span><span>0</span><span>; i <span>2</span><span>; i++) {
          context.fillRect((i * </span><span>2 </span><span>+ </span><span>1</span><span>) * GAP, </span><span>0</span><span>, GAP, </span><span>480</span><span>);
      }
</span></span></span>

四.重新渲染的范围尽量小

错误代码：

  <span>context.fillRect(</span><span>0</span><span>, </span><span>0</span><span>, canvas.width, canvas.height); 
</span>

正确代码：

      <span>context.fillRect(</span><span>20</span><span>, </span><span>20</span><span>, </span><span>100</span><span>, </span><span>100</span><span>);
</span>

五.复杂场景使用多层画布

 <span><span>canvas </span><span>width=</span><span>"600" </span><span>height=</span><span>"400" </span><span>style=</span><span>"</span><span>position</span><span>: absolute; </span><span>z-index</span><span>: 0"</span><span>>
</span><span>canvas</span><span>>
<span>canvas </span><span>width=</span><span>"600" </span><span>height=</span><span>"400" </span><span>style=</span><span>"</span><span>position</span><span>: absolute; </span><span>z-index</span><span>: 1"</span><span>>
</span><span>canvas</span><span>>
</span></span></span>

六.不要使用阴影

      <span>context.shadowOffsetX = </span><span>5</span><span>;
      context.shadowOffsetY = </span><span>5</span><span>;
      context.shadowBlur = </span><span>4</span><span>;
      context.shadowColor = </span><span>'rgba(255, 0, 0, 0.5)'</span><span>;
      context.fillRect(</span><span>20</span><span>, </span><span>20</span><span>, </span><span>150</span><span>, </span><span>100</span><span>);
</span>

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：

<span>rounded = (</span><span>0.5 </span><span>+ somenum) | </span><span>0</span><span>;
</span><span>rounded = ~ ~(</span><span>0.5 </span><span>+ somenum);
</span><span>rounded = (</span><span>0.5 </span><span>+ somenum) <span>0</span><span>;
</span></span>

九.使用requestAnimationFrame制作游戏或动画

<p></p><p>        (<span>function</span> () {<br>            <span>var</span> lastTime = 0;<br>            <span>var</span> vendors = ['ms', 'moz', 'webkit', 'o'];<br>            <span>for</span> (<span>var</span> x = 0; x                 window.requestAnimationFrame = window[vendors[x] + 'RequestAnimationFrame'];<br>                window.cancelAnimationFrame =<br>          window[vendors[x] + 'CancelAnimationFrame'] || window[vendors[x] + 'CancelRequestAnimationFrame'];<br>            }<br><br>            <span>if</span> (!window.requestAnimationFrame)<br>                window.requestAnimationFrame = <span>function</span> (callback, element) {<br>                    <span>var</span> currTime = <span>new</span> Date().getTime();<br>                    <span>var</span> timeToCall = Math.max(0, 16 - (currTime - lastTime));<br>                    <span>var</span> id = window.setTimeout(<span>function</span> () { callback(currTime + timeToCall); },<br>              timeToCall);<br>                    lastTime = currTime + timeToCall;<br>                    <span>return</span> id;<br>                };<br><br>            <span>if</span> (!window.cancelAnimationFrame)<br>                window.cancelAnimationFrame = <span>function</span> (id) {<br>                    clearTimeout(id);<br>                };<br>        } ());</p>

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

本文已同步更新至：

HTML5实验室【目录】:   http://www.cnblogs.com/iamzhanglei/archive/2011/11/06/2237870.html