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对一个矢量图动画，开启位图缓存能大大提高运行效率。所谓开启位图缓存，其实要自己动手，先创建一个临时canvas，然后把矢量图绘制到这个canvas上，到了实际绘制时，直接把这个临时canvas拷贝到真正canvas上。而位图拷贝的速度是非常快的，比重新绘制矢量图要快很多。
三部曲：
1、建立临时canvas（位图缓存）
p.cache = function(x, y, width, height, scale) { // draw to canvas. scale = scale||1; if (!this.cacheCanvas) { this.cacheCanvas = document.createElement("canvas");} this._cacheWidth = width; this._cacheHeight = height; this._cacheOffsetX = x; this._cacheOffsetY = y; this._cacheScale = scale; this.updateCache(); }
2、绘制到临时canvas
p.updateCache = function(compositeOperation) { var cacheCanvas = this.cacheCanvas, scale = this._cacheScale, offX = this._cacheOffsetX*scale, offY = this._cacheOffsetY*scale; var w = this._cacheWidth, h = this._cacheHeight, fBounds; if (!cacheCanvas) return; var ctx = cacheCanvas.getContext("2d"); w = Math.ceil(w*scale); h = Math.ceil(h*scale); if (w != cacheCanvas.width || h != cacheCanvas.height) { cacheCanvas.width = w; cacheCanvas.height = h; } else if (!compositeOperation) { ctx.clearRect(0, 0, w+1, h+1); } ctx.save(); ctx.globalCompositeOperation = compositeOperation; ctx.setTransform(scale, 0, 0, scale, -offX, -offY); this.draw(ctx, true); this._applyFilters(); ctx.restore(); };
3、copy到真正canvas
p.drawFromCache = function(ctx) { var cacheCanvas = this.cacheCanvas; if (!cacheCanvas) { return false; } var scale = this._cacheScale, offX = this._cacheOffsetX, offY = this._cacheOffsetY, fBounds; if (fBounds = this._applyFilterBounds(offX, offY, 0, 0)) { offX = fBounds.x; offY = fBounds.y; } ctx.drawImage(cacheCanvas, offX, offY, cacheCanvas.width/scale, cacheCanvas.height/scale); return true; };
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原文地址：http://www.cnblogs.com/kenkofox/p/4624362.html
