Try canvas2svg.js. [Demo]
I ran into needing this myself and ended up writing a library for this. At the time, the other libraries were a bit sparse, or didn't generate valid SVG.
The basic concept is the same though. You create a mock canvas 2D context and then generate an SVG scene graph as you call canvas drawing commands. Basically you can reuse the same drawing function. Depending on what context you pass to it, you either draw to a standard 2D canvas or generate an SVG document that you can serialize.
You can't actually "transform" a canvas element that's been drawn to, as it's just a bitmap, so keep that in mind. When you export to SVG you're really just calling the same drawing function again using a fake context.
So as a quick example:
//create a canvas2svg mock context
var myMockContext = new C2S(500,500); //pass in your desired SVG document width/height
var draw = function(ctx) {
//do your normal drawing
ctx.fillRect(0,0,200,200);
//etc...
}
draw(myMockContext);
myMockContext.getSerializedSvg(); //returns the serialized SVG document
myMockContext.getSvg(); //inline svg
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