how to make a fog effect or blur effect in canvas with js?

Question

I'm using javascript and THREE.js 3d engine,

and I want to do a fog effect,

here's an example http://mrdoob.github.com/three.js/examples/webgl_geometry_terrain_fog.html

but it only support WebGL,

so is there any way to simulate fog effect, or a blur effect with javascript and canvas?

thanks alot.