sharpen() CSS filter

In http://lists.w3.org/Archives/Public/public-fx/2011OctDec/0031.html, I suggested removing sharpen() from the CSS filter list. I'd like to break that issue out here for discussion.

My reason for suggesting this is because sharpen (actually an unsharp mask) requires several steps: blur, subtract, multiply and add (or something like that). This can be difficult to hardware accelerate on constrained systems, like mobile platforms. Also, sharpen doesn't seem to be a "core effect" like drop-shadow, blur and the color-matrix effects.

I think we should eventually add sharpen, as well as bloom, displacement, lighting and some of the other more complex effects. But for now I think we should stick with a small set that is easily accelerated.

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~Chris
cmarrin@apple.com