Nope, can't be done.

The lens is like a screen within a screen. You're viewing a picture of what is on that screen. The only information it has to work with is what color is on the screen, and then it gets to modify what that color is.

Any thing like edge detection, blurring, etc. requires that you be able to examine more than one point in the scene. But the lens shader only gets data from one point at a time.

Worse, I have no access to anything but the color of the scene element behind the screen. I can't ask how far away it is, or what direction it is facing, or anything like that. If I could, there are many interesting effects I could do, not just with this lens but with translucent/transparent materials in general. Even better, if I could launch multiple rays of my own, in any direction I wnat, I could implement true subsurface scattering.

The issue of toon lines has come up many times. Even if I could access the edge-blend information of what is behind the screen, it wouldn't help much. I can put a shader directly on the prop and still not do a good job with toon lines. If you could, you'd see better toon shaders already.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)