For Cello Fortress I came up with a new technique for Depth of Field blur: area based. (Or at least: as far as I know it is a new technique...) The idea is quite simple so I have written a blogpost explaining how it works:

Area based depth of field blur

Here's a short summary of the idea (more the details and high-res images at the link):

This concept can also be applied to other shapes than boxes. For example, you can also have a sphere within which everything is sharp, while everything outside that sphere is blurred. One can even have several such spheres. How about a visual style where everything is blurred except for the areas around the characters? I think some other interesting visual styles can be made this way, especially in combination with very strong blur.

Feel free to use this idea in your own projects. Let me know if you give it a try! Also, if this turns out to already be an existing technique, let me know so that I can update my blogpost accordingly.

Yeah, I wondered about that: is standard realistic DoF supposed to be at a fixed distance to the camera, or at a fixed distance to the camera plane? The first would result in a curved focal plane (is in my top image), while the second would result in a flat focal plane.

I just gave it a try with a real lens (I have a special portrait lens for my camera that has really strong DoF). I am pretty sure I see a curved focal plane, so the focal plane should indeed be based on the distance to the camera.

Yeah, I know, that is why I was also confused and had to try with a real camera to see how it really works. :)

Technically I guess you areright this isn't DoF, but the post effect shader used is exactly DoF. This is not simply a blend between a blurred and a non-blurred image (the fastest and most common form of DoF): my implementation takes into account the depth of the pixels around and thus makes sure sharp objects not bleed over a blurred background. What I have made is DoF with simply a modified depth buffer. Whether it is still called DoF then, I don't know.