How do you render images with realistic depth-of-field defocus?
You can do a kind of reasonable out-of-focus blur with a Gaussian blur, although the actual OTF of an out-of-focus camera is more like a circular box filter. And you can do a pretty good approximation of a Gaussian blur with three (ordinary, non-circular) box filters. But the diameter of the blur changes according to how far things are from the focal plane.
It occurs to me that maybe you can adjust the width of those box filters dynamically as you move over the image, and that maybe this will give you a reasonable-looking approximation of depth-of-field blur for many images, though not all. I’m thinking that you could have a “weight” factor for each pixel that is highest at the focal point and decreases as you move further from the focal point, and you maintain a constant “weight” within the moving-average sliding window as you slide it, sliding one or the other edge faster as necessary to keep the weight inside the window constant. This way, the window is very narrow when it's at the focal plane, and very wide when it's far away.
(This still results in computing far too many pixels for the out-of-focus stuff, and doesn't help with blurring of reflected images.)