Wasn't it Warp who wrote:
>  I think it would be possible to render first a non-antialiased image
>and then make a second pass shooting more rays where necessary in order to
>antialias the image.
>
>  POV-Ray still works as it worked 10 years ago, when computers had just a
>few megabytes of memory (and thus loading a big image to memory was just
>out of question). Nowadays keeping the entire image in memory (as an option)
>would not be such a burden as it was 10 years ago and thus all kind of
>extra passes would be possible.
>  Perhaps POV-Ray 4 will change to this ideology...
>

Permitting the continuation of a render that was stopped part way
through would need extra attention. If the render was stopped during the
AA pass, then the continuation would need to find files containing both
the result of the first pass and the partial render of the AA pass.

I guess that there might also be some subtle problems with things like
isosurface cacheing, particularly if the image has large areas where the
values don't change enough to cause many extra rays to be cast on the
second pass.

I also suspect that the half-pixel fix (change 1340) might possibly be
related to the way that antialiassing works, and might need to be
addressed again.

None of these would be showstoppers, but would require careful
consideration and extra programming effort.

I also suspect that an antialliassing algorithm that really did look in
all four directions would take longer to render, and produce a result
that looks worse (or at least no better) than the existing algorithm.

With the present algorithm, a vertical or horizontal black/white
transition results in one pixel being supersampled - the one that has
the colour transition on its top or left. The suggested algorithm would
also supersample the pixel that has the colour transition on its bottom
or right. So I would expect this to result in twice the number of
supersample calculations being performed, and double the width of the
grey zone, which may well be enough to make the image look out of focus.

-- 
Mike Williams
Gentleman of Leisure

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