I know the standard answer to this sort of thing is "code it yourself", 
but since I can't do that, I might as well get the idea out...

A post on p.b.i reminded me of something that's been bugging me for a 
while: when using mosaic rendering, each successive pass recalculates 
every pixel. Wouldn't it be a great deal faster to save the data and 
only calculate color information for new pixels? Granted, this wouldn't 
work with antialiasing, but I would think that for un-AA'd test renders 
(which should ideally be as fast as possible, right?) this would be a 
huge improvement; every rendering pass after the first one would be 
twice as fast!

While I'm at it.. does POV-Ray really compare rendered pixels to 
determine if AA needs to be performed? If so, how does it compare a 
given pixel to an unrendered one that's on its right or bottom? And if 
you're comparing rendered pixels, would it be any slower to render an 
un-AA'd image first, then perform a separate AA pass with that data? If 
not, why not rework the AA code, essentially extending the "mosaic 
rendering" concept to the subpixel level? Would this be a bitch to 
implement?

-Xplo

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