>> libpng only needs the current and previous row to run the pixel filter.
>> I have no idea what the DEFLATE compressor needs. (Other than a complete
>> redesign...) All assuming you're not going to generate interlaced PNG,
>> that is.
> 
> Well, with antialiasing off, it should be possible (in theory) to produce an
> interlaced PNG with O(1) memory too. You'd have to modify POV-Ray to
> compute the pixels in the same order they're stored in the PNG file.

If you generate a PNG image without any fancy filtering, then you don't 
need any extra memory except for the DEFLATE compressor. But usually PNG 
files use filtering to increase compression. (I don't know, but I'd 
*hope* libpng provides a way to control this...) If filtering, you may 
need the previous row of pixels [only].

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