🚀 go-pugleaf

Tom_Lane@G.GP.CS.CMU.EDU writes:

>The free JPEG image compression software currently uses traditional
>Floyd-Steinberg dithering when reducing the number of colors in its output.

>Even with careful coding, F-S dithering is a remarkably expensive
>proposition... in our current release, dithering *alone* (not counting color
>selection) takes nearly as much time as JPEG decompression proper.

>If anyone is aware of cheaper dithering techniques that give results as good
>as F-S, or of implementation tricks to speed up F-S, I'd appreciate hearing
>about them.

I once made a variant of F-S that was somewhat faster: instead of
distributing the error to four neighbouring pixels, distribute it to
only one. The idea is to make the choice of which pixel to distribute
to random: in the simplest case you randomly choose either the next
pixel on the same scan-line or the same pixel on the next scan-line.
You need a fast way to generate random bits, bit a simple
shift-register method works reasonably well. If you want slightly
better quality, you randomly choose one of the four pixels that F-S
would distribute the error to.

The quality is not quite up to the standard of F-S, but nearly so.

	Torben Mogensen (torbenm@diku.dk)

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