Cyrille,
> It currently only support dithering to 8bit. The only
export filter that is
> capable of creating a palette from an image is PNG, and
PNG is limited to
> 8bits palette. But there is nothing in the algorithm
that prevent to use
> bigger palette when have export filters which can work
with higher depth
> palette.
yep, but try displaying a PNG dithererd to 256 colors chosen
from 16.8
millions on a device (e.g. a mobile phone) that can only
display 262144
colors -- if it has smooth ramps you will likely get
horrible banding
and ugly colors as the device clamps a color to the next
available one.
A good feature for starters is hence to allow dithering to
256 out of
32768/65536/262144 evenly distributed colors (as opposed to
16.8 million
evenly distributed ones).
The second feature is not dithering to a palette but
dithering to a
lesser number of colors. I.e. dithering from 24 bits to e.g.
15 bits, 5
red, 5 green 5 blue.
The easiest way of doing this clamping (this is what most
hardware does)
and it looks rather unpleasant.
Next is error diffusion with random(), then comes 'clever'
error
diffusion with an algorithm like Floyd-Steinberg.
Alternatively there is
pattern diffusion with Bayer which gives the nice "8
bit console/game
graphics" look.
For dithering to n bits via simple error diffusion:
assuming we have a color channel represented as a floating
point number
in 'value' and 'one' contains 2 ^ bits - 1, i.e. the number
of colors - 1:
intvalue = clamp( round( one * value + 0.5 * randfloat()
), 0, one );
with randfloat() returning a number between 0..1:
float randfloat() {
return rand() / ( float( RAND_MAX ) + 1 );
}
Hope that makes more sense. 
.mm
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