Re: [Gimp-print-devel] Article: adaptive methods for dithering color images

[Thomas T. <tt...@bi...>]

Hrafnkell Eiriksson wrote:

> Yes photos and all natural images are "lowpass" in nature, that is they
> are mostly smooth regions. Spreading the error over a large area is ok there.
> Line art has hard edges, hard edges means high frequency content,
> so spreading the error there is worse, its a kind of lowpass filtering.

Actually, it is not lowpass filtering, but it is determining how far the
noise propagates. There is a thing called a 'Riemersma dither' that uses
a table to find the density printed so far. This means that your errors
do not prapagate all over the page. If you think about it, it is a bit
strange what happens if you print two dots. If one dot is printed a
little too small because of rounding, the other dot has a good chance of
getting a little bigger!

What Riemersma dither does is look back at (say) the errors of the last
100 pixels printed, weigh each with a scale factor, and use this as
error when deciding to print the current pixel.

> What viewing distance do you juse to judge the performance of
> dithering?

JPEG used 8x8 pixel blocks to prevent fringing to occur too far from the
hard edge that caused it. With dither, the mechanism is rather
different. If less than half the error is transferred to the next pixel
horrible effects can occur. Say only 10% is transferred to the next
pixel, 10% two pixels to the right, etc. If we would print 60% gray
level, the pixel would be black, The second would be black too, as only
10% of the 40% that was printed too much is transferred there as error.
Only when one gets on the order of 10 pixels away do white pixels start
to form. This is actually okay for very light tints.

I did make a suggestion to Robert that it might be possible to do
something with the error buffer. Before using the value in the error
buffer, distribute it partially away. The result is a very large
distribution area.

By adjusting the amount that is re-distributed according to the density
of the current color pixel, one can prevent the above 'clipping' effect
from occuring.

Another suggestion I had was to use the required gray value (of error +
input) to decide whether to print, and when it is decided to print, lay
down color dot of the color that needs to be printed most from a color
error perspective. Keep laying down different color dots in this
position until the gray value is as it should be. This has the advantage
of following the human eye sensitivity better than working per color.
But it probably is very difficult to integrate with variable dot size
and other clever rastering support.

An extension of that model would be something that actually knows about
printer and observer capabilities, calculates the value of what has been
produced so far, and bases a decision on that. That could be in the form
of a heuristic that looks at the input value, looks at the immediate
area that has been printed so far and compare to what it should have
been, and determines what option would be the best fit. That could be as
simple as a two-dimensional version of Riemersma, or as complex as
something that also looks at contrast and disregards sharply contrasting
areas (i.e. if the background was just a bit too light here, do not make
the text on it any darker).

The problem with the above is that for every dot to be printed, one has
to build a perception model and compare this with the perception model
of the original image. This is very slow compared to the few shifts and
adds that make up error distribution (although the implememntation as it
is now also has quite some decisionmaking built in).


Thomas