Re: [Gimp-print-devel] Black generation, continued

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

Robert L Krawitz wrote:

> 1) The black is coming in too quickly on the 870, as you surmised.  I
>    haven't tried the EX yet, but I suspect it would be even more
>    noticeable.

Wouldn't this be a matter of adjusting the lower and upper bounds for
black? Mmmm... maybe we need a smoother start at the beginning of the
curve as well - maybe I should take tk to the power of 2, so we get a
smooth start anyway - but I'd like the elimination of the final bits of
color also to be smooth. Now the curve that generates bk is like:

       ____
      /
     /
____/

While I'd rather have those corners smoothed. Any idea for a quick
function? I'm thinking about scaling the beginning and end of the values
to -1 and +1, and then doing 3*tk - tk*tk*tk as a transfer function.
Makes a smooth curve and is not too slow.

> 2) The behavior at 720 dpi is significantly different -- there's much
>    more black, and very dark regions look like they're not getting
>    enough ink.

I do not notice this with the 600, but that may be because I leave
density at 1. Could be some problem with using 65536 versus d->density,
but I thought I had that right by now. In any case, previously the
driver would take away topo much CMY for every K part added: when using
more black than default, a grey wedge would show a light band where
black cut in - so I am a bit surpised that now you see this effect.

> 3) I think the ink reduction might be a bit too effective -- there
>    just isn't enough ink in dark solid colors.

I was thinking, on the way home, that the ink reduction could better
happen after we make up our optimal CMYK valye. We'd add the ink amount,
find how much we're over budget, and subtract half of that from CMY and
add to K. The budget value could probably be derived from the upper
bound for black. That would mean we'd remove the functionality from
where it is now and move it to the absolute end.

I think I have something to try tomorrow. Enough new ideas. Jean-Marc, I
had a quick glance at your code but do not see any difference in the way
you generate black (i.e. the part directly following  if (black !=
NULL)  in dither_cmyk. Is that correct?

In any case, I was happy to be able to use a lot of black if I wanted: I
like to check pathological cases - they make me understand what goes
wrong. But in this case, using a lot of black seemed to work fine.

I also hope the 257 x 257 matrix will be ready tomorrow. Curious what it
will bring...


Thomas