[argyllcms] Re: RGB Printer profile shadow detail issues
- From: Graeme Gill <graeme@xxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Wed, 8 Jul 2020 12:28:48 +1000
Brandon Bates wrote:
Printfab is actually raising the black
level L value a bit and thus brings it in line with CMY a little better (By
laying down
more CMY at the low black levels) I would be happy with this but instead of
my shadows
being too open now they are a just a bit crushed. I can comp this manually
with some
levels/curves in photoshop but the point of a good profile is not to have to
do that...
Quite often this is a workflow/gamut mapping issue.
Typical source images are in an RGB space with an almost perfect black point
(i.e. the reference black areas corresponds to L* = 0).
Default workflows simply dump this into the profile B2A table, and
using relative colorimetric or some random perceptual table this
simply clips the black to the printer black, and you get crushed shadows.
The correct fix is that the blacks of the source have to be adapted to
the gamut limits of printer. Adobe tools provide a cheat for this called BPC.
You could also use the gamut mapping controls in Argyll to create a perceptual
table that will correctly map the black points.
[ Of course there are other secondary effects such as viewing conditions etc.
that can have an influence on the visual result. Assume a bright viewing
condition when you have a not so bright one, and you get poor shadow detail
too. etc. ]
So
on to new territory for me, CMYK printing. PrintFab has both RGB and CMYK
printing
options and so far I have used RGB. I printed and read a CMYK chart and I'm
finding that
the K by itself is MUCH darker in L value than CMK (even within the Cone Pro
CMY inkset)
Not unusual. For CMY to block all wavelengths equally, the wavelength
transitions from
reflect to block would have to perfectly complement each other. That's a big
ask amongst
all the other goals for formulating an ink, and probably falls into the low
priority
and very hard to achieve basket. Black on the other hand can be loaded with
something that blocks light uniformly across the spectrum. This is precisely
the reason that K is added to a CMY print system.
so that if any CMY gets added to the K it makes it lighter (Goes from a
maximum K of just
under L3 up to L6 instead)
That's not so expected. Normally the assumption is that printing inks are
subtractive,
so adding more ink makes things darker. Either the nature of the inks is not so
good
(i.e. CMY is adding a reflective component that is lightening things up), or
perhaps
you are operating within a fixed ink loading budget (i.e. % coverage) where you
are
actually trading K for CMY, and this is lightening the result up because per ink
coverage % CMY is a lot less potent (1/3 and worse).
So given that most inksets don't behave that way, my new
question is how do I adjust the black generation curve in Argyll to allow for
only K black
at the darkest colors? I'm sorry I'm a complete noob when it comes to color
separations.
Is -kx where I want to head?
It's a place to start, but max K typically isn't ideal for photographic
like reproduction. To be fair, Argyll's black generation controls are
difficult to use. The tutorial gives some guidance, but the place to
start is to choose an ink coverage (ink limit), and then a CMY/K balance
for your black, and then try and get a well behaved transition along
the neutral axis.
Also seems like I loose a lot of gamut in CMYK for some
reason but there a lot of knobs to tweak so I'm not sure I have it right yet
(I may have
turned the driver ink limit down too far to avoid pooling but killed gamut,
need to
test...)
You shouldn't loose gamut, you should be able to improve it a bit (or at
least make your own choices in trading gamut for other properties).
But setting up a raw print system is a bit of an art. Setting per
channel maximum ink densities will affect gamut, as well as
having a bearing on maximum ink loading possible for black.
Typically dot gain compensation is very useful too, in linearizing
the printer behavior before profiling.
(Argyll's print calibration goes some way in automating this sort of thing.)
Cheers,
Graeme Gill.
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