Graeme,
I'm very sorry about the "device values question" I missed that completely,
most of what read is from Mark I. Nelson or from his own sources and I have
a very bad memory, so I can't be trusted (LOL). I went back to verify and
here is his explanation for using color to make our negatives, this date
back to 2004.
"Inkjet printers are designed to make very good positive prints, not
negatives. They use a CMYK model, which employs cyan (C), magenta (M),
yellow (Y) and black (K) inks to form colors and tones. In order to print
good shadow detail and a dense black, inkjet printers add the black ink to
the CMY in the lower tones. This is great for making good inkjet prints, but
it is terrible for making good negatives. The black ink does not work the
same in negatives as the C, M, or Y inks, it is opaque rather than
translucent and has a very high UV density, in some cases over log 4.0.
These characteristics of the black ink make it undesirable to use in
negatives. Inkjet negatives and negatives rendered with other color capable
digital output devices function in a different manner than imagesetter or
conventional film negatives. I tend to think of the combination of film
substrate and ink as a new form of film negative. However there is a
fundamental difference with this type of negative that can be used to great
advantage. Instead of blocking ultraviolet light with varying levels of
opacity, inks, both dye based and pigment based, are translucent and they
filter light. As stated above, this is true of all the inks except black.
Thus printing negatives without black ink provides a much more controllable
negative. The color filtering affect of inks works in a very predictable and
precise manner, which allows you to control the density range of negatives
over a broad range to match a variety of alternative process Exposure
Scales."
I don't know that his statement about black ink is true or not. I haven't
tried any of this myself as of now, there are thousands of folks using his
method and it seems to make an excellent job. But I can't help myself to not
like it much when I see the kind of curve needed to linearize the process of
making these negative. Even at 16 bit per channel some parts of the curve
used (dependent on the exposure response of the photographic process
involved) are so steep or quasi flat that it is not what I would called
"fine tuning" or "optimal control".
Before I forget, he showed a graph that black made as a combination of CMY
inks as up to 3.4 (Log UV transmissive density) which is plenty enough. The
process I which to use is called Kallitype and requires a negative UV
density range of about 1.8.
In your previous message:
" ArgyllCMS doesn't currently support Monochrome ICC profiles, so there's no
straightforward workflow. There may be ways of fudging it though, using
calibration curves. It would be something like: create a .cal file by hand
(perhaps using a spreadsheet to normalize to the white value) from the
measurements of an R=G=B step wedge. Incorporate it into a profile (say sRGB
?) using applycal, and then do a cctiff conversion of your modified sRGB
profile as input, and an un-modified one as output profile, to get the
ArgyllCMS machinery to invert the per-channel curves while processing your
image. (You may have to convert your input image to R=G=B before the
conversion too.)"
I went to have a look at the two .cal files provided with Argyll and I
apologize before I ask this stupid question but what is the " RGB_I" column
for?
If I understand correctly, what you propose above is kind of a work around
to create a profile (relatively easily) for making the negative I'll need to
print the (R=G=B) image I see on my screen on my Kallitype print, right?
I suppose there are no in depth writing on this approach anywhere, right?
The only thing I'm sure I understand a 100% in this approach is starting
with a file having (R=G=B), that's the easy part, the rest well I'm sure I
know just enough to be the dog in the bowling alley and not much more.
I assume the (R=G=B) step wedge is the input image I'll use to make the
negative and then use it to make the Kallitype (K) print, right? If so do
you have any recommendation on how to create this file? How many steps do
you think are required? Do I need to invert it when I print the negative or
do I do this using the curve I'll create later?
******* You don't have to answer the above questions, just thinking aloud
******
I'm not sure I would do it if I were in your shoes as they say. But I'll say
this, if this approach as any chance of giving us a better mean "to control
the density range of negatives over a broad range to match a variety of
alternative process Exposure Scales" as Mark Nelson says, well I want to
learn how to do it the proper way.
Thanks,
YG
-----Original Message-----
From: argyllcms-bounce@xxxxxxxxxxxxx [mailto:argyllcms-bounce@xxxxxxxxxxxxx]
On Behalf Of Graeme Gill
Sent: Monday, November 20, 2017 10:21 PM
To: argyllcms@xxxxxxxxxxxxx
Subject: [argyllcms] Re: How to do this?
Yves Gauvreau wrote:
Hi,
++ Just any mixture of non equal RGB values. Not sure what you mean by
device values though?
All my images are in a RGB colorspace even when R=G=B. I'm not sure I
understand the R=G=B step wedge, if this is what I need to print on my
transparencies then I already know it won't work. Even full black ink
or a neutral combination of CMY doesn't provide enough density on the
negative when using UV light to expose the print.
All the methods I've seen to create
digital negative use a greenish step wedge and Photoshop curves to get
this wedge to print correctly.
I suppose the calibration curve are more or less equivalent to
Photoshop curve but they are applied in a profile instead of on an
adjustment layer?
