[argyllcms] Re: How to do this?
- From: Graeme Gill <graeme@xxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Mon, 20 Nov 2017 11:11:06 +1100
Yves Gauvreau wrote:
Hi,
1- I have a patch sets, I print this on somekind of transparent material.
I measure the resulting print,
Now I have the relation: original patch color => transparency printed
values,
right?
depends what you mean by "color". You have a relation between the device values
and
the visual color, yes.
2- I use this transparency as a negative to make a B&W photographic print.
Then I measure the results of this B&W print.
And now I have the relation: transparency printed values (colors) => B&W
photographic print values (grayscale), right?
If you want, yes. But if your intention is to know the mapping between the
device
values and the B&W print values, why not skip measuring the transparency ?
Now using the above data, is there a way to convert a B&W original image to
the colored
negative I need to print this image on photographic paper using an ICC
profile? How do I > create one?
Working in RGB unnecessarily complicates things - you can't map uniquely from
monochrome
to RGB values, so the best approach is to use the RGB as if it was monochrome -
i.e.
create a test wedge with R=G=B, print it, make your B&W print, and then measure
the
print. You can now invert the print reflectance/Y value to R=G=B value curve.
I see this here in Argyll docs, CCTIFF -> Color convert a TIFF or JPEG file
using a
sequence of ICC device, device link, abstract profiles and calibration files.
Some of you will ask why bother to do this, well all the methods I've seen to
create a
digital negative as most of them call it, use a scanner as measuring tool and
none of
them use any form of a ICC profile for this, just curves inside Photoshop or
again
curves at the RIP level. It works and it may be just fine.
I'm just curious to see if using another method and of course a profile would
be
an improvement in any way shape or form.
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.)
Cheers,
Graeme Gill.
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