The way we solve this at the moment is to choose an RGB colour from a test
print which is usually has a much stronger chroma than the resulting spot,
However with Perceptual compression it will print accurately. This is a
manual process of looking up colours on charts.
Another possibility would be to print the same test print on fabric and
match the spot color directly
from that. This would save a lot of steps and give a visually accurate
result, which profiles are not going to do.
This is basically what we do, I have an RGB colour chart printed on
Fabric, We then use an Spectromter to measure the charts and find the
RGB colour with the lowest Delta-E, In Effect an Absolute Colorimetric
Match. The results are recorded in a Database for future use, The
Problem is with so many different fabrics this is a time consuming
process that could be automated.
So, in effect you want to "overdrive" the preceptual intent conversion, so
that it ends up with the same color you would have gotten, had been able to
set the RIP to relcol.
Thats right, I want to find an RGB colour that when printed via a
peceptural intent gives me the right spot match.
there is an argyll command line utility by Graeme Gill that does exactly
this. Lcms will then allow you to concatenate transforms ad infinitum, so
you could create what amounts to a device profile that converts from your
original RGB color, to your "disperse relcol" , identity transform back to
Lab, and from there to "disprese preceptual". Once the transform is set up,
you use it like any other transform.
Thats Mike, I will look up Argyll. This may work as it avoids the low
resolution B2A table.
Another thought would be to modify the profile used by the RIP. Replace the
perceptial LUT with the relcol LUT.
I can just set this on the RIP, But then I end up with the problem of
the colours clipping again in RelCol.
Yes, basically you have to turn carwheels to get around the perceptual
intent of the RIP. Your task would be infinitely simpler and more accurate
if there were no conversion in the RIP at all, and you could provide a 6
channel image, one per jet
I would love to do this, But it would basically involve making my own
Colour managed Postscript RIP less the print driver. No small task. Even
Ghostscript only has basic support for ICC profiles.
If only my RIP had decent support for modifying Named Spot Colours reliably.
First, I would print your RGB test image via the RIP, and use the swatch
book to manually match each color using a light source that closely matches
the final viewing conditions. Then reate a test image that contained a
swatch for each spot color, print it, and verify that each swatch matches it
counterpart in the swatch book reasonably well. Then I would tweak the RGB
values of the swatch colors that were still significantly off until they
were acceptably close. Lather, rinse, repeat.
Thats a lot of time ... Perhaps something I can delegate an apprentice
can do. :-)
If the images are photographs that include spot colors, then I would add
additional "pseudo spot" colors for neutral gray, a skin tone, foliage, and
other important colors in your images. For each image to be printed, check
each spot color manually, including the hue (not brightness) of each pseudo
spot color, and use curves to set it to the value from the swatch image. If
the number of images is large, I would use the same curve on all of them, or
have several curves, one for each category of image.
Thanks for your help Mike.
One other option I was just thinking about is to print out all the RGB
colours in 17 steps and measure them, Something like 4913 swatches. Then
build a high resolution Lab>RGB lookup table. This is a little beyond me
at the moment however this process sounds a lot like creating a profile
- I could use the AbsCol intent especially if the PCS of the Profile is
D50. So what options are there for creating such a High Resolution RGB
Profile which had a High Res B2A table? If so I guess I would simply
create a LabD50 > RGB Transform?
Glenn
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