Re: Color Correction by the Numbers for Ink Jets

Lee Varis

hfdomke wrote:

Dan Margulis's writes ...the Highlight should be
5C2M2Y and the Shadow should probably be something like
If I go into Photoshop's color picker and enter those numbers
and convert them to RGB I get these numbers:
Highlight: R248 G250 B255
Shadow: R0 G1 B12

If I can pick my black point and white point dropper values to use
with curves ... would these be reasonable number to enter if my
output device will be a 6-ink Epson Inkjet printer
You can not extrapolate accurate RGB numbers from CMYK aim points. The
numbers you cite are highly dependent on what RGB & CMYK workspace
profiles you are using. I'm guessing that Dan's numbers are based on a
different kind of CMYK setup than what you've got set in your PS Color
Settings (you don't mention what you are using there- when I do this in
my set up I get 251R 249G 248B for white). Also the CMYK gamut is
smaller in most areas of color and several RGB colors may have the same
CMYK equivalents (dependent on rendering intent settings that affect
gamut compression) - the numbers you end up with won't necessarily match
the ideal RGB starting numbers for these CMYK aim points. I would prefer
to find neutral black and white points for RGB images, something like:
R240 G240 B240 for white (sometimes as high as 250 ea.) and R5 G5 B5 for
black (sometimes I like values as high as 10 for black) These numbers
are for highlight and shadow where you want to hold some sense of detail
or texture - a specular highlight should still top out at 255 ea. with 0
ea. for deep black shadows. Most of the time you want something neutral
in these areas and neutral in any of the standard PS workspace profiles
will be equal values in RGB. In CMYK if your numbers in these extreme
areas are off a few points it won't be that noticeable but your RGB
numbers seem to be definitely on the cool side and may force a cool bias
in other areas of the image if you use them to auto correct.

I also question the accuracy of the color picker to do conversion
calculations anyway - try creating patches of color in RGB and then
actually convert to CMYK to test this out. I find that the resulting
numbers are sometimes off a little bit from the color pickers
"prediction" (though not so much with these "end point" numbers). I'd be
interested to hear from someone at Adobe how Photoshop arrives at the
color equivalent numbers in the color picker.


Lee Varis

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