Question about tone remapping with Relative Colorimetric Rendering
Question about tone remapping with Relative Colorimetric Rendering
- Subject: Question about tone remapping with Relative Colorimetric Rendering
- From: Louis Dina <email@hidden>
- Date: Sat, 01 Jun 2013 09:35:35 -0500
I'm trying to understand how tones are remapped when going from file to
print, using an RGB driver. I'll use an example so my question is clear.
I created a black to white gradient in Photoshop, Adobe RGB. 11 step
posterization. I purposely chose to keep color out of this test, since I
wanted to keep it simple to understand how the luminance values get
mapped. That gives RGB numbers, from dark to light, of 0 25 51 76 102 128
153 178 204 229 255. The info palette shows L* values of 0 6 20 32 43 54 64
73 83 91 100.
I *converted* the file to a custom RGB profile (using the epson printer
driver), Relative Colorimetric, BPC on, of a weak Dmax matte fine art
paper. This paper achieves about 18L* for Dmax and has a white of about
96L*. I chose a weak paper on purpose to amplify the effect.
The info palette shows changes in the two darkest patches ONLY. Black goes
from L* of 0 to 2, and the next lighter patch goes from 6 to 8. All other
patches remain the same. The RGB numbers change too reflecting profile
tables to adjust brightness and neutrality. I believe these are the new RGB
numbers that will be sent to the printer driver, correct? They obviously
aren't the achieved values in the print, because the paper can achieve only
18L* black and 96L* white.
Is there a way to know what the final L* values in the print will be using
Photoshop, or in looking at the profile? If so, how? I assume this
information must be in the profile. (I built this printer profile using
i1Profiler).
OK, switch to the original RGB numbers (before conversion) and go to Proof
Preview instead of Convert to Profile. If I proof preview using RC intent,
BPC turned On, the file display shows the two darkest patches brightening
up (same as the conversion). However, since I am not converting, the
numbers in the info palette are the original numbers.
Still in Proof Preview, RC, BPC On, if I turn on Simulate Black Ink, the
entire tonal scale lightens up, most notably, from blacks to the midtones,
but even a little bit in the 1/4 tones. Obviously, the profile knows this,
but it doesn't show me the numbers.
If I then turn on Simulate Paper White, the display reflects the dingy
yellow-white of the paper (96L/0a/3b or so). Now, the light end of the
tonal scale is scaled back, giving a more accurate view of the real tonal
range of the final print. Unfortunately, the numbers in the info palette
still show the original numbers. Is there a way to see the final numbers I
would read in the print if using a spectro (assuming a perfect profile, of
course)?
Sorry for the long email, but I want it to be clear. RC Intent has always
been described as accurately reproducing in-gamut colors, then clipping all
out-of-gamut colors and tones, bringing them back to the edge of the
destination gamut. Obviously, the whitest white this paper can produce is
96L/0a/3b, and the blackest black is 18L*/0a/0b.
Here's the real question...Will all the grayscale tones in my step wedge
from 18L to 96L reproduce accurately on the final paper using RC Intent? Or
will they be scaled somehow? The Proof Preview shows them ALL changing.
And will my two black patches (original numbers of 0L and 6L) both end up
being 18L? Again, the proof preview says No. I'd like to believe the
midtones will stay locked, so 50L in my info palette will read 50L in the
print, but I am not convinced this is the case.
Thanks for any responses, and sorry again about the length of this email. I
hope it was clear.
Lou Dina
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