On Jan 5, 2018, at 3:19 PM, Ben Goren <ben@trumpetpower.com> wrote:
On Jan 5, 2018, at 12:18 PM, forums@walkerblackwell.com wrote:
I’m not looking for a visual match. That’s what normal ICCs do.
[...]
Let’e say our dMax is L* 14.45 on Matte Paper and our dMin (paper white) is L* 96.5
...but that's the problem! L* _is_ visual! That's the entire point of the definition. The math has grown much more sophisticated in recent refinements, but the basic "unit" of L* is designed to be the minimum humanly-perceptible difference between two samples -- and it just happens that there's "close enough" to 100 such "units" in typical viewing environments that the standard was defined as such.
L* is an entirely _perceptual_ measure, and only becomes absolute when coupled with viewing conditions.
As in...D65 L*=100, a*=0, b*=0 is a very noticeably bluer color than D50 L*=100, a*=0, b*=0. Both D50 and D65 are defined with certain illumination (brightness) levels; swap them out for actual illuminants with similar spectral distributions but different brightnesses (as in, move the dimmer switch up and down) and the colors thus defined will be perceptually dramatically brighter and dimmer, even though every sample in this exercise remains L*=100, a*=0, b*=0.
I’m standardizing to a D50 env. And am not too worried about a/b axis right now (black ink to paper white). For now it’s just not part of the worry . . .
The most common absolute, non-perceptual measure is XYZ...but it's something useless and unintuitive to artists (though it's really good for mathematicians).
What I’m talking about is simple. Is there a consistent way to calibrate with an ICC such that numbers above print like this when measured with a spectro (approximately as the real values have been rounded to 2 decimals):
If I understand right, simplified, you want the following:
R=G=B=0 => D50 L*=14.45, a*=0, b*=0 [...] R=G=B=120 => D50 L*=53.09, a*=0, b*=0 [...] R=G=B=255 => D50 L*= 96.5, a*=0, b*=0
YEP! (But a/b can kinda go wherever they go based on paper and black ink.)
A good perceptual rendering will come close...but, immediately, you've got some big honkin' practical problems. Maximum density is almost always far from neutral, and paper white is almost always different from neutral in a different direction. If you want closest possible to the neutral axis, you're going to have to clip both...meaning your maximum density is going to be a lot lighter, and you'll have to lay down at least some ink over the entire page. Your dMax suddenly in reality isn't as dark, and your dMin isn't as bright. Or, you can decide to have off-neutral maxima and minima...but how do you transition to and from them? Do you draw a straight line from the color of dMax to paper white? That rarely produces pleasing results. More common is to keep most of the neutral axis neutral, and then bend it to the extremes. But at what point to start bending, and how much to bend? Such considerations are what separate the good color management systems from the ugly -- and where Grame's skill (and aesthetic) in ArgyllCMS really shines through.
Yeah. Not worried about black ink color, etc as above.
Further complicating is that, though Lab units are supposed to be visually identical in all dimensions, such that a change of one a* unit is perceptually as "dramatic" at all points as a change of one L* unit or one b* unit...that's not actually the case. So you either need a better color space (they exist, and Graeme makes effective use of them behind the scenes) or you have to use "special sauce" magic that might result in perceptual uniformity at the expense of linearity in Lab space.
Just neutral (ish) axis for now to simplify things.
And yet another complication...R=G=B=0 in your monitor's native (unprofiled) space is going to have some L* value that isn't actually 14.45, and R=G=B=255, again won't be L*=96.5. In a really tightly controlled viewing environment, your monitor _might_ have a profiled R=G=B=255 => L*=100, but I'd honestly be surprised if any such actually exists. That means that you're mapping R=G=B=255 => L*=482.7, a*=1.7, b*=-8.4 (to pick a random but realistic number) on your monitor to R=G=B=255 => L*=239.8, a*=-0.8, b*=2.5 on your printer (again random but realistic, considering typical office illumination). Hopefully, you can extrapolate the madness that ensues….
This is where soft-proofing with Preserve RGB numbers comes into play. Yeah, I do this already for linear environments . . ., etc.
The typical approach to what I think you're trying to accomplish is what's called, "soft proofing." You can supply Photoshop with your printer's profile and tell it to render the image as closely as possible to how it'll print. Since monitors typically have larger gamuts than printers, especially near the neutral axis, this usually works well. The soft proof will look washed out compared to the rest of the screen...but, in a well-controlled environment, the print placed next to the monitor will look exactly as washed-out.
yep. But not worried just wondering if it can be done
The real-world problem is that your print is going to, again, look completely different when you take it out of your perfectly-controlled proofing booth and into the real world. It's why National Geographic looks drop-dead gorgeous in a viewing booth but is nearly uselessly dark in the typical living room in the evening. You could actually make a print that looks drop-dead gorgeous in the living room, but it's going to look washed-out and faint in that viewing booth….
Again. I’m not thinking about this in a viewing booth. For sake of argument let’s say I don’t give a single damn about how good or bad it looks under any condition. I just want the printed neutrals (or near neutrals) to measure L* linear when measured with a spectrophotometer that has been calibrated against tile white. I’m not thinking about monitor brightness, contrast conditions, or anything in the monitor. Just hard photoshop numbers translated to hard printed values. Simple. All other things can go where they go for now. If the print is near the sun I’m fine with it blinding me, if the print is in shadow I’m fine with it looking too dark. But linear printed L* values (percentage of reflected light) related to a TILE WHITE of a spectro is what I’m after.
If I might suggest...the perfect starting point for you, right this instant, would be to grab a roll of PTFE (teflon) thread tape. It's as close as you're going to get to a perfect lambertian reflector without spending thousands of dollars -- and you need expensive equipment to tell the difference. Fill your monitor with white, and hold the roll in front of the monitor. The extent to which there's a visual mismatch between the two is the extent to which your viewing environment is compromised. Until the thread tape and the monitor match, nothing you ever get out of the printer will match.
It's a brutal test for all its simplicity and cost-effectiveness. It's also quite enlightening....
Cheers,
b&