U of S | Mailing List Archive | alt-photo-process-l | Re: Gum Calibration 2 (How to read color samples?)

Re: Gum Calibration 2 (How to read color samples?)

On Oct 24, 2008, at 11:06 AM, davidhatton@totalise.co.uk wrote:
On Oct 24 2008, Loris Medici wrote:

This goes to those who calibrate with PDN / ChartThrob and use a scanner
for making readings from the printed step tablet:

What adjustment(s) you do, to the scan of the step tablet? Do you
desaturate it only?

I will re-calibrate from scratch (and separately for each color), using
stock paint:gum solutions, that's why I'm asking...

I was thinking to change the image mode to Lab, split channels, and then
use the Luminosity image (mode changed to RGB) for taking the readings.
That way, the basis of the calibration would be Luminosity. That sounds
right to me, since we're trying to match both chroma and luminosity for a
good color balance, isn't it?

I just don't like the idea of desaturating, since it's not obvious how
Photoshop does it, and feel like it can give quite different results
depending on color... (I'm not sure saying that -> just a feeling ->
doesn't seem right.)

Any thoughts?
I was hoping someone else would answer this; I was curious what the answer would be.

I'm not entirely sure what you're asking, overall, but as far as what happens when you desaturate, thought I'd offer the discovery I made a couple of years ago that if you set your monochrome working space to grey gamma 2.2, there's no discernable difference between desaturating and simply changing to greyscale mode, except that if you change to greyscale you lose two channels, which can be useful if you're working with big files. But as far as the pixel values themselves, they are identical (I checked this myself, moving back and forth between the modes). That may not hold if you're working in some unusual color space, but it works for Adobe RGB and sRGB.

As for changing to Lab mode and splitting out a channel, I'm not sure why you'd do that. My understanding is that Lab Luminosity and HSB Brightness are the same, at any rate for either one of them you can read off the values in the info box.

In ChartThrob, to answer your bigger question, I don't do anything to the scanned printed step tablet because ChartThrob can calculate the curve from the RGB file, so I'm not sure what the purpose of the desaturation step is and where it fits in the calibration process. So maybe PDN is so different from ChartThrob that it doesn't make sense to ask a question asking for input from users of both. I don't know, I'm just wondering.

Hope any of that is the least bit helpful,