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

[Katharine Thayer <kthayer@pacifier.com>]

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,
Katharine