colorimeter corrections white patch

[Tilman Holzhauer]

I would like to know how the
measurments of the white patch is calculated into the 3 triplets for R,
G, B. Would it be possible to get that informacion?
Greetings,
Tilman

[Graeme Gill]

Post by Tilman Holzhauer
I would like to know how the
measurments of the white patch is calculated into the 3 triplets for R,
G, B. Would it be possible to get that informacion?

Hi,
RGB is a device dependent space, so you can only convert from
a color measurement if you know exactly what the behavior of that
space is.

If you have an ICC profile describing the space, then you can use something
like icclu to convert a measurement to an RGB value.
See <http://www.argyllcms.com/doc/icclu.html>

Graeme Gill.

[Tilman Holzhauer]

Hello Gill,

Thank you for the answer, but I am not sure if that was my question:) I actually asked that in the dispcalGUI forum and Florian recommended asking you here. When I create corrections with a spectro for my colorimeter in dispcalGUI, R, G, B and a white patch are measured, right? But in the end I get only xy and Y values for R, G, B...how are the results of the white patch calculated into the xyY RGB values?

Thank you for your great work.

Tilman

Sent from Yahoo Mail on Android

[Graeme Gill]

Post by Tilman Holzhauer
Thank you for the answer, but I am not sure if that was my question:) I actually asked
that in the dispcalGUI forum and Florian recommended asking you here. When I create
corrections with a spectro for my colorimeter in dispcalGUI, R, G, B and a white patch
are measured, right?

Sorry, but you didn't indicate the context.

A 3x3 matrix is created to best fit the measurements of the spectrometer
patch XYZ to the colorimeter XYZ values.

Post by Tilman Holzhauer
But in the end I get only xy and Y values for R, G, B...how are
the results of the white patch calculated into the xyY RGB values?

It's not entirely clear what you mean. Your corrected measurement
XYZ values are the instrument measured XYZ multiplied by the correction
matrix. It's hard to say exactly what contribution the white patch makes
to the matrix - the fit is least squares sum of the delta E94 of all the
instrument calibration patches.

Graeme Gill.

[Tilman Holzhauer]

Hello Gill,

Ok. I was just curious. In lightspace, you generate an *.edr file (one for the colorimeter, one for the spectro) that contains four triplets,
xyY(XYZ) for R, G, B AND white. Those two *.edr files get then compared in some way to get the correction matrix (which I don't actually see in lightspace). In dispcalGUI I never see any values for white in the matrix (the *.ccmx) but a patch does get measured (I think in Calman also?) So I was asking myself if the method is actually the same or how I could compare correction offsets created with lightspace to a ccmx correction file created in Argyll/didpcalGUI.

Greetings,
Tilman

Sent from Yahoo Mail on Android

[Graeme Gill]

Post by Tilman Holzhauer
Ok. I was just curious. In lightspace, you generate an *.edr file (one for the
colorimeter, one for the spectro) that contains four triplets, xyY(XYZ) for R, G, B AND
white. Those two *.edr files get then compared in some way to get the correction matrix
(which I don't actually see in lightspace). In dispcalGUI I never see any values for
white in the matrix (the *.ccmx) but a patch does get measured (I think in Calman
also?) So I was asking myself if the method is actually the same or how I could compare
correction offsets created with lightspace to a ccmx correction file created in
Argyll/didpcalGUI.

Well, you can do it in smaller steps if you want - measure the two
instruments on the same patch colors and then use makeccxx on the two
.ti3 files to create the .ccmx. As far as I know lightspace doesn't
use exactly the same method, it uses a direct XYZ based solution
rather than one based on perceptual errors, but if you want to
be certain, you should ask them.

Graeme Gill.