[argyllcms] Re: Need help interpretating results of camera profiling

  • From: Bernard <bdebreil@xxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Fri, 20 Jun 2014 11:03:58 +0200

Thanks for this reply. It appears that color management is a very complex topic, especially when applied to photography. It gets even more complicated when dealing with tools that are not those with which most studies and tests have been carried, namely Photoshop and other MSWIN or Mac Intosh tools. I worked with 'dcraw', 'ufraw' and 'darktable'. More further down...

Graeme Gill wrote:

I would have thought that
changing the white balance of the test image was not the right thing to do. 
I think the target workflow should be to run the raw image through the profile 
to the
working space, and then do any subjective adjustments in the working space. 
the grey balance of the test image will completely mess this up, since for the 
to remain valid, you have to apply exactly the same adjustment every time you 
the profile.

I will therefore try to do adjust WB and L (L a b) on the exported tiff image (in the color working space), not on the raw image in linear RGB absolute colorimetrics. But Pascal De bruijn, in his topic on camera color profiling from june 2010 using Darktable and Argyll, states that WB and Lightness of the brightest spot should be done prior to exporting

Input profile
was 'linear RGB', gamma correction OFF, no 'base curve', output profile set to 
colorimetrics' and 'Linear RGB'.

Hmm. Is that Darktable setting ? How sure are you that it was doing no color 
conversion ?
Ideally there would be a "calibrate" setting that disables all color conversions
I am not sure of anything... :=)

This being achieved, and prior to export to a 16-bit TIFF file in above 
mentioned settngs
and profiles, curiosity lead me to measure a few spots, using darktable tools, 
and compare
results with the values found in the manufacturer's file and in the Argyll 
file. The results surprised me... but please bear in mind that I know next to 
about color measurement and standards ; I only learnt whatever one can grab 
reading a few
dozens of pages. So, besides the white spot R13, I tested Q19 (a red patch), 
C10 (a green
patch), and E4 (a blue patch). RGB measurements were a good surprise, since 
were found minimal between measured values and values stated in reference files 
for the
same patches. Indeed it was so close that I could hardly believe it:

I guess I'm unclear which part of the process you are describing. Is this the 
workflow of grabbing the test chart camera RGB values,
Yes, it is. One may think that is evident that in such context I would find the color values stated by the target manufacturer... but I thought I would check, just in case the Darktable measuring tools would be inappropriate

Now, Graeme Gill writes in his tutorial: 'Colprof: average errors of 5 or less 
and maximum
errors of 15 would normally be expected'. My typical peak error is 29, and the 
error of 8.53. Is this too much ?

Input profiles often have larger fitting errors than characterizing output 
devices, but
DE of 29 is extreme. Most often this is due to a patch identification mixup.
I found why these results were so bad. When cropping the image of the target, I had kept the top part which identifies the manufacturer and his logo with other fancy icons. I have now removed everything except the color patches and their coordinates axis. My peak error is now 13.367 and the average error 2.716

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