Color profile from Tiff file

Software RawTherapee
15

Hello Jacques, hello to all
It was a real pleasure to discuss with you by phone. Thank you for your advice. Here is an update on my work using RT 5.10, as this version corrects L* a* b* bug. Sorry for the delay, it’s not my only project.

ICC profile creation
1) Reference image with RT
Starting with BabelColor Macbeth chart, I saved it as a reference image. It’s identical to the original image, still a gamma-encoded one and not a linear one.

2) Linear with RT
input profile: AdobeRGB; abstract profile: linear; output profile: Prophoto. The resulting image was processed with DcamProf.

scanin.exe -v -p -dipn -F %corners% target.tif ColorChecker.cht cc24_ref-new.cie
dcamprof.exe make-profile -g cc24-layout.json target.ti3 target.json
dcamprof.exe make-icc -t linear -W -n target_dcam target.json target_dcam.icc

Here is the profile displayed with GamutVision, in dotted line. The plain line is the AdobeRGB profile. As you can see, they are strongly different.
gv_lin_RT

3) RT with linear Gimp
input profile: AdobeRGB; abstract profile: none (=sRGB); output profile: RTv4_medium. The image is then converted to linear using Gimp. There is almost perfect agreement between the two profiles.
gv_nometa_RT_lin

4) DcamProf options
Adobe RGB and its equivalent RTv4_medium use D65 as the illuminant. Adding this option to make-profile induces a shift of green vertex to the right (not shown).

dcamprof.exe make-profile -i D65 -g cc24-layout.json target.ti3 target.json

acr option is often suggested for make-icc. The result is shown below, clearly degrading the profile.

dcamprof.exe make-icc -t acr -W -n target_dcam target.json target_dcam.icc

gv_nometa_RT_lin_acr

ICC profile usage
Input profile: the one obtained in step 3 above; abstract profile: linear; output profile: RTv4_medium. L* a* b* values are as follows:

  • A1 dark skin: 37.2/13.6/13.5 (ΔE_2000 = 0.95), expected values are 37.54/14.37/14.92
  • F3 cyan: 46.8/-35.9/-31.0 (ΔE_2000 = 3.95), expected values are 49.57/-29.71/-28.32

Color accuracy is excellent on dark skin patch. Cyan patch is not as good but is expected to be difficult as it is outside of the sRGB color space. Next step is to check this protocol with a TIFF obtained from a flatbed scanner. Finally, I will be able to apply it on my microscope images which are very similar. I’ll keep you in touch.

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