I am a French PhD student currently struggling with colorimetry, and especially converting from RGB to Lab.
1°) to take a sRGB photo as close as possible to the real colors
2°) to decompose my sRGB photo in mathematically right values of L*, a* and b*
3°) to get the values of L*, a* and b* for every pixel (2 columns of data: the value of L* and the number of pixel corresponding to this value - the same for a* and b*)
With this values, I will then be able to make a statistical study of the distribution of the colors.
My current method is the following:
My lab owns a box with constant lighting in which I can put at once my product. A SLR Canon camera is placed on top of the box and takes a photograph through a hole. I take photos in sRGB color space, but could you please tell me if you think I should use Adobe RGB instead ?
Then, I upload the photos of my computer, and convert them from CR2 (RAW format of Canon) to Tiff with the software XnConvert. I am not sure that the ICC profile given to the photo by XnConvert is sRGB and that the gamma correction is applied. Could you please advise me another free software that does well the conversion ?
Then, I use the plugin of ImageJ 2.0 “Conversions between RGB color space and Lab color space” (Plugins / Image / Color / Convert RGB image to Lab Stack v1.00)
Finally, Analyze / Histogram / Copy and then I paste the values in a spreadsheet which allows me to compute the mean, the standard deviation, etc…
I read on the website http://www.ericbrasseur.org/gamma.html that many softwares (including ImageJ 1.40) produce gamma error in picture scaling.
I am using ImageJ 2.0, do you know if in this version of ImageJ the gamma correction is applied ?
ImageJ 2.0 uses 16-bits and 32-bits, so no problem of posterization if I understand well.
I have a few more questions:
The LAB given by GIMP 2.10 is supposed to be Hunter Lab color space or CIE Lab (1979) color space ?*
I would like to use GIMP 2.10 to compare to the results I got with ImageJ 2.0. When I open a photo in GIMP 2.0, I am asked to convert the image to built-in sRGB or keep the current ICC. I guess I should convert to built-in sRGB, but what should I choose for the rendering intent (perceptual, relative colorimetric, saturation, or absolute colorimetric ?), and should I check or uncheck the black point compensation ? The informations I found on the web concerning the black point compensation only referred to printing and I don’t need to print.
Is it possible to obtain the different values of L a b of the pixels and / or the histograms with GIMP 2.10 ? Or should I do the conversion to LAB with GIMP 2.10, save the 3 images and then obtain the values with ImageJ (but what if ImageJ doesn’t apply the gamma correction)?
Thank you very much for reading up until this point,
And thanks to @Elle for bringing me to this forum,