Preparing/using a camera color profile using IT8 target on Linux/Darktable

Question

In my photo processing workflow I use Darktable to handle the CR2-to-TIFF/JPEG conversion, however I frequently struggle with getting the colors right. As the first step toward correct colors I am using a grey card to get an idea about white balance, but I wanted to go a step further and start getting a color profile.

My understanding of the process is that I get myself a (physical) IT8 target from some manufacturer, like Wolf Faust; they supply me with the printed target and a set of values specifying the calibrated color values for all the patches on the target. The process is then similar (in spirit) to using a grey card: I need to take a photograph of the IT8 target each time lighting conditions change.

Assuming the above description of camera profiling is correct, given a photograph of the target under the same lighting conditions as the photographs I want to color correct, how do I create a camera profile under Linux?

What software tools and in what way should I use? Can someone recommend a blog post/tutorial describing the process?

Answer 1

First, as to whether you'll want to shoot an IT8 target as often as you would a white balance target, the answer is "probably not". Getting a good IT8 shot involves significantly more consideration than shooting a gray card, and once you have a good profile made with a good full-spectrum light source, it will be perfectly usable for photos taken under most lighting conditions, adjusting only for white balance. If you have, say, an LED or fluorescent light source that you use often for important pictures, you might want to create a specific profile for that case.

As to what software, you'll probably be using the ArgyllCMS software tools to process an image file carefully prepared in your choice of RAW developers. Specifically, the scanin tool will attempt to recognize the layout of color swatches in the target image and read their color values into a data (*.ti3) file, and the colorprof tool will then attempt to generate a profile that maps those values to the nominal chart values with a minimal degree of error.

As to the actual process... it's complicated. The procedure can be as simple as in the link provided by @chili555 above, but somewhat better results can be acheived with some extra effort during the capturing as well as during the processing. The short version is that you'll produce a minimally processed TIFF from a carefully exposed RAW shot, in linear color space, which is white balanced for the light source and exposure compensated to match the documented tonal range of the IT8 card. The linked articles are a good starting point for diving in as deep as you're comfortable with.

As to personal experience, I found that the first profile I generated using the simpler approach was a substantial improvement over the stock profile provided in darktable (which, from what I understand, is drawn from the same source used by most other software, i.e. Adobe). The results of doing the extra work for a "well behaved" profile were subtler, but corrected some troubling issues related to color neutral values going into the profile not coming out neutral (using the spot white balance on a gray card, for instance, would not produce a gray output that was RGB neutral when sampled). My conclusion is that profiling is worthwhile if you're not happy with the results you're getting from software-provided profiles, but the results can vary depending on the effort invested.