Hello, I have bought a set of gray, white, and black cards from a camera accessories shop. Assume that I take a portrait of a person holding the three cards, or do product photography with the three cards present in every photo. How exactly do I exploit the presence of the three cards to make the colors in the photos more accurate when postprocessing in darktable? Which module(s) do I use?
I am new to the use of these white balance cards, and I think I may be misunderstanding the purpose of these cards. Thank you for helping!
You can use it like described in the documentation: CAT tab workflow
“Alternatively you can use the picker (to the right of the color patch) to select a neutral color from the image or, if one is unavailable, select the entire image.”
You use them as reference for the white and black point when you set the exposure and tone mapper. I.e., you adjust the exposure/contrast/white point/black point until the white card looks white and the black looks black (or the white and black on the card look like the white/black point that you are after).
Black and white references are tricky: setting them from a card supposes that they represent the brighest and darkest areas in your image, but that’s not necessarily true.
If you have some shadow parts where you want detail, your black reference may have to be set lower than the card indicates. Conversely, some bright areas can be brighter than the white card, so those would turn flat white when you use the white card as a reference.
The method of setting black and white references from such cards is valid for e.g. reproduction of flat originals, but requires thinking when used on a 3D scene, especially with multiple lights.
Even the grey card requires some attention: its illumination must be the same as for the part in your subject you want represented by the card.
Also, most of such white/black/gray cards are not intended for white balancing, the colours are not guaranteed to be neutral (unless the card manufacturer tells you they are neutral).
