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Comment by stavros | Hacker News Reader

Not all color, but you don't need all color. You need color temperature and maybe luminosity, and it's good for that.

> You need color temperature

For artificial light (or mixed sources, e.g. from reflections), it is not correct. The light source is characterized by its whole distribution (which can be anything) rather than one temperature parameter (for the Planck distribution).

An easy example is a fluorescent lamp spectrum (https://commons.wikimedia.org/wiki/File:Fluorescent_lighting...).

Unfortunately, cameras only capture RGB values, not a whole spectrum. It is impossible to perfectly color-correct the image unless you know both the spectrum of the light source and of the materials in the scene. A reference card with lots of colors still only provides you with the known spectra of the colors on that card, it still might not contain a color with the exact same spectrum as that flower you are trying to photograph for example.

Another issue is that even if you have the perfect photography + post-processing setup, when I see the result it might be on a non-calibrated monitor, or on paper but in a room with non-perfect lighting, and perhaps I was just looking at something else a second before and my brain's auto white-balance hasn't caught up yet. So I might still not perceive it as was intended. This is why that gold dress looks blue to some people.

The patch of grey on the reference scale + good lighting of the scene to begin with will get you 99% of the way, the remaining 1% is extremely hard to get right, if at all possible.

Well, in principle you get 3 degrees of freedom instead of 1 (though, I would love to see camers with more than 3 color filters).

In practice you do 2 anyway (with exposure correction being used even more often than white balance). So I agree that more than that is usually not worth the effort.

From my amateour photography experince - with strange lighting you cannot do much anyway.

With multiple light sources, the white balance is undefined and you need to compromise somehow, yeah.

Even a scene where there is strong sunlight and a bright blue sky will make white balancing quite difficult, as the shadows are lit with sky blue skylight and the sunlit parts are lit with the sun.

I use darktable and it's possible to correct the blue tint in shadows, but like... it's annoying. And at some point I'd say fuck it and just accept that photography is not a science.

You're going to have a hard time calibrating that without a whole bunch of patches with known reflectance/transmission profiles, in practice you just have to color-correct scenes under such light by eye and there will be things that look wrong. When I did this professionally I'd still use the grey as a primary reference along with skin, since we're very attuned to what those should look like.

> You're going to have a hard time calibrating that without a whole bunch of patches with known reflectance/transmission profiles

If you care about accurate color, you do use a card with "whole bunch of patches" of different colors. For example, these British Library guidelines on photographing and scanning archival material say:

"An industry-approved colour chart or greyscale reference card is essential in order to check the accuracy of the colour captured in the photographs. This is a physical card that is available to purchase on the internet. It cannot be downloaded from the internet and printed, or inserted into your image using software. Approved colour checkers that have been used in EAP projects include: Gretag Macbeth ColorChecker, QP cards, Kodak Color Control Q13 card, Past Horizons Bookmark Photo Scale."

(https://eap.bl.uk/sites/default/files/Guidelines%20for%20pho...)

ColorChecker (“Macbeth”) Chart: https://poynton.ca/notes/color/GretagMacbeth-ColorChecker.ht...

QP cards: https://www.argraph.com/QP202.html

Kodak Q13: https://www.kodak.com/en/motion/page/color-separation-guides...

Bookmark photo scale: https://www.pasthorizonstools.com/Photo_scale_bookmark_p/boo...

Thanks, forgot what they were called. Maybe the state of the art has moved in the ~10 years since I got out, or it's just the part of it I was in (fine art, prints for the magazine to reproduce (also forgot what those were called...)), but correcting to a color checker was manual and usually inexact anyway so I'd cue off of the grey and whatever thing in the image had a tone we'd have a strong expectation of how it should look (again, usually skin).

As you quote they get used in archiving but those also tend to be done under well-characterized lighting and if not, it's something that's worth spending the time to tweak every little thing.

It's a long shot from being able to apply automatic perspective and grey-point corrections plus a little bit of tweaking so the colors look "right" (or as right as possible under the wonky spectral distributions of most lights these days).

I did manage to convince the company to buy the transparency so we could calibrate the film scanner's output.

There’s a wide variety of photographic color meter light calibration charts, see https://www.amazon.com/gp/bestsellers/electronics/3109910011...

My favorites are from Datacolor or X-Rite.

Calibration indeed presumes shooting a multi-color target. I personally use the process[0] from DCamProf docs.

[0] https://torger.se/anders/dcamprof.html#workflow_dcp

> skin

Is there a standard skin color or something? How do you deal with different skin colors?

No standard color of course, but we're so attuned to looking at people that we internalize certain ideas of what skin looks like (or should look like), more so than other objects out in the world. The big things I'd avoid are cooler tones (blue, green) as well as yellow; a touch of red or magenta is tolerated much better. In fact because there's no standard skin tone, that provides latitude to get something that looks right without nailing it exactly as long as the pitfalls I mentioned above are avoided.

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