Recently I’ve seen two kinds of posts that share the same fundamental flaw. This is not the first time I’ve seen either of these mental errors in action, and I’m sure it won’t be the last. I’ve had a hard time getting some of the people making the errors to see what’s wrong with their reasoning and methodology, and I’d like to do what I can to prevent recurrence of this kind of overly facile thinking in the future. Hence this post.
Here are the two types of posts:
- Drawing conclusions about how a camera and/or raw developer renders the colors in human skin by comparing the portraits in the DPR studio scene across cameras and raw developers.
- Creating color profiles for cameras by photographing targets created on an inkjet printer.
Before I get into the details of the mental error common to both exercises, please let me tell a joke, well known among engineers, that illustrates at a high level what’s going on.
It’s midnight. There’s a bar, a streetlight, and a cop. A drunk staggers out of the shadows, and starts to poke around under the streetlight. The cop asks him what he’s doing. The drunk replies, “I’m looking for my keys.”
“Did you lose them here?” says the cop.
“No”, says the drunk. “I lost them over there, but the light is much better here.”
Here, in a nutshell, is how the two types of posts are looking for the keys under the lamp post.
- The portraits in the DPR studio scene are printed on a photographic printer; the colors in those portraits are, at best, metameric matches to the colors of actual human skin.
- Unless you’re photographing inkjet printer output, the colors in the camera calibration target in the experiment above are, at best, metameric matches to the colors of your actual subjects.
There’s a big word in both bullets above: metameric. What’s it mean in this context? Specifically, what’s a metameric match and why is that inadequate?
Metamerism is a property of the human vision system, and occurs because the color-normal (insert caveats here) human reduces spectra falling on the retina to three coordinates, and thus there are an infinite number to spectra that can produce colors that look the same to our putatively color-normal person. That phenomenon is called metamerism, and spectra that resolve to the same color are called metamers. A metameric match occurs when two samples with different spectra are perceived by a color-normal person as the same color. Metameric failure is the lack of occurrence of a metameric match when one might be expected.
Let me do a bit of self-plagiarism from an earlier blog post and give you an example that illustrates several kinds of metameric failure.
Let’s imagine that we’re engineers working for an automobile manufacturer and developing a vehicle with a carbon-fiber roof, aluminum fenders, and plastic bumper covers. The marketing folks have picked out ten paint colors. They don’t care that we meet their color specs exactly, but they do care that the roof matches the fenders and the bumpers match both. We meet with our paint suppliers, and find out that the paint formulations have to be slightly different so that they’ll adhere properly to the three different substrates, and so the bumpers can survive small impacts without the paint flaking off, and so the weave of the carbon fiber won’t show through and wreck the infinitely-deep pool high gloss effect that we’re going for.
Turns out that the reflectance spectra of all three paints are somewhat different, and we mix the paints so that we get a metameric match for a color normal person for each of the ten colors when the car is parked in bright Detroit sunshine.
What can go wrong?
We have prototypes painted in each of the colors, park them in front of the development building on a sunny day and call in the brass. Two of them, both men are what we used to unfeelingly call color blind. One suffers from red-green colorblindness protanopia (no rho cell contribution), and one the other red-green colorblindness, deuteranopia (no gamma cell contribution). About one percent of males suffer from each.
Each of the color-blind persons says that some of the paints don’t match, but they identify different paint pairs as being the problem ones. This is called observer metameric failure. Everybody else says that the colors match just fine and can’t figure out what’s wrong with the two guys who are colorblind. There’s one woman who has a rare condition called tetrachromacy (four kinds of cone cells), and none of the color pairs match for her. That’s another kind of observer metameric failure.
Now we call in the photographers and have them take pictures of the cars in bright sunshine. In some of the pictures, the color pairs don’t match. This is called capture metameric failure. The odd thing is that the colors that don’t match for the Phase One shooter are different from the guy with the Nikon.
We bring the cars indoors for a focus group. We carefully screen the group to eliminate all colorblind people and tetrachromats. The indoor lighting is a mixture of halogen and fluorescent lighting. Several people complain that the colors on many of the cars don’t match each other. When this is pointed out to the focus group as a potential problem, all agree that some colors don’t match, and they all agree on which colors they are. This is called illuminant metameric failure.
The photographers take pictures of the cars in the studio using high CRI 3200K LED lighting, and a bunch of colors don’t match, but they’re not all the same colors that didn’t match when the same photographers used the same cameras to take pictures of the cars outside. This is a combination of illuminant metameric failure and capture metameric failure.
We find a set of pictures where the colors match, and the photographer prints them out on an inkjet printer. We look at the prints in the 5000K proofing booth, and the silver car looks neutral. We take the prints into a meeting with the ad agency in a fluorescent-lit conference room, and the silver car looks yellowish. All the observers are color normal. This is a combination of one or more instances of illuminant metameric failure. In the viewing booth, the observer is adapting to the white surround, and the spectrum of the inks depicting the silver car resolves to a color with a chromaticity similar to the surround. In the conference room, the observer is adapting to the white surround, and the spectrum of the inks depicting the silver car resolves to a color with a chromaticity different from the surround. The fact that the printer uses fluorescent yellow ink and the paper has optical brighteners doesn’t help matters.
With that in mind, let’s take a look at metamerism in the two cases above.
In the first case, the portraits in the DPR test scene, there are some obvious difficulties even before we get to metamerism. The captured image from which the print in the scene was made may have color errors in the developed file, stemming from a combination of the color filter array (CFA) of the camera used to make the portraits and the raw developer and color profile employed. The print may not faithfully represent the colors in the developed file. The print may have faded over time. But let’s set all that aside and assume that the colors in the portrait photos properly represent the colors of the subject. Even if that’s true, the spectra of the inkjet print will be different from that of the subjects’ skin, and thus the match between the print and the subject will be a metameric one. So when the studio scene is captured by the camera under test, the colors in the developed capture will reflect how accurately the camera and associated raw developer render colors in the skin-tone range rendered by an inkjet printer, not how the camera/developer combination renders the colors in actual human skin.
In order to test how well the camera/developer reproduce human skin tones, the subject needs to be either an actual human being, or a test chart constructed so that each patch’s spectrum matches that of some human skin. The lighting spectrum needs to be the same as that of light that the user of the studio scene cares about, but that’s usually less of an issue. If that is done, the capture metameric error of the studio scene would match the capture metameric error associated with an actual human subject, and the skin tones in the scene will reflect how the actual camera captures skin tones. As things stand now, unless your chosen use for the camera is making pictures of inkjet prints, the DPR studio scene tells you nothing about how well the camera and associated raw developer will render human skin tones.
Finding the keys would be difficult if we used actual human subjects in the DPR test scene or came up with test patches with the required reflectance spectra. It’s much easier to look under the lamp post and use an inkjet print.
In the second case, we wish to present known spectra to the camera so that we can see how the camera responds to those spectra and use that information to make either an accurate color profile for the camera, or make a color profile whose inaccuracies are by design. The readily available test targets have either 24 to less than 200 patches, and many of those are grays. We’d also like more patches. Some people are making more patches by printing their test targets with an inkjet printer. That gives more colors, but all the colors are created from the same 4 to 8 inks, applied at various droplet sizes with various dilutions. Those inks mix on the paper in a was more complex than the additive mixing of RGB light, but the spectra of the mixed colors is a function of the spectra of the individual inks. When you print targets of several hundred patches with an inkjet printer, you are by no means getting several hundred independent spectra, and you are getting spectra that are not related to those of natural-world subjects.
In order to make a profile useful for natural-world subject matter, the patches need to be either natural-world objects, or a test chart constructed so that each patch’s spectrum matches that of some human skin. The lighting spectrum needs to be the same as that of light that the user of the profile cares about. If that is done, the capture metameric error of the patches would match the capture metameric error associated with an the intended subject matter, and the colors in the captured scene will be acceptably accurate or biased away from accuracy in the desired direction. With inkjet-printed targets, unless your chosen use for the camera is making pictures of inkjet prints, the profiles will produce colors that depart for the real ones in unknown and uncontrollable ways.
Finding the keys would be difficult if we used actual natural objects in the test target or came up with test patches with the required reflectance spectra. It’s much easier to look under the lamp post and use an inkjet print for a target.