Extraordinary claims. Ordinary investigations.

Special sauce


What do you see above? Beef patties, lettuce, tomato and cheese on a bun, right? The colors seem somewhat strange, and though we may suspect there’s special sauce, there’s no sesame seed visible.

But what’s really missing here is color, since the image above has only one true color.

Zooming the highlighted square, with some tomato, lettuce and bun, we can see that in fact the image is composed solely of red tones, interlaced with tones of gray:


There’s no green lettuce, yellow bun or brown beef, it’s all an illusion. But this illusion is not so much a “bug” of our visual system, it’s actually evidence of a special feature, that makes it far superior than any current digital camera.

Digital cameras do have a primitive version of this feature: it’s the color balance. It’s very important, because the raw color of an object does not depend solely on the way it reflects light, it also depends on the light that hit the object in the first place. A room lit by a yellow incandescent bulb will look different if it’s lit by a cold fluorescent light. If you don’t adjust the color balance, the colors of a photo or video will just look wrong, either too blue or too red, for instance.

Things is, they may look wrong, but that’s the real colors of the light being reflected by the objects, and as they say, the camera never lies.

Incidentally, the problem this represents for what the “real” or “corrected” colors of a photo should be means that even to this day there’s some valid discussion as to exactly how Mars will actually look to the first humans that will land there.

Well, we don’t have a manual color balance adjustment on our eyes. It’s all automatically done by our brains, because we almost always see the colors the way they should be. This may look like a tautology — of course the way the colors “should look like” is the way we usually see them — but it’s not so far off.

We do have color constancy, and that’s a very important evolutionary feature, because if we couldn’t trust our eyes to differentiate a red fruit from a blue one in varying conditions of illumination, we would probably be more prone to intoxicate ourselves, for instance. And we are not the only ones who possess this “special feature”, probably all animals with color vision have this too. Color vision would actually be almost useless without it.



Amazingly, color constancy was only carefully studied in the last few decades, and it was only in mid-1960s that Edwin Land, inventor of the Polaroid photography, actually proposed a theory to explain it, the Retinex.

According to Land’s theory, our color perception is not just a passive detection by our retinas of the different wavelengths of light. If that were the case, as explained above, it would be a cheap camera, and with no color correction, almost useless in the real world.

Our color perception actually involves the complex processing of the information captured by the retina, to extract the necessary information to color correct it. Land proposed an algorithm for that, and successfully demonstrated how it could generate color constancy by analyzing the relationship between the raw primary colors captured by light sensors.

Land’s Retinex theory would also explain why we see colors in the hamburger that are not really there. In this case, the gray interlaced tones were actually encoded with the green color information of the original full color image. Our brain process it the same way, and from that information it makes us perceive colors that are encoded there as information, but not as real colors.

The implications of this are amazing, because in a way it means that colors do not actually exist, at least the way we usually assume (or are even taught). Green is not just a mixture of blue and yellow, purple is not just red and blue light. None of these colors exist as a defined wavelength captured by the eye. That is only valid if the original wavelength was pure white.

Colors are actually a determined relationship of wavelengths processed by our brains. A kind of special sauce.

– – –

Be sure to check Land’s original papers:

Further study of the visual perception revealed more about the complexity of the whole process — and also some cool optical illusions.

[The hamburger image was created by Chris Taylor]

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