Comment by andrewla | Hacker News Reader

Comment by andrewla | Hacker News Reader

Saying black is the "absence of color" only makes sense when we talk about additive color. For subtractive color, black is a color like any other, just like white is.

The entire area of dyes and pigments is intensely fascinating. I went down this rabbit hole while trying to get a set of paints for a toddler that mimicked process color primaries rather than the traditional but inaccurate red/yellow/blue primaries.

Additive color is in a sense a solved problem -- we can create monochromatic light (or at least very narrowly band-limited light) in all possible wavelengths, and all human perception of color is based on a fixed set of wavelengths. It is possible to produce lights that will mimic any color that humans can perceive.

Subtractive colors, on the other hand, are dependent on finding physical substances with emission spectra that roughly map to the gamut of human perception. There are no magic substances that produce a monochromatic reflective baseline, instead, we are dependent on finding substances that have emission spectra that are simply close enough to the complements of the human visual primaries.

That's a hard enough problem, much less dealing with the fact that actually applying dye or pigment to another physical substance involves chemical reactions that may affect the color of the pigment, as may exposure to ultraviolet or visible light, or changes in temperature. At one point I had a Canon inkjet printer and I switched to using off-brand ink, because it was so much cheaper, and looking at the photos now (~5 years later) the difference is stark -- the cyans of the knockoff ink have almost completely vanished, even from photos that were not exposed to sunlight or significant direct light, while the original Canon ink is still true to the original photos.

Color are colors only because visible light reaches your sensor (eye, camera) and it's interpreted as one color or another. And this should already hint at where I'm going.

So whether you have a laser emitting a specific visible wavelength, or a sheet of paper absorbing some, in the end visible light will reach you and you can say "hey look, it's red". Different visible wavelength reaches your retina, different color.

Now imagine you had true black light source (paradoxically) "emitting" no visible wavelength. Or you had a sheet of paper covered with true black ink, absorbing all visible wavelengths. What kind of visible photons would you register? What wavelength of visible light would reach you?

Since no photons get to your sensor how exactly do you define that color other than "absence of color"? Or are you you talking about a convention that "the absence of color" would in itself be called a color, in this case black?

I don't see the necessity to pick out one point in a multidimensional space and go: "All these other are colors, but this one is not!".

Put differently [1]: A flat emission spectrum of zero energy is still an emission spectrum.

[1] Modulo how color is sensed.

It's not a necessity, but it is not unnatural either. A flat emission spectrum of zero energy is the only spectrum that corresponds to zero emissions.

Don't multiply entities beyond necessity?

Not sure what that means? There are many languages and interpretations on this planet, and boiling the ocean to remove the redundancy would be futile, so my advice would be to learn to ride the waves instead.

It means you should not add complexity to a model if it does not improve the predictive power of the model.

> Color are colors only because visible light reaches your sensor

You're playing a semantic game. In short, I dispute this definition. This is a reasonable definition of additive-based color mechanics. It is otherwise an arbitrary definition of the word "color" that does not match common usage of the term.

To expand the term, "color" is often used to mean the perceived spectral emissions from a surface under white-ish illumination.

If a wall is "red", and you turn off the lights, is it still red? By your definition, no, which means the tautology you extend is meaningless to me, as my definition of color would trivially admit that the wall remains red.

The wall retains the capacity to reflect red light. If it were to change its spectral response while the lights are off it doesn't persist in being red.

Subtractitive color is only a thing because of additive colors. By adding extra pigments, you are trapping photons, and preventing light from getting to the eye. Black is the color that occurs when your pigments (or whatever other tools you are using) trap enough light that little-to-no light reaches the eye

The question is not "what color is the wall?" but "what color are you seeing?". So what color are you seeing when looking at the red wall in perfect darkness? Better yet, are you seeing anything at all? Are you claiming that anything you can't see, on the other side of the wall, is also black because you can't perceive any spectral emissions from their surface?

Earlier you said the wall should be black because black is a color and that's what you'd be seeing. But now you say the actual color is red but only using prior knowledge. And if the wall actually changed colors (I quickly painted it blue just to prove you wrong) what color would you say it is? Black? Red? Blue? So your example is purely philosophical and doesn't support your claim one bit.

Is the absence of light a type of light? Can we call the absence of any photon a "close04-ray" in honor of it's distinguished inventor, me?

A room doesn't contain void, it simply lacks everything else. Water is odor-less and taste-less. Silence isn't a type of sound, it's the lack of sound. And a completely empty fridge doesn't contain food. Your definition of a color while correct sabotages your conclusion that "detecting no spectral emissions" should be called the color black.

And please stop using additive/subtractive as if this should change how colors are perceived. It changes how they are produced but your eye will still detect light at a specific wavelength that you'll call a color.

Colloquially people will still call it the color black. But it's just a convention also meant to support the fact that we call a wide range of greys and dark colors as "black". It's for ease of speech and description, not for accuracy. In reality true black is simply the absence of any perceived light.

Wrong. Human perception of color is not physics dependant. And perception is everything.

> and all human perception of color is based on a fixed set of wavelengths.

Minor clarification: Multiple wavelengths at the same time, which are drawn from the fixed set/range. Magenta, for example, requires multiple wavelengths, which is why you can't create a band of it from a simple prism.

Many people -- perhaps influenced by color-wheel charts -- assume a false 1:1 relationship between the physics and the sensations. The analogy I like to use is that wavelengths are to colors as chemicals are to smells.

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