Wednesday, April 2, 2008

Administrative note and color lawsuits

First an administrative note. Most feedback we get from you, our esteemed readers, is in the form of personal email. Only rarely are we able to generate sufficient controversy to spark a debate in the blog comment section, such as with Non-local realism, An On-Line Color Thesaurus, or yesterday's Revolutionary White Reflectance Standard for Metrology. Therefore, we are happy for every good comment we get. However, as you are aware our blog server is rather crafty, and it is difficult for us to find comments when you replace the post title with your own title. This summer HP will be upgrading to commercial blogging software and this blog will run smoother, hopefully even multilingually. In the meantime here is my answer to a comment on color lawsuits I was unable to locate.

IANAL (I am not a lawyer), so I cannot tell you how many colors the Constitution on the Laws thinks you are seeing or entitled to seeing. From a color science point of view, color does not exist in nature, it is an illusion that is elicited in our visual system.

Colorimetry is the art to predict an illusion from a physical measurement, hence what we do in color reproduction is to try to build models that allow us to make statistical predictions of this illusion. Our supreme authority is the Commission Internationale de l'Éclairage (CIE), which in Definition 845-02-18 defines (perceived) color as follows:

Attribute of a visual perception consisting of any combination of chromatic and achromatic content. This attribute can be described by chromatic color names such as yellow, orange, brown, red, pink, green, blue, purple, etc., or by achromatic color names such as white, gray, black, etc., and qualified by bright, dim, light, dark etc., or by combinations of such names

Perceived color depends on the spectral distribution of the color stimulus, on the size, shape, structure and surround of the stimulus area, on the state of adaptation of the observer’s visual system, and on the observer’s experience of the prevailing and similar situations of observation

Perceived color may appear in several modes of appearance. The names for various modes of appearance are intended to distinguish among qualitative and geometric differences of color perceptions

As all the et cœteras in the definition reveal, there is nothing that allows you to count how many colors you can see. A metric you could use is to enumerate all the color names you can tell, i.e., the size of your color lexicon. However, the color lexicon is acquired, so its cardinality depends on your experience. The cardinality also depends on time, as we name more colors the more evolved the civilization gets.

For example, 2000 years ago, the Romans could not distinguish between blue and green. More recently, 1000 years ago, the Japanese, which had three colors — white/pure (shiroi), black/dark (kuroi), and colorful/red (akai) — added a fourth color to their vocabulary because tree leaves, sky, and the sea are colorful but not red, hence aoi became the name for those things, without distinction between green and blue. Even today midori (green) is not an attribute but a substantive; the color of an unripe apple is blue (aoi), not green (midori) because it does not make sense from a grammatical point of view.

aoi ringo

We can use psychophysics to start with one color, then change is slightly until we perceive a just noticeable difference (JND), increment the counter by one, and start over with the next iteration step. This way we could determine that we can see something between 6,000 and 10,000 different colors. But when you look what populations name distinctly in a color thesaurus, you typically find a 700 to 900 word dictionary.

If we go back to the lawsuit discussed in that blog comment, we could take the position of the electrical engineer and look at the addressable number of colors. When a display can address 16,777,216 colors, this is many more than you can actually see, and even 262,144 is much larger than 10,000. From a color science point of view, the point is moot.

Turning the argument around, a printer can only put down or not put down cyan, magenta, yellow, or black marks. Yet you would not claim the printer is capable of printing only four colors. The trick is that the human visual system (HVS) has a limited resolution and therefore you can halftone colors with dithers.

In display monitors and TV the spatial resolution is much lower than in printers, so instead of spatial dithering, temporal dithering is used, but you still see the same color, it was just cooked differently. Remember, color is just an illusion elicited in the HVS.

One complaint in that post is that you can see artifacts when you can address less than 16,777,216 colors. That is moot too. Give me any two colors on an 8 bit display capable of displaying only 256 colors and I can produce a completely smooth gradient between your two colors, with each step below one JND.

This is the HVS as we know it in color science. IANAL, and I do not know if the law refers to a human visual system. For that matter, I do not even know if lawyers might live in a world where the photons are colored.

To close the loop, if you have comments, do not send me email, post them to the blog without changing the title.