Friday, September 26, 2014

Unique hues

Color does not exist in nature, it is an illusion happening in our mind. Therefore, we can not solve color appearance problems with physics alone. The situation gets even hairier when we converse about color, because color names are just an ephemeral convention. For example, it is easy to "synonimize" blue and cyan or red and magenta.

Especially the latter is not out of the blue. Back in the days when I was doing research in color science, I had an OSA-UCS atlas. Periodically, I was getting updates of color tiles. One tile was standing out: the one that Boynton and Olson had determined to be the best red, with coordinates (L, j, g) = (–3.6, 1.5, –6.9), representing the consensus for the basic color term "red." Not only was it changing, but is was changing frequently. When I lined the updates up in the light-booth, the best red was shifting towards magenta.

I was so puzzled about the OSA changing the tile for the best red, that I called them up. I found out that they periodically where getting new tiles from their supplier Dupont when they developed new pigments. The OSA would then scale them visually and emit updates to the atlas. Therefore, the shift of basic red was a sign that industry was getting better at creating what humans consider to be the best red, and the hue was shifting from yellow towards magenta.

This indicates that the above synonymity is not arbitrary.

Art teachers are the ones most victimized by the non physicality of color, especially when they have to teach the difference between additive and subtractive color, as I wrote in "Is it turquoise + fuchsia = purple or is it turquoise + fuchsia = blue?" a few years ago. Recently I received the following question: do you have any ideas as to how I can prove that red, green, yellow and blue are the unique hues?

It is not that Goethe's yellow-red-blue (today we would write yellow-magenta-cyan) primary theory is wrong. His 1810 Farbenlehre and his battle against Newton with the theory of red-green-blue primaries was a little weird, especially considering Newton (1642–1727) had long been dead when Goethe started fighting against him.

Go back to the 1470s in Florence. The 1453 collapse of the Roman Empire of the Orient (Byzantium) had generated an influx of scientists fleeing from Constantinople to Venice, who took along their over 1500 year old libraries. Aldo Manuzio, exploited this trove of works and cheap scientists to create the new profession of "publisher" and popularized this knowledge that had gone lost in the west. This lead to the Renaissance in Italy, which essentially consisted in taking a field, studying all books about it, and formulating a theory about this field, which can be used to apply it over and over. An example was Niccolò Machiavelli studying politics and creating the field of political science.

At that time, somebody we would now call "engineer" did not go to study at a university but to a place called a "bottega" or "workshop" in English. One of the most famous ones in Florence at that time was the Bottega del Verrocchio, run by Andrea di Michele di Francesco de' Cioni. Verrocchio was his nickname and can be translated as "true eye."

In the pictorial arts, at that time the hot trend was to use very vivid colors and to try to reproduce reality as faithfully as possible. In Verrocchio's bottega, students would learn about pigments and binders and research the creation of new paints to realize highly vivid colors.

One of these students was a lad from the village of Vinci, called Leonardo. Leonardo was passionate about this quest for realism and would dissect cadavers at the hospital to learn how muscles and tendons worked anatomically. For color, he invented a new method to analyze a scene consisting in viewing it through a series of stained glasses (a common novelty in the street markets of the 1470s). Today we would call this a spectroradiometer.

From his studies, Leonardo da Vinci formed a new theory that color is not a physical quality mastered by studying pigment admixtures, rather it is a new phenomenon he called "perception." Perception is the capacity to have a sensate or sentient experience, what philosophers call quale (the plural is qualia). Quale is what something feels like; for example, the smell of a rose or the taste of wine as specific sensations or feelings. We experience the world through qualia, but qualia are not patterns of bits in memory, far from it.

From this he developed a methodology to paint the light illuminating a scene instead of the scene itself. The technique he developed was to paint in many layers with very little pigment. He called this technique "sfumato" and the result "sfumatura."

Working with the spectroradiometer, he realized that colors are not a discrete set with the names of pigments. Rather it was a set of opponents. Doing his drawings he realized, that white and black are actually colors (in 1475 they were not considered to be colors) and they are opposites he called "chiaro" and "scuro", resulting into a technique he called "chiaroscuro."

Once you realize colors are not related to pigments, you have to come up with a representation based on perception. This is where language comes in. When describing a color in 1475, you would say for example "this color is a vermilion with a hint of lapislazzuli" to describe a certain pink. So in 1475 the primary colors were the pure pigments, like oltremare (lapislazzuli), azzurro (azzurrite), indaco, tornasole, vermilion, gold, etc.

Some pigments like lapislazzuli were very expensive and others like azzurrite were more affordable. Admixture would allow you to obtain a given color with a cheaper paint that elicits the same perception. Therefore, Leonardo concluded that the old naming scheme is not useful. To find a representation, he then set out to find if there are any perceived colors that you would never describe as an admixture of two or more colors. This lead him to red, green, yellow, blue, black and white, which are his unique hues.

Further, he discovered that you would never use descriptions like reddish green, bluish yellow, or dark white. This is what he called the "color opponent system" and allowed him to describe perceived color as points in a 3-dimensional space. This is the basis for CIELAB and for the NCS color atlas. For the Munsell color tree there is an extra step for purple.

Leonardo da Vinci’s opponent colors, rendered here using his chiaro-scuro technique to suggest 3-dimensionality. Left: view from top. Center: view from slight left. Right: view from slight right.

With this you have the answer to the above question: you have to distinguish between the creation of color with colorants and the perception of color.

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