Friday, November 2, 2012

Autumn Contrasts

Once in a while, deference to the season is good for us…

I just returned from the Manchester, NH, ISCC meeting, and from some leaf-peeking in the windy (in both senses) corridor from Saratoga Springs, NY to Manchester. Based on the season and on the picturesque meeting venue, I thought it might be time to talk about autumn colors, in a slightly more speculative vein than Mark Fairchild’s Metameric Blacks column two autumns ago (see ISCC News # 448 p. 5). The dance of autumn leaf colors as a kind of co-evolution among species seemed sufficiently provocative. Consider the words of Michael Pollan [1], "We’re prone to overestimate our own agency in nature. […] In a coevolutionary relationship every subject is also an object, every object a subject. That’s why it makes just as much sense to think of agriculture as something the grasses did to people as a way to conquer the trees."

How do autumn leaves participate in co-evolution? For one thing, there’s foliar fruit flagging [2]: bright leaf colors call attention to berries containing seeds that birds and other animals can disperse (by the usual “indoor” route). And there seems to be a code whereby insects recognize which trees will tolerate them as parasites through the winter.  That code is mediated by chemicals called anthocyanins (typically red), which unlike yellow and orange carotenoids, are not present in a leaf throughout the year but appear in particular plants at the time of chlorophyll depletion. The balance between anthocyanins and carotenoids leads to quite a large palette of colors that could guide an insect or other color-perceiving animal. In addition to waving a “red flag” at aphids and other parasites to repel them, the anthocyanins in maple trees also seem to stunt the growth of nearby shrubbery, hence adding still more survival potential to the maple [3]. 

The trouble with this interesting dance is that it is hard to prove the cause-and-effect relationship of co-evolution. Oddly, one counter-question is “Do the dancers mean it?”  It doesn’t take much metabolic expense for maples to make anthocyanins, because the pigments evolve from direct reaction between sunlight and other leaf chemicals when the chlorophyll depletes and with it the phosphate supply. So maybe the trees aren’t intending the color code, and are therefore the signal isn't honest [4]---or, one might say, is insincere. In counterargument, the maples are paying an opportunity cost in shedding their chlorophyll early, thereby allowing the red to happen when there is a maximal contrast with surrounding green plants.

Faced with learned arguments about the sincerity (or lack of it) in plants, I have to retreat to the more familiar territory of vision science. Fortunately, my Manchester trip rewarded me with a vision science epiphany in a steam bath in Saratoga Springs. Upon entering the steam bath, I was unable to see anything but white steam. But in a minute or so, some ghostly shapes emerged and I could see other occupants and even a vacant place to sit down. The light was such that I might even have been able to read a newspaper---or maybe just the headlines!  Such adaptation to fog (low contrast) was not exactly a textbook visual effect. It even raised some interesting questions about a paper I was to co-author with Rob Carter, in Manchester (“The incredible lightness of the power law”).  The Weber-law behavior we were describing had some interesting experimental quirks, but none exactly like the fog-adaptation I was now experiencing.

Upon returning to New Jersey, I found a 2005 CIC13 paper by Mark Fairchild (yes, the same Mark Fairchild) called “On the salience of novel stimuli: adaptation and image noise.” Adaptation to persistent features of an image (such as blur, horizontal striations, or point-like noise) seems to be similar to chromatic adaptation, but it affects spatio-temporal as well as color channels. Could this kind of mechanism be responsible for fog adaptation?  I must defer any answer, or risk missing the deadline of this column.

Meanwhile, the rest of my trip (after the steam room) amply confirmed that fog-adaptation is not a universal law. After a spectacular sunlit view of fall foliage in Saratoga Springs, we drove across Vermont and New Hampshire in a dismal, fog-beset drizzle. The brightest anthocyanin in the world could not have provided signal or cheer in that miasma. And it didn’t get better with the passing hours.

After a while I gave up, and---lo and behold---the leaves had fallen. I had missed my opportunity. If I had been a bug or a squirrel, I would have fallen all over the dance-floor of co-evolution, unable to reap its benefits but completely sincere.

Michael H. Brill

1. Pollan M, The Botany of Desire: A Plant’s-Eye View of the World. Random House, 2001, p. xxi.
2.  Stiles, EW, Fruit flags: two hypotheses. The American Naturalist 120, 500-509 (1982). See also
4. Archetti M, Brown SP, The coevolution of autumn colors. Proc. Roy. Soc. Lond. B 271, 1219-1223 (2004),

Tuesday, September 11, 2012

What’s the big deal about Riemannian color space?

It’s time for a serious essay, for a change…

At the last ISCC Annual Meeting (Nov. 2011, San Jose), Mark Fairchild presented a paper called, “Is there really such a thing as color space? Foundation of uni-dimensional appearance spaces.” I quote from the abstract: “Color science is not devoid of examples of so-called color spaces that were actually descriptions of color perception one dimension at a time.” Mark’s examples ranged from the Munsell system (hue, value, chroma) to CIECAM02 (brightness, lightness, colorfulness, saturation, chroma, hue).

I believe Mark has begun the important exercise of deconstructing color science so as to reconstruct it in more fundamental terms without extraneous formalism. In a sense, we all must undertake such an exercise when asked to explain to a non-expert a complicated concept in color science. What is the essential purpose of the concept? What benefits does it confer, and conversely, what would happen if we abandoned the concept?

I was recently asked to explain to a non-expert audience the concept of a three-dimensional Riemannian color space, as used by two current authors. The authors simply wrote that the workings of tensor calculus made the concept “useful.” This hardly answered the question of purpose or benefit. To establish the historical connection and mathematical correctness, I looked at a retrospective work attempting to explain color-space geometry to theoretical physicists who were already comfortable with Riemannian spaces in other contexts:

“In 1666 Newton discovered that colors form a convex pencil in a linear vector space of three dimensions. In 1891 Helmholtz suggested a metric for that space; and by 1942 MacAdam had surveyed the experimental values of the metric. Meanwhile, Riemann, at the opening of his epoch-making address in 1854 on the foundations of differential geometry, had singled out the space of positions of objects and the space of colors as the only continuous manifolds of several dimensions in common experience. Later in that address he pointed out that a metric represents the effect of factors extraneous to the space itself---the result of what he called ‘forces’ acting upon it to bind it together. Riemann's suggestion has been brilliantly fulfilled in general relativity, where gravitation is the force that imparts a significant metric to space-time. In the space of colors, however, little has been done […] This paper outlines a theory of color space in which Riemann's ‘forces’ are essentially those of natural selection.” [1]

Although dazzling, this passage doesn’t get to the benefit or purpose. And certainly it doesn’t give an intuitive sense of Riemannian geometry per se.

Accordingly, I made a try at an explanation. When we imagine color in a three-dimensional space, we are looking for a consistent arithmetic of color differences expressed as distances in the space. It is not enough to be able to compute a distance between any two colors (as can be done for such formulas as CIE DE2000). Intuition from, say, a topographical map in two dimensions, suggests (i) that there should be a path between the two colors along which the distances add; (ii) that path should comprise the shortest distance between the colors; and (iii) between any two points along the path, the shortest distance should be the part of that same path that connects the two points. Spaces with such defined paths are called Riemannian. If the paths are all straight lines in some coordinate system (as in CIELAB) the space is Euclidean.

After basking in the afterglow of effort, I realized that I still hadn’t answered the original question. Why is the intuition of a contour map especially helpful in color space? How is that need met by a three-dimensional space with a Riemannian distance?  

Perhaps we should start more primitively, and define the utility of a three-dimensional color-order system without the additional encumbrance of a metric. Johann Lambert in his 1760 book, Photometria, gave a compelling use case (quoted by Rolf Kuehni): "Caroline wants to have a dress like Selinda's. She memorizes the color number from the pyramid and will be sure to have the same color. Should the color need to be darker or go more in the direction of another color, this will not pose a problem." [2].

So if you order your colors into a three-dimensional pyramid, you can find the color you want easily (without getting lost) by iterated change in the sensible directions. Seeing all the neighbors of a provisionally-chosen color gives guidance for the next iteration.

Now suddenly it is clear to me what Mark Fairchild’s uni-dimensional scales lack. They don’t allow you to see all of a color’s neighbors. But here’s a surprise: Neither does the Riemannian space, especially in the dehydrated metric-tensor form in which we usually see it. Truly, I can’t determine all of a color’s neighbors from that tensor. If I travel on a shortest path (geodesic) through color space, a neighbor I left long ago may be a neighbor I have right now, yet the locally characterized metric won’t explicitly reveal it. As new Riemannian color spaces emerge (e.g., from “Riemannizing” color-difference formulas) we should be mindful of that fact.

I think Johann Lambert and---a fortiori---Rolf Kuehni identified the purpose of three dimensions, Riemannian spaces, and---most especially---pictures that transcend local description. What every shade sorter knows, we theoreticians should re-learn.

[1] Weinberg JW. The geometry of colors. Gen Rel Grav 1976; 7: 135-169. [I omit references cited by the quoted passage on p. 135.]
[2] Kuehni RG. Color Space and its Divisions. 2003; Hoboken, NJ: Wiley, p. 55.

Michael H. Brill

Tuesday, July 17, 2012

Perils of the TLA

There’s many a slip twixt the intention and the abbreviation.

In the course of my duties as a subcommittee chair of the ASTM International, I recently shepherded a ballot to withdraw a standard on host computer communications because “the standard is not needed: manufacturers use their own SDKs and users can select an SDK from a menu.” One voter complained that he couldn’t find “SDK” in any dictionary but did find two definitions on the Internet: “Software Development Kit” and “Super Donkey Kong.” He presumed correctly that the former was intended, but he made his point: We should spell out our abbreviations at first occurrence, even if we think everyone should know them.

It’s easy to imagine amusing coincidences from such ambiguity, e.g., the ASA rating on a photographic medium used to record the luncheon meeting of the Acoustical Society of America. Or the National Science Foundation being caught at the bank with NSF (non-sufficient funds). Or the CIA spy who hangs out at the Culinary Institute of America. We could go on and it would look like fun.

Sometimes it isn’t fun. Remember the famous legal trademark dispute between the World Wrestling Federation and the World Wildlife Fund? The giant panda won the WWF fight, as WWE know.
Once I was scheduled to fly from Baltimore-Washington Airport to Miami for a large religious meeting. In preparation for that meeting (and to avoid sunburn), I wore a baseball hat that bore the large letters “NSA.” When I mistakenly ended up at the gate of Fort Meade, Maryland (right next to the airport), I got a strange look from the security guard. Fortunately, I still made my plane.

Undeclared abbreviations can cause confusion even in a narrow field. In connection with medical imagery, I have to inquire regularly whether the American College being called ACR is Radiology or Rheumatology. But my close encounter of the worst kind concerned two methods of solving differential equations, both called SDA: Strong Discontinuity Approach and Spectral Domain Approach. The name of the Russian mathematician Boris Galerkin is associated with both, which deepened the confusion.

“Super Donkey Kong” doesn’t sound so funny anymore.

I think we ought to avoid double meanings of abbreviations at least in the same field. To this aim, I offer the public service of pointing out a new spell-out of LCD that emerged at a recent solid-state-lighting committee meeting: the Light Code Designation (LCD) system for LEDs.  The term doesn’t seem to have reached Google yet, so there may be hope for liquid-crystal displays if we can head this one off at the pass.

By the way, in the title of this essay, TLA means “Three-letter abbreviation.” I haven’t even mentioned two-letter abbreviations (numerous PCs, nm as nanometers versus nautical miles) or four-letter abbreviations with multiple meanings (most notoriously the ISCC).

What are some amusing/confusing TLA’s in your field?

Michael H. Brill

[Note: When I began this essay, I incorrectly used “acronym” in place of “abbreviation.” An acronym is a very special abbreviation that spells a pronounceable word. For example, when NBS changed to NIST (rhymes with “mist”), it graduated to an acronym. NRC and NPL are still just abbreviations. MHB]

Tuesday, May 8, 2012

A Horsehead Nebula of a Different Color

Having recently undertaken to summarize some astrophysical topics for color scientists, I am confronting another language barrier between neighboring fields.

About ten years ago, astrophysicists Karl Glazebrook and Ivan Baldry introduced the world to “the color of the cosmos”---an average color of all the stars, corrected for red shift. I reviewed that work as it evolved in interaction with ISCC feedback [ISCC News # 397, May/June 2002]. The astrophysicists’ first answer, “turquoise,” changed quite a bit as the data conversion to color was refined. The final accepted color, obtained via a color-appearance model, was declared to be “beige” or “cosmic latté.” Some of us dissented: The answer “salmon” was consistent with the correct chromaticity, and the answer “black” could not be ruled out because, relative to a screen white, the average night sky is---after all---black.

The dialogue about cosmic color was instructive to everyone, partly because it revealed a difference of language and thinking between astrophysics and color science. Within the ISCC community, we already know some peculiarities of language---e.g., colors that are warm to artists have low color temperatures for scientists; and contrast in the display industry is defined to be greater than 1, whereas contrast in the paint industry is defined to be less than 1. These are familiar enough. But since we of the ISCC don’t often talk with astrophysicists, we may be unaware of some language stumbling blocks between these communities.

One example is “luminosity,” which to us means integrating a spectral power distribution weighted by a bell-shaped function that peaks at 555 nm.  However, in astrophysics, “luminosity” means the power integrated over the whole electromagnetic spectrum. Sometimes, astrophysicists prefix their “luminosity” with “bolometric,” and that removes the confusion. However, we still should watch out, as such niceties as a prefix are often lost in a technical discussion.

Another term, “brown dwarf,” refers to an infrared-radiating object that has too little mass to burn as a star (i.e., to sustain nuclear fusion). To color scientists, it is an oxymoron to call a self-luminous object brown. Some astrophysicists are aware of this problem. For example, Kenneth Brecher of Boston University, in a talk called “How Now, Brown Dwarfs,” referred to Joseph Silk’s objection that “brown is not a color.” (Well, actually brown is a color, but Silk had a point.) Brecher concluded that an isolated brown dwarf would look similar to a neon gas-discharge light.

Still another astrophysical term that will bemuse color scientists is the “green valley” of a galaxy color-magnitude diagram.  A color-magnitude diagram is a plot of galaxy color (actually difference between logarithms of light received through a blue and a violet filter) and luminosity (the bolometric kind, if you please). At the top of the diagram is the “red sequence” of galaxies, at the bottom is the “blue cloud” of galaxies, and in between is the “green valley”. In one sense the term should not confuse, because the “green valley” is a place with a conspicuous lack of galaxies, that lack being because there are no green black-body radiators. However, to name a thing for an absent attribute is a bit of a brain-boggler.

The galaxy color-magnitude diagram is an example of the use of color versus brightness to sleuth out the evolution of distant astronomical objects. An earlier such diagram, designed for individual stars rather than galaxies, is the Hertzsprung-Russell diagram, which has existed in various forms for about 100 years. The sleuthing process is quite intricate, given our limited perspective on the universe, and I am trying to learn more about it. For readers who want a reasonable introduction, I recommend the Wikipedia articles on the Hertzsprung-Russell diagram and the galaxy color-magnitude diagram.

As a final example of a word whose meaning becomes less certain in the astrophysical arena, consider how to render a “true” color. Together with such familiar issues as camera-to-tristimulus transformation, one wonders whether to correct the red shift of a distant receding object. If we do, then the object has the color we would have seen in its vicinity long ago; if not, then it’s the color we see now from afar. Fortunately that issue doesn’t affect near objects such as the Horsehead Nebula (a mere 1500 light-years away). Even so, Internet search reveals a variety of colors for the Horsehead Nebula. The colors may not be “true,” but are surely “different”---hence the title of this essay.

Michael H. Brill

Friday, March 9, 2012

Don’t Try This at Home

Are you impressed by the afterimage studies done by George Brindley [1] in which he stared at an automobile headlight to observe the effects of afterimages? Did you cringe upon discovering that Alfred Yarbus [2] invented a suction cap to attach optical apparatus to his own eye so as to retinally stabilize the images from that apparatus? Then imagine the following exploration done by Isaac Newton and documented in a recent book by Edward Dolnick [3]:

“To see whether the shape of the eyeball had anything to do with how we perceive color, Newton wedged a bodkin---essentially a blunt-ended nail file---under his own eyeball and pressed hard against his eye. ‘I took a bodkin & put it betwixt my eye & ye bone as neare to ye backside of my eye as I could,’ he wrote in his notebook, as if nothing could be more natural, ‘and pressing my eye with ye end of it…there appeared several darke and coloured circles.’ Relentlessly, he followed up his original experiment with one painful variation after another. What happened, he wondered, ‘when I continued to rub my eye with ye point of ye bodkin’? Did it make a difference ‘if I held my eye and ye bodkin still’? In his zeal to learn about light, Newton risked permanent darkness.” (pp. 48-49)

Newton’s adventurous spirit brought great reward. As Dolnick (p. 74) quotes from I. Bernard Cohen, Newton’s 1672 article in the Transactions of the Royal Society (reporting that white light contains all the colors of the spectrum) was “the first time that a major scientific discovery was announced in print in a periodical.” Previously, it was believed that publication would adulterate the personal benefit of having an idea.

Newton and his scientific contemporaries were adventurous in their ways of thinking as well as in their daring experiments. In the 17th century, it was adventurous to explore the world with experiment at all, because it was considered somewhat heretical to try to read God’s mind and to question the wisdom of the ancients. Dolnick’s articulate description of how “they were not like us” made me ask: What kind of thinking do we now consider living on the edge?

Some hints come from casual examples. A few writers of physics have the ambitious goal to change the very fabric of reality. That is what I call writing on (as opposed to reading) the mind of God. For example, Hermann Minkowski told the 80th Assembly of German Natural Scientists and Physicians in 1908: "Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality." Much more recently, two book titles also declared a rewriting of reality: How I Killed Pluto (by Mike Brown, 2010) and The Dreams that Stuff is Made Of (edited by Stephen Hawking, 2011).

There are trepidations about such hubris. After quoting Minkowski’s famous remark in an introductory chapter to a book on Einstein [4], the anonymous author says in the very next sentence, “Four months later, he [Minkowski] died very prematurely from appendicitis.”

I don’t think Minkowski suffered a divine reprisal, but does adopting his spirit mean we are kidding ourselves? I’ll leave it up to readers of this column to answer the question, “should we try this at home?” Meanwhile, I’ll continue trying to figure out why additivity of color matches doesn’t quite work.

Michael H. Brill

1. G. S. Brindley, Two new properties of foveal after-images and a photochemical hypothesis to explain them, J. Physiol. 164 (1962), 168-179.
2. A. Yarbus, Eye Movements and Vision. New York: Plenum Press, 1967. (Translated from Russian by Basil Haigh. Original Russian edition published in Moscow in 1965.)
3. E. Dolnick, The Clockwork Universe: Isaac Newton, the Royal Society, and the Birth of the Modern World. New York, HarperCollins, 2011.
4. Einstein’s 1912 Manuscript on the Special Theory of Relativity, George Braziller, Publishers in association with the Edmond J. Safra Philanthropic Foundation, Introduction: Provenance and Description of Einstein’s 1912 Manuscript on the Special Theory of Relativity, p. 18.

Friday, February 10, 2012

A Russian answer to Escher’s day/night print

In the last issue of ISCC News (# 454), Hugh Fairman reported on Henry Hemmendinger’s search for an M.C. Escher print that seemed to transform from a day-lit scene to a night scene when the illumination spectrum changed. Was this a deliberate trick from clever use of more than three colorants? As is clear from Hugh’s article, it was not, but before I learned that fact, I extrapolated Henry’s search far afield, to the heartland of Russia.

In the summer of 2008 I was in Tambov, teaching Russian college students how to use English to advance their science careers (see Issue # 435, p. 6). For three weeks I stayed in the same dorm room, and every morning and evening had the same view from the same chair of a picture on the wall. By day the picture was a sun-lit landscape with some water and green shrubbery in the foreground. By night (in tungsten light) the picture appeared instead to be moon-lit, partly because the sky around the sun/moon orb was darker, but mostly because the green of the shrubbery appeared relatively lighter. The tungsten light was evidently rendering the blue sky darkly, but was raising the lightness of the shrubbery as if to mimic the Purkinje shift (shift to rod dominance in low light levels---hence greater lightness of green). Of course, the Purkinje shift here was illusory and not real, because the light was still bright enough to render colors: my cones still ruled the night.

I thought this might be an example of the art object Henry sought that conveyed two scenes in two lights due to colorant manipulation. I considered trying to purchase the picture, but even if I got a fair deal on it the trip home would not be easy. As I sat in that same chair one evening, I thought I’d have a closer look at the picture before I made my purchase offer. So I rose from the chair. Instantly the green shrubbery darkened.

Oops! This wasn’t related to metamerism at all. The shrubbery was brighter when I sat in the chair because I was receiving a specular reflection from the tungsten light. In daylight, I didn’t get a specular reflection, so that is why the shrubbery looked darker by day.

I had to marvel at this picture, which had different gloss in different areas. The shrubbery had the greatest gloss. Do reproductions in Russian dorm rooms have such texture and gloss differentiation, or was I looking at an original painting? Later I learned that, in printing, ink over-loading (hence gloss) is more likely in the green (and purple) than in other colors because more than one colorant maximizes its load. But meanwhile, I had reached the end of my skills as an art connoisseur, and the end of my time as well---I had to return home the next day.

Was this day/night trick deliberately set up? Perhaps not. Was it related to metamerism? Definitely not, unless you count the much-disparaged term, “geometric metamerism.” Alas, I couldn’t discuss the matter with Henry Hemmendinger, who by that time had gone where colors are more real and permanent.

Michael H. Brill