Tuesday, May 17, 2022

The Oldest Hack in Color Engineering

  (Send contributions to mbrill@datacolor.com)

 

When I was at MIT, I learned what it meant to call something “a hack.” The term referred to a patch in software (or hardware) that was ad hoc but clever and fixed a problem. There are many other definitions of “hack” (the noun), but I don’t use those here. To me, “hack” is not necessarily pejorative, but descriptive.

 

In color engineering there have been many hacks, originating even before the term “color engineering” was popular. Two rather good ones are Cal McCamy’s approximation for correlated color temperature of a light given its chromaticity [1] and the CIELAB L* function as an approximate inversion of a fifth-degree polynomial that characterized the Munsell Renotation System lightness scale [2].

 

Just for fun, I now ask you: What is the oldest hack in color engineering? One could nominate Newton’s representation of color as a closed circle. Newton must have been aware that the purples are not elementary colors that he could see with his prisms. But Newton and others may have adopted the circle as an instructive idealization, and that is not a hack.

 

To get the ball rolling (I expect hundreds of reader responses), I nominate the object-color tristimulus value (X, Y or Z, for any illuminant and observer you like). This quantity is defined as a specific ratio (see ASTM E-308, any edition). The numerator is the wavelength integral of the product of illuminant power density, reflectance and color-matching function (generally x-bar, y-bar or z-bar). The denominator is the wavelength integral of the product of the illuminant power density and the function y-bar. There’s also a factor of 100, but that doesn’t matter.

 

What problem does this definition cleverly solve? In a single stroke, it renders the object-color tristimulus value dimensionless and independent of the absolute light intensity, as is its cousin – the emissive-mode tristimulus value. (This latter fact emerges from the grounding of emissive-mode tristimulus values on the emissive-mode color-matching experiments that underlie basic colorimetry. In a short paper I just submitted to Color Research and Application, I explain how the titration in a color match leads to cancelation of all dimensions.) Creating the object-mode ratio makes the tristimulus values from reflected lights appear comparable to the values from emitted lights and emissive displays. Certainly color-matching equivalence classes of reflected lights are not disturbed by the ratio. But, as many color-management experts have warned, we should be careful in asserting exact colorimetric color reproduction between emissive and reflective media. For one thing, only the reflective tristimulus space has an unambiguous white point.

 

How is the object-color tristimulus value a hack? It is ad hoc, solves the units problem, and is clever enough to survive generations of standards bodies like the ASTM and the CIE. The ad hoc quality manifests when we observe that the object-color tristimulus values have a distinct and asymmetric dependence on Y wrought by the denominator, yet that asymmetry was not based on color matching (as should befit a true tristimulus value). Furthermore, as the current object-color tristimulus value is a ratio between two spectrum integrals, it will have a weird illuminant-invariance such as I have found for Von Kries adapted tristimulus values [3] and band ratios [4]. If the illuminant is restricted to a finite linear function space, changing its coefficients will not alter the object-color tristimulus value if the reflectance is outside a forbidden function subspace. There’s no room for such games in the emissive-mode tristimulus values.

 

So that is why I consider the object-color tristimulus value as a hack. My only remaining question is, how old is it? Wyszecki and Stiles [5] point to the dawn of the 1931 CIE system of colorimetry. I have not investigated further, but it’s entirely possible that this hack precedes not only the term “color engineering,” but also the name “tristimulus value” itself. I’ll leave that subject to serious historians in our ranks.

 

[1] McCamy CS, Color Res Appl 17 (1992), 142-144 (with erratum in Color Res Appl 18 (1993), 150.

[2] Newhall SM, Nickerson D and Judd DB, Final report of the OSA subcommittee on spacing of the Munsell colors, J Opt Soc Am 33 (1943), 385-418.

[3] Brill MH, Minimal Von-Kries illuminant invariance, Color Res Appl 33 (2008), 320-323.

[4] Brill MH, "Can color-space transformation improve color computations other than von Kries?" in:  Human Vision, Visual Processing, and Digital Display IV, J. P. Allebach and B. E. Rogowitz, Editors, Proc. SPIE 1913,485-494 (1993).

[5] Wyszecki G and Stiles WS, Color Science, 1st ed. New York: Wiley, 1967, p. 279.

 

 

Michael H. Brill

Datacolor

 

 


Wednesday, March 2, 2022

My Big Win in Vegas!

  (Send contributions to mbrill@datacolor.com)

When I find myself in a casino, it’s always because of a professional meeting. My reaction to these casino visits is perhaps predictable.

 

In 1979, I attended the Illuminating Engineering Society meeting in Atlantic City. There I dropped a coin in a slot machine, and then became so absorbed in a conversation with Bill Thornton that I was oblivious when a red carpet was unrolled behind me and Sammy Davis, Jr. walked from one end to the other.

 

In 1985, I attended the quadrennial AIC meeting in Monte Carlo. There I sacrificed another coin to a one-armed bandit, and then became so absorbed in a conversation with Claude Pelissier that I didn’t see the sights all around me on the “topless” beach.

 

In 1992, a field trip after an Acoustical Society meeting in Salt Lake City lodged me at an inexpensive casino/hotel in Mesquite, NV. A somewhat larger sacrifice went to a slot machine there. I was so impressed by the casino’s use of light and color to disorient patrons so they would gamble, that I presented a summary of the ruse at an ISCC Interest Group III panel discussion (see ISCC News Issue 340). Nobody told me what glamorous opportunity I missed in Mesquite, but I must have missed something because I got lost on the way back to my table from the restaurant salad bar due to the evil genius of the casino architects.

 

In 2005, I returned to Nevada, this time to an ASTM E12 meeting in Reno. By now my obligatory slot sacrifice swelled to five dollars. I’m reasonably sure I didn’t miss any glamorous sights, and I was aware that, in all my experiences with casinos, I hadn’t retrieved a penny from the slot machines.

 

But then, in September 2021, I visited Las Vegas and had my Big Win. The meeting this time (American Urology Association) was not mine but my partner Karen’s. After she made travel arrangements with me in tow, the AUA thought it prudent to make the meeting virtual. Karen was determined not to change our plans, so we checked into the Venetian, the conference hotel. (It was on the opposite end of the price scale from the hotel I’d stayed at in Mesquite in 1992.) Karen attended the virtual meeting from our hotel room. To me it was a bit eerie, like a séance with ghosts from an alternate universe. Meanwhile, I explored the hotel/casino complex. True to my experience in Mesquite, I became repeatedly lost on a grand scale. One small but effective confusion was to call two of the three hotels in the complex “Venetian” and “Venezia.” The architecture and a sinuous indoor canal spoke “Venice,” but straight paths and right angles were rare, and the site map had little visual correspondence to the site itself. Even employees of some of the concessions could not describe how to get to other places under the same roof.

 

Later, we embarked on an evening tour on a double-decker bus, and I had an opportunity to meet my obligatory slot machine. The tour guide gave us an hour in the environs of the Golden Nugget Casino. Stepping inside, I found that the only acceptable way to sit down for an hour was to gamble. Karen slid a $20 bill into a machine, and we managed to keep busy for more than a half hour. Her goal was to get as much run-time as possible out of that investment. She found that strategy was great fun when she visited Las Vegas with her mother several years earlier. When I slid my $20 bill into the same machine, I had a different goal. After a while I had won almost ten dollars, whereupon I decided to cash out. A little slip of paper appeared, and I grabbed it and asked the nearest bystanders where I could find the cashier prior to the imminent departure of our bus. Peering with cow-like eyes, they told me they didn’t know. How could anyone gamble without a plan for even the first two minutes that would ensue if they actually won? It was sad.

 

Out of time, I returned to the bus and read the paper slip from the machine: Void after 30 days! I’d have to come back to the Golden Nugget later in my trip. But I never did.

 

So that was how I scored my Big Win in Vegas. Well, maybe a Bigger Win was to have made the trip despite some significant health issues and with the overarching Covid risk. Some people thought I was crazy to do it. Maybe it’s good to indulge such craziness just once amid the larger-than-life habitués of Vegas.

 

Michael H. Brill

Datacolor