- Simultaneous contrast is a visual perception phenomenon.
- The visual system is composed by mechanical, optical, chemical and electrical subsets.
- The evolution of the visual system didn’t focus on precision, but rather on usefulness.
- For survival it is much more important to distinguish (or separate) a lion in the middle of savannah, than it is to be able to appreciate two identical yellows.
- This behaviour, though, introduce a relativism in our color perception.
- This relativism can, often, be used to improve our images in photographic post-production.
Should a colorist even learn about the visual system?
Learning color correction, I will never stop saying this, should be a practical experience. For the luckiest, we could say practical as well.
Some topics sure are necessary, for example what is the best approach for a successful backup for a photographer. Some theories, like the zonal system, are interesting, from an historic point of view as well. But the visual system, and the visual perception, should probably be learned first. I’m using “probably”, because in these years I saw users often unable to contain the enthusiasm, with the risk of overdoing what should be a subtle interpretation.
I researched many visual perception phenomena (crispening, spreading, Bezold-Brücke, Abney, Helmholtz–Kohlrausch, color constance, Hunt, Stevens, Helson-Judd, Bartleson-Breneman and others), and even if all of them are interesting on paper, just few are actually usable in color correction. Simultaneous contrast is interesting and it is useful as well. I will write about all of these as well at a later time.
Challanges in learning about visual perception
Charles Darwin wasn’t able to reconcile the evolution of the eye given its complexities and diversity of eye designs.
So it was a major surprise for us that we have found what appears to be a clear progression from a simple eye to a complex eye, which occurred over a relatively short period (30 million years) in evolutionary history.
Professor Shaun Collin, from UQ’s School of Biomedical Sciences
When we study the visual system, we do not learn just about the eyes, but a complex combination of eyes and brain (and even this is blatantly oversimplify). Our eyes gather data and send the information to the brain where they are interpreted, and manipulated with a purpose that is driven and refined by our species evolution.
We can quantify simultaneous contrast only through comparison
In our everyday color perception the cognitive processes are extremely important. We should repeat now that we are talking about visual perception and not how we measure color, because we are absolutely unable to.
New evidence suggests that the human visual system incorporates
a high-level, functionally specialised system for monitoring animals. Such a mechanism may have evolved to direct attention differentially to ancestrally important categories of objects, regardless of their current relevance.
Geraint Rees – Vision: The Evolution of Change Detection
The simultaneous contrast
The fact that the after-image or simultaneous contrast is a psycho-physiological phenomenon should prove that no normal eye, not even the most trained one, is foolproof against color deception. He who claims to see color independent of their illusionary changes fools only himself, and no one else.
Interaction of Color, Josef Albers 1963
Simultaneous contrast is a phenomenon that happens when two adjacent colors influence each other, changing our perception of these colors (more or less saturated, more or less bright). It can be observed both with different hues, or luminosities.
Also, and this is a very important point, this is a phenomenon that can be evaluated only with comparison.
Something that is true for all perception phenomena. We are not able to say, or define, how much an item may be cold or how much a color may be saturated. We can only say that something is colder than something else, or more saturated than.
And this is where an excess of relativism can be dangerous. Let’s say we show an image that we color corrected to someone that does not work with color or photography.
This observer, doesn’t like the photo and tells us that the subject, a car, is oversaturated. How should we react to this feedback?
The most prominent among the phenomena of interaction is, of course, color contrast. The principle received its classical formulation by Michel Eugene Chevreul, the French chemist and director of the Gobelin tapestry works. He described simultaneous contrast as follows: ”If one views at the same time two areas of different brightness but of the same hue, or of the same brightness but of different hue, in juxtaposition, i.e., bordering on each other, the eye will observe (provided the areas are not too large) modifications that bear in the first case on the intensity of the color and in the second on the optical composition of the two juxtaposed colors.”
Since the effect of color contrast operates in the direction of physiological complementarity, it serves to heighten it where it already exists, e.g., in the relation between blue and yellow, or to modify colors in the direction of such complementarity if they are reasonably close to it. Von Allesch experimented with a greenish yellow and a reddish yellow whose admixtures were so slight that when inspected separately both colors looked like pure yellows. Brought together they tended to emphasize their distinctness, looking dearly greenish and reddish and presumably producing the kind of clash already discussed as the effect of "Similarity of the Dominant." But if a third yellow of intermediate hue was placed between the two, the contrast diminished and the total arrangement showed a more unified yellow. Such effects of assimilation are also observed when, for example, one strongly red patch in a painting brings out subtly red components in the colors around it.
Art and visual perception, Rudolf Arnheim, 1954
Back to the feedback that our observer gave us. First it is an opinion, and it has a value. We will start from who gave the opinion to us, someone that is not a professional. To this person, though, we didn't ask a feedback on sharpening or local contrast, both elements that requires an education in color.
That opinion is simple, and on a very common object, a car, that everyone see on daily basis. Let’s not hide behind the usual “I wanted this image exactly like this”, or “I understand, but there is a simultaneous contrast effect that you do not know about”. A good correction shouldn’t be explained.
To a colorist, every opinion, each feedback, is precious. We need to learn to listen, to be able to verify, to make choices and to achieve results. To listen often means to translate feedbacks from an obscure language (this image is too loud, or I think it should be more crunchy), to verify with the tools we know, and to apply corrections if needed.
And this is even more important if we are dealing with a photographer.
The appearance of a colour can be greatly affected by the presence of other colours around it; this is termed simultaneous contrast (or chromatic induction). Simultaneous contrast can result in large changes in the appearance of colours in items such as woven fabrics and tapestries. The French chemist Michel Eugene Chevreul, as director of Gobelin, the famous carpet manufacturer, was one of the first to investigate the phenomenon; in 1839 he introduced his law: “two adjacent colours, when seen by the eye, will appear as dissimilar as possible”. Chevreul, 1839.
In Figure 14.3 the effect of simultaneous contrast on lightness is demonstrated. It is well known that a dark surround makes a colour look lighter, and a light surround makes it look darker. But it is also true that a dark surround lowers apparent contrast, and this is evident in the right-hand part of the figure. This contrast-lowering effect occurs quite strongly when pictures are projected in cinemas, and has to be countered by increasing the contrast of the picture being projected.
Hunt – Measuring Color
A practical example
To sum it up, simultaneous contrast is a visual perception phenomenon. The term already suggest a relativism. Our body, including the visual system, evolved to perceive the differences, especially when these became very subtle. The best way to achieve this was to introduce, in ourselves, a subjectivity. But let’s try also to not be limited by this, let’s give a clear foolproof example. We can see boxes of squares and colors in many places of the web, but I will use something from Josef Albers’ book that is even better.
The author asks his student to take three buckets of water. The first one contains cold water, the third one hot water. The middle one contains half of cold and half of hot water. He then asks to a student to place one hand in the cold water, and one hand in the hot. It is a clear to the student what is being perceived, one hand is feeling cold, one hot.
At this point the student is asked to remove both hands, and put them in the middle bucket. And here we can understand easily the whole issue. The hand that was before in the cold water is now feeling warmer, but at the same time the hand that was in the hot water is perceiving cold.
This simultaneous perception of different quantities from the same entity shines as an example of how our body works. Hot, cold, are only words that we use to express something that we cannot measure, but that at the same time has a deep meaning for us.