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The Painting Lesson

by Linda Carson

big black pig studio
98 King St. N., Waterloo Ontario Canada
www.bigblackpig.com


Contrast & visual illusions

Vision scientists and artists all know that what we see is affected by the context in which we see it. A red rose looks lighter, brighter somehow, when we see it against a black background...

Red rose at night; acrylic/collage on hardboard;
6 inches X 6 inches; Carson 2002

...than it does when we see the exact same rose against a white background...

Red rose on snow; acrylic/collage on hardboard;
6 inches X 6 inches; Carson 2002

...but it's exactly the same colour in both images. (In fact, it's an adhesive decal, factory-produced. I didn't believe I could paint the rose twice in precisely the same size, same shape and same colour.)

Our brain's visual system does a lot of fine-tuning to enhance our ability to pick out objects from the background and spot patterns in what we see. As a result, how we see hue, value, saturation and the edges of objects is context-dependent.

Some visual illusions reveal this fine-tuning. For instance, lateral inhibition is a mechanism of the visual system which boosts contrast at the edges of shapes. This usually just makes it easier to see shapes in dim or hazy conditions. However, if you look at these flat stripes below, you may also notice that some of them seem to lighten up along the edge where they meet a dark stripe. They don't really. This image—called Mach banding—is a special case where we notice the visual system over-emphasizing the edge a bit.

Mach banding

Should painters care? The purple stripes on the flag below are all the same colour. However, to most human eyes, they look different because of the surrounding black and gray stripes. That's the sort of thing painters definitely care about. (Try this: roll your mouse over the picture to see the same purple stripes with a plain black background. Now they look alike, don't they?)

Some of these visual effects have names. French physicist Michel-Eugene Chevreul first identified this general set of experiences in his influential model of colour theory (1839). He observed that two colours seen side-by-side will appear as different as possible, and called that the law of simultaneous contrast. A light colour looks even lighter when adjacent to a dark colour. A yellow colour looks even more yellow when adjacent to its complementary colour, purple. Successive contrast describes the visual effects, and aftereffects, of the eye adapting to the colours it sees. Successive contrast makes the area immediately surrounding a colour gradually take on the cast of that colour's complement. For instance, a neutral gray mat around a yellow painting will take on a faint purple hue after you look at it for a while. This effect also accounts for visual illusions of afterimages. I try not to get too hung up on the names. I've seen both of these effects called simultaneous contrast in different sources. I've also seen at least one source refer to extended contrast, a variant of simultaneous contrast.

Sometimes the light/dark contrast effect is called irradiation—it makes colours look darker against a light background than they do against a dark background. A shape entirely surrounded by a light colour looks smaller than the same shape surrounded by dark. Looking at the picture below, most people think the rose on the white background looks smaller than the one on the black background.

Roses; acrylic/collage on hardboard; two panels, each 6 inches X 6 inches; Carson 2002

There will be no Contrast Quiz. I'm not advising you to memorize the different types of contrast. In fact, I'm beginning to think that there are some confused—and confusing—definitions out there, depending on where you look. The important thing to remember is:

Colour is context-dependent.

It's not enough for the colour to look right in the store, in the tube, or on the brush. It needs to look right, in context, on the painting.

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Art & Text (C) Linda Carson 2002

Loosely translated, that means:
"Please don't copy this material or redistribute it in some other form, for any reason. This is my livelihood."