COLOUR CONSTANCY

Colour constancy is the feature of our visual system that allows us to recognize the local colour of objects under light of differing hue, brightness, and saturation. In normal or related viewing, our visual system seems to make a judgement as to what a white surface would look like in any part of the visual field. A colours that is too bright, compared to this inferred white surface, to be the result of reflection is seen in illuminant mode, that is, as an independent light. Other colours, whose brightness is judged in the same way to be consistent with their being surfaces reflecting light, are seen in surface mode, i.e. as surfaces having a local colour that is judged by comparison with the inferred white.

Several dramatic optical illusions demonstrate colour constancy in action. In the checkerboard illusion by Edward Adelson's (Figure 3.5A), the two squares marked A and B are actually identical in lightness on the image, but our visual system calculates that in a shadow area this grey must belong to a white surface, while in the lit area the same grey must belong to a black surface, and that is how we see them. In the same way, in the cube illusion by R. Beau Lotto (Figure 3.5B), our visual system sees the same image colour as being dark brown in the context of strong lighting, and light orange where the same image colour appears in a deeply shaded context. In the cross-piece illusion , also by Lotto (Figure 3.5C), the colour at the intersection of the two rods is actually an identical grey in both cases, but in the context of apparently yellow illumination on the left and blue illumination on the right, this is judged, and seen, to be the reflectance of a blue-grey object and a yellow object respectively.

Figure 3.5. Three optical illusions demonstrating colour constancy in action (follow links for larger images). A. The checkerboard illusion of Edward Adelson. B. The cube illusion of R. Beau Lotto. C. The cross-piece illusion of R . Beau Lotto

In each case these comparisons are made unconsciously, and what we see in our normal way of looking is the inferred local colour. Tonal painters have to learn above all to look at their subjects with a different attitude to normal viewing, in order to judge objectively the hue, "colorfulness", and brightness of the light coming to their eyes from each point in the subject, in relation to the full range of subject as a whole.

At this point the beginning painter might ask: "well, if that's the way it looks to my eyes, shouldn't I paint it that way?" The answer to this is a definite no - if we can recreate the stimulus that created the appearance, we will create the effect the we are after; if we instead chase the appearance we will create something different.

Given practice we can learn to switch at will between this painter's way of seeing and our normal mode of vision. But we always need to be on guard against the tendency to slip into judging colours in constancy mode, that is, to paint their perceived local colour, instead of the colour that we need to create the illusion of that colour. The problem is very similar to the difficulties encountered in foreshortening in drawing, where we need to learn to see and draw what is actually in front of our eyes, and not what our brain works out for us.

Certain mechanical tricks or devices that are sometimes recommended to students for observing colour objectively can be workable, but most have serious difficulties and limitations. For example, the idea that you can hold up paint on a brush, palette knife or other device and match it with your subject is in general workable only if you have some way of turning up the illumination on your brush until you can match the brightest highlight on your subject with the tone of your paint, and can keep the illumination at the same level while you compare the other colours. (One teacher currently advertising such a method on the internet seems to get around this problem by having his students paint only dimly lit subjects). These methods of course also eliminate any option of translating the tonal range of the subject into a your own choice of tonal level and range in your painting. Devices involving an aperture in a card that bears a greyscale or colour chips for comparison suffer from the same difficulties and limitations, and in addition run the risk of giving an excessive impression of the brightness and "colorfulness" of colours seen in isolation, which can be avoided only if the colours are continually compared with the brightest colours in the subject. The latter comparison can be made very effectively however by using a blank card with two apertures, which can be moved towards or away from the observer in order to compare more and less separated points.

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