Our visual system does more than provide us with an image of the world. An image may contain an array of light and dark areas, but which of these areas represent shadows, and which represent dark-coloured objects? Does a red area in the image represent a red object in white light, or a white object in red light? Is a particularly bright point in the image a light-coloured surface, or a light source? To separate the effects of lighting and surface colour, the image must be interpreted. Since we normally experience objects as having definite colours without having to make a conscious effort of interpretation, it follows that this interpretation must be occurring subconsciously as part of the processing of the image by our visual system. This feature of our visual system allows us to recognize without conscious effort the local colour of objects under lights of differing hue, brightness, and saturation, a capability referred to as colour constancy.

Colour constancy is never more than partial. Under weakly to moderately strongly coloured illuminants, an array of surface colours may approximately maintain their perceived hues, but changes in apparent lightness in relation to each other are inevitable (see Figure 10.8, slider near middle of scale). Colours close to the hue of the illuminant tend to appear relatively lighter; dissimilar hues tend to appear darker. The effect however depends on the exact spectral power distributions both of the illuminants and of the surfaces. Two surfaces that match under a white light may not match under a coloured light, or even under another white light with a different spectral power distribution, a phenomenon known as metamerism. These factors naturally play havoc with the tonal scheme of a painting, which is why paintings should if possible always be executed under similar lighting conditions to those under which they will be viewed.

Adaptation to the colour of a light source, which modifies the data provided by the eye by adjusting the relative sensitivities of the three cone types, should not be confused with this process of interpreting that data into illuminant and surface colour. Adaptation to the colour of weakly to moderately strongly coloured light sources assists the process of interpreting surface colour by boosting the available information on colour differences, while causing us to underestimate the strength of colour of the illuminant. Surfaces under a monochromatic light, however, can convey no information on differences in hue and chroma to the eye, and so differ visibly only in lightness (see Figure 10.8, slider at right end of scale).

Given its ability to automatically interpret surface colour for us, we can infer that our visual system must in some way be making a judgement as to what a white surface would look like in any part of the visual field, and judging all colours by comparison with this white. A surfaces giving off light whose brightness is consistent with being the result of diffuse reflection is seen thus as coloured surface having a greater or lesser degree of "greyness". A colour that is too bright, compared to this inferred white surface, to be the result of diffuse reflection is seen either as a fluorescent surface, an independent light, or a specular reflection of an independent light. Evans (1974) suggested the term brilliance for this scale of appearance, and used the term zero grey point for the point where colours exhibit neither greyness nor fluorescence/luminescence. I argue here that in a digital image, all colours having the same relative brightness ("B" in HSB) as a white surface at the same location will be seen as having zero greyness.

Several dramatic optical illusions demonstrate colour constancy in action. In the checkerboard illusion by Edward Adelson's (Figure 3.6A), the two squares marked A and B are actually identical in lightness on the image (i.e. they are the same grey), 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 dark surface, and that is how we see them. In the same way, in the cube illusion by R. Beau Lotto (Figure 3.6B), 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 3.6C), the colour at the intersection of the two rods is actually an identical colour (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 3.6. 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 from each point in the subject. That is, we have to switch off one kind of processing - one that is built into our visual system, wherein each colour is compared with an inferred white - and learn a completely different kind of processing, where each colour is compared with the full range of colours in subject as a whole. With 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 also need to learn to see and draw what is actually in front of our eyes, and not what our brain works out for us.

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 see in our subject; if we instead chase the appearance, we will create something different.

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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