PART 1. INTRODUCTION

1.1 COLOURS IN SPACE - Three introductory essays

Hue, value and chroma

All painters, whether working in traditional or digital media, are in a real sense navigators in space. Whether they are aware of it or not, each touch of colour they apply can be considered, using various systems, as a point within a space defined by three dimensions.
  

Figure 1.1.1. Left: Portrait of Vincent Van Gogh by Henri Toulouse Lautrec. Pastel, 1887. Right: RGB colours from this image, plotted in YCbCr colour space using the program ColorSpace by Philippe Colantoni. (www.couleur.org).

The system most generally familiar to painters has dimensions of hue, value and chroma. Hue refers to the circular scale of "pure" or “saturated” colours formed by the colours seen in the spectrum (red, orange, yellow, green, cyan, blue and violet), together with the non-spectral colours like magenta, seen when the two ends of the spectrum are mixed. Value (also known as tone, greyscale value, and lightness) refers to the scale from black to white. Chroma or colour strength refers to the amount of visual difference from a grey of the same value. In short then, the system may be said to classify any given colour according to the closest full or “saturated” colour, the closest grey, and the visual difference from that grey (Fig. 1.1.2).

Figure 1.1.2. Explanation of Munsell dimensions of hue, value and chroma. Grey scale and 10YR hue page from the Munsell Book of Color (glossy edition).

This three-dimensional classification and its key concept of chroma were devised by the American artist and art teacher, Albert Munsell (1858-1918), in a booklet entitled A Color Notation (Munsell, 1905). Munsell then published quantitative scales of hue, value and chroma in a small Atlas (Munsell, 1907, 1915), which after his death was elaborated by the Munsell Color Company (directed by his son Alex) as The Munsell Book of Colour (1929). This was again revised in 1943 following thorough testing by the US Bureau of Standards, and this 1943 "renotation" forms the basis of all subsequent editions. The current edition has 40 hue pages (Fig. 1.1.3), and is available in a choice of editions having either matt or glossy colour chips.

Figure 1.1.3. The forty hue pages of a modern edition of the Munsell Book of Color (glossy version). Click on each hue page to enlarge, and scroll down for more pages. In the matt version the range of colours is generally a little greater among the light colours and a little less among the dark colours.

Because the maximum attainable chroma varies with value, and varies in a different way for each hue, Munsell hue pages form an irregular, tree-like space when arranged radially around the central value axis (Fig. 1.1.4A). The specific, quantitative scales of Munsell hue, Munsell value and Munsell chroma will be discussed in more detail later.

The dimensions of value and chroma apply specifically to colours perceived as qualities of objects, as opposed to lights. Colours perceived as qualities of lights (Fig. 1.1.4B) can be described in terms of three dimensions of hue, brightness (perceived amount of light), and either saturation (colour purity, i.e. perceived freedom from admixed white light) or "colorfulness" (colour strength, a function of both brightness and saturation; see Fairchild, 2004, 2005).

Figure 1.1.4. Fundamental dimensions for colours perceived as properties of (A) objects and (B) independent light. Saturation refers to purity of colour of light, and can vary throughout its range (white to monochromatic) at any level of brightness; it is represented in B by the angle from the neutral axis. "Colorfulness" refers to strength of colour of light, and can be thought of as saturation times brightness; it is represented in B by the distance from the neutral axis. Chroma (strength of colour of objects) depends on the "colorfulness" (saturation and brightness) of the light given off by an object for a given level of illumination. Chroma is necessarily zero at maximum and minimum lightness (white and black respectively), and reaches its maximum range at intermediate lightness levels.

Value and chroma apply to object colours seen in nature, or depicted in an image, or making up an image, as long as these are viewed in such a way (called related viewing) that they perceived as colours of objects and not light. This is true whether the image surface reflects light (e.g. a photograph, painting, or projector screen), transmits light (e.g. a stained glass window) or emits light (e.g. a computer monitor or TV screen). However, all object colours can also be seen as light if viewed in a manner that is called unrelated, for example through a small aperture. This again is true irrespective of whether the object reflects, transmits, or actually emits light. The dimensions of brightness, saturation, and "colorfulness" therefore can apply not only to independent light sources, but also to object colours viewed as light.

Painting colours in space

Any painter familiar with the colour wheel and tonal scale of traditional "colour theory" would surely be aware to some extent of the spatial aspect of their activities, but many do not seem to derive much advantage from this awareness. Such painters may for example vaguely observe that a given colour needs to be "warmer" or "cooler", without troubling to apply the more precise concepts of hue and chroma. Many think of colour mixing as applying colour "recipes", obtained either secondhand from art instruction books, or from their own elaborate mixing charts showing how they mixed a particular colour previously. Typically, painters of this sort have little knowledge of the physical principles involved in creating effects of light and shade, and often rely for such effects on crude and inaccurate formulae, such as to "get the shadow colour by adding the complementary colour", and so on. A hallmark of this uninformed approach to colour mixing is the traditional admonition "Don't use black!". The real problem is not black paint, but the painter's inability to visualize any unintended effect of adding black paint as an easily corrected shift within colour space.

In contrast, other painters find it invaluable to think consciously of their colouring activities as maneuvering through a three-dimensional colour space. Most think of this space in terms of relative hue, value and chroma, but there are also many who in effect train themselves to think in terms of absolute scales of these dimensions, such as those of the Munsell Book of Colour (q.v. Graydon Parrish and Steve Linberg's Classical Lab). The glossy version of the Munsell "big book" is favoured among oil painters because paint mixtures can be tested on the individual removable colour chips and then wiped off safely. Painters who stop short of going the full Munsell often find it very helpful to at least think of value in terms of an absolute scale of some kind.

A spatial conception of colour assists painters by providing a framework for observing colour relationships, for selecting and mixing colours, and for creating colour relationships from the imagination.

1. As a framework for observing colour relationships.

Most painters of this latter kind do not try to copy each colour in their subject in isolation (the strategy of every beginner). Instead, they use the concept of colour space as a frame of reference for grasping the relationship of each colour to the totality of colours present. Tonal realist painters, for example, typically observe colour relationships in the light from their subject, and then, by a process of either conscious or unconscious translation, identify each individual colour in terms of the hue, value and chroma of the paint colour they will need to use in order that the whole ensemble replicates the visual appearance of the subject as closely as possible. In practice, this usually involves first selecting the most important (say) six to ten colours in the subject, and finding the place of these in relation to each other (Fig. 1.1.5). This begins the process of building what I call a scaffolding for progressively finding the place of all remaining colours, most of which can usually be considered as variations on, or intermediates between, these scaffolding colours.

Figure 1.1.5. : Left: Lyndall by David Briggs, 2005, oil on canvas. Right: plan view (above)
and side view (below) of ten selected colours from the image plotted in YCbCr space using
the programme ColorSpace by Philippe Colantoni. (Note that, as in most illustrations on this
site, the CbCr plane is shown in reverse to its standard orientation, to place the spectral
sequence of colours in clockwise order, following Newton, Munsell, and the hue circle in HSB
colour space, among others).

2. As a framework for selecting and mixing colour.

Artists who think in terms of colour space do not need to remember recipes for mixing colours: they understand that most colours can be mixed from any number of combinations of paints, as long as the target colour is within the three-dimensional gamut of those paints. They literally visualize colour mixing as moving colour from place to place through colour space. They decide on the changes in hue, chroma and value required, and predict in advance what effect various additions are are likely to have. These crafty painters often, for example, premix a pool of colour on the other side of a target colour, and add this in stages to draw the colour methodically towards its target (Fig. 1.1.6).

Figure 1.1.6. A very simple example of the use of the concept of colour space in colour mixing. The target colour (B) is observed to have higher lightness and lower chroma than the starting colour (A). Adding white to A is expected to make both of these changes, but when tried is found to produce mixtures that are still too high in chroma when they reach the lightness of B. Pre-mixing a pool of light grey at around the lightness of C, and adding this in stages to A, should bring the mixture closer to the target.

3. As a framework for creating colour relationships from the imagination.

The dimensions of colour form an essential conceptual framework for any kind of activity that involves creating colour relationships from the imagination. In the nineteenth and early twentieth centuries, much thought on colour spaces (including Munsell's own writings) was directed towards discovering rules of "colour harmony", and there are still many echoes of this kind of investigation today. On this site however I am much more concerned with the application of colour space to the creation of convincing effects of light. The concept of colour space provides an essential quantitative framework for applying the simple physical laws that govern the behaviour of light and colour. If the artist gets these seemongly technical details right in a painting, the payoff can be a vivid glow of light. And, just as with, for example, perspective and anatomy, having the understanding that allows you to do something from the imagination makes working from nature far more efficient.

Figure 1.1.7. Imaginary sphere under three imaginary light sources, painted as three layers in screen mode (one for each light source). David Briggs, 2007, Photoshop CS2.

The dimensions of what, exactly?

In Book VII of The Republic, Plato tells that Socrates likened "the world of sight" to shadows cast on the wall of a cave by things we are unable to see directly. The comparison forms part of a larger political argument, and was not intended to apply specifically to colour vision; nevertheless it has turned out to be a very apt analogy for colour perception. It was Isaac Newton who revealed the identity of these unseen "things" we experience as colour: for a light it is the balance of spectral components or "rays" (today we would say wavelengths) present, and for an object it is the balance of spectral components that the object is disposed to reflect. The shadow analogy is made all the more apt by another of Newton's discoveries, the feature of our colour vision known as metamerism: just as different things can cast the same shadow shape, different combinations of spectral components can be indistingusihable to human vision, and are seen as exactly the same colour. By the end of the eighteenth century, observers like Monge and Rumford had established the converse: just as the same thing can be made to cast different shadow shapes, the same spectral distribution can be made to appear different colours. A colour is not the same thing as a spectral distribution, any more than the shadows in Plato's cave are the same as the things that cast them: a colour is the response of a visual system to a spectral distribution. But, just like the inhabitants of Plato's cave, we accept our colour perceptions as objective reality, so much so that we can become quite unsettled when they are changed by an "optical illusion", or when another person stubbornly sees an object as the "wrong" colour.

Newton explicitly upheld the distinction between physical stimulus and mental perception, arguing that, just as vibrations exist in air but sound exists only in the mind, "the Rays to speak properly are not coloured. In them there is nothing else than a certain Power and Disposition to stir up a Sensation of this or that Colour" (Newton, Opticks, Book 1). Nevertheless, scientists from Newton to Helmholtz assumed a quite direct relationship between the two (as in fact exists between vibrations and sound), and even today one still encounters the notion that colour vision involves of mixtures of red, green and blue/violet sensations arising directly from three types of cone cells that detect rays of those colours. However according to the modern zone model of colour vision, each cone cell type responds to a very broad range of wavelengths, and hue perceptions are created within the visual system itself as red vs green and yellow vs blue opponent signals, based through an indirect process on differences in cone cell responses.

Object colours are subjective creations in an additional sense. Because of an extremely useful feature of our visual system known as colour constancy, the colour that we perceive an object to be - its hue, value and chroma - depends more on its own spectral reflectance (the proportion of the incident light throughout the spectrum that the object reflects) than on the wavelengths actually reflected under a given light. This feature depends on the remarkable ability of our visual system to in effect estimate how the light coming from an object differs from what would come from a white object under the same lighting. The colours we see as properties of objects are actually perceptions created by our visual system to represent this "inferred" difference to us; though they seem to be sensed directly, they are really more like elements of a 3D computer model projected by our minds onto, and fitted to, the external world.

Strictly speaking then, hue, value, chroma, brightness, saturation, and "colorfulness" are all psychological dimensions of subjective perception. When we talk about them quantitatively, we are really talking in each case about what is called a psychophysical dimension, calculated based on measurable physical properties, but intended to correlate for a "standard" observer with the actual psychological dimension.

The next page presents an overview of the main dimensions of colour. A warning: this page is a rather compact summary of some of the main ideas covered on this site, and may require some perseverence from those new to the subject!

 
 
 
Modified April 12, 2012.. Original text here.


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