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Proceedings of DSC 2003 2, Figure 1 A graphical representation of the human colour axes. blue violet part of the spectrum This adaptation provides us with the ability to. discriminate colour on a yellow blue axis It is interesting to note that this colour. axis seems to be associated with our notion of warm and cool colours 11. A second adaptation occurred in primates about 30 million years ago This. divided the yellow green sensitive cone cells into two kinds of cells one more sen. sitive to green and the other more sensitive to red The adaptation provides very. fine colour discrimination in the red green region of the spectrum Such an adapta. tion is clearly advantageous when assessing the ripeness of many kinds of fruit and. it hardly a surprise that it should have arisen in primates This new adaptation. provides us with colour discrimination on a red green colour axis. Because there are three natural colour axes it is natural to describe colours as. locations in a three dimensional space A representation of the plane spanned by. the yellow blue and red green axes is shown in figure 1 Although these axes have a. natural physiological explanation they do not correspond to our natural perception. of colour Our perception seems to correspond to the use of polar coordinates in. this plane The angle counter clockwise from red to a given colour correlates with. our notion of colour hue and the radial distance from the origin to a given colour. correlates with our notion of chroma or colourfulness. 2 Colour Synthesis and Colour Matching, The three dimensional nature of colour means that a wide range of colour sensations. can be generated by mixing different amounts of three primary colours Any three. colours can serve as primaries but the widest set of colour sensations can be created. by single wavelength light sources placed close to the peak sensitivities of the three. kinds of cone cells present in the eye The most common choice for primary colours. in lighting and display technology consists of monochromatic red green and blue. Much of what we know about our colour vision systems is based on colour. matching experiments which seek to quantify the amounts of three given primaries. needed to match a given colour or set of colours Suppose that to match a given. colour we require amounts R G and B of a particular set of red green and blue. primaries The values R G and B are called the tristimulus values for the match. Proceedings of DSC 2003 3, One possible matching experiment consists of seeing how single wavelength. colours can be matched by a given set of primaries For a given wavelength. suppose that the tristimulus values for a match are r b and g Regarded as. functions of r b and g are known as the colour matching functions for the. given set of primaries, Colour matching functions are important because it has been observed that. colour matching is linear This means that it is possible to determine the match. for a mixture of wavelengths by using the matching functions to obtain a match at. each frequency and then summing across frequencies If the function s gives the. amount of each wavelength present in a colour then the tristimulus values for the. mixture are given by,R s r d G s g d B s b d, It is common to normalise the tristimulus values by dividing them by their sum.
to obtain the relative amounts of red green and blue required to obtain a match. The normalised values,R G B R G B R G B, are referred to as the chomaticities for the match Because chromaticities sum to. one it is possible to compute any one of them from the other two It is usual to. reduce dimensionality by discarding the third chromaticity This makes it possible. to plot colours in two dimensions which is often convenient. If two colours have tristimulus values C1 R1 G1 B1 and C2 R2 G2 B2. then any mixture of these colours must have a tristimulus value which lies on the. line joining C1 and C2 This property is also preserved for chromaticities which. makes chromaticity plots useful for describing colour mixtures. 3 Colour Standards, In the 1920s and 1930s a series of colour matching experiments by W D Wright. and J Guild showed that there was enough consistency in colour matching for it to. be possible to define a standard observer whose hypothetical colour matching. functions would represent a typical human colour response and provide an abso. lute reference standard for colour This standardisation was carried out under the. auspices of the Commission Internationale de l E clairage the CIE in 1931. Although a standard observer had been agreed upon there was still the issue. of which set of primaries should be used for colour matching With any real set. of primaries it is not possible to match all possible colours there will always. be some colours which can only be matched after adding a small amount of one of. the primaries to that colour In such cases the tristimulus value of that primary. is taken to be negative This leads to matching functions which take on negative. values for some ranges of, The CIE felt that the use of negative matching functions was likely to be a. source of confusion and errors Their solution to this problem was to use a set of. imaginary primaries to obtain colour matching functions This made it possible to. define all colours with positive tristimulus values and hence positive chromaticities. Proceedings of DSC 2003 4, Roughly speaking the CIE primaries can be thought of as supersaturated shades. of violet blue yellow green and orange The tristimulus values corresponding to. these primaries are denoted by X Y and Z and chromaticities by x y and z A. full colour description can be obtained by plotting the tristimulus values in a three. dimensional space and a partial one by plotting the x and y chromaticity values on. a two dimensional chromaticity diagram As part of their choice of primaries the. CIE arranged that the Y tristimulus value correspond to the apparent brightness. of colours, While the CIE tristimulus values provide a complete colour description they do.
not correspond to the way we perceive colours In particular distances between. tristimulus triples need not reflect the degree to which we perceive the colours to. differ The CIE recognised this and introduced new spaces which more closely reflect. the way we perceive colour, The two perceptually based spaces introduced by the CIE in 1976 are the. CIELUV and CIELAB spaces The CIELUV space is generally preferred by those. who work with emissive colour technologies such as computer displays and the. CIELAB space is preferred by those working with dyes and pigments such as in. the printing and textile industries, I will confine my remarks to the CIELUV space This is because the presentation. graphics I will describe are typically created on computer displays and this is where. colour experimentation takes place, The CIELUV space is based on the uniform chromaticity coordinates. u0 4X X 15Y 3Z,v 0 9Y X 15Y 3Z, These coordinates correspond to positions on a red green and yellow blue axes To. produce a full colour description these coordinates must be scaled and combined. with brightness information The full CIELUV spaces is defined by the coordinates. 116 Y Yn 1 3 16 for Y Yn 0 008856,903 3 Y Yn otherwise.
u 13L u0 u0n,v 13L v 0 vn0, where Yn is the Y tristimulus value and u0n and vn0 are the uniform chromaticities. for the white point of the display, This parameterisation means that the u axis corresponds to the horizontal axis. of figure 1 and the v axis to the vertical one The intersection of the two axes takes. place at the colour neutral white point,4 Device Dependent Colour. Cheap colour computer displays became widely available in the 1980s The earli. est displays were very limited in the colours which could be displayed Typically. such displays were limited to a fixed set of just 8 or 16 rather lurid colours A. second generation of cheap colour displays could display 256 colours While this. Proceedings of DSC 2003 5, is considerably better than first generation displays it is still quite limiting The. restriction of web pages to a fixed palette of 216 colours is a legacy which dates. from the heyday of this kind of display, The current generation of colour displays is much more satisfactory They can.
display as many colours it is possible to discriminate with the human eye Unfor. tunately the software which controls these displays often retains the effects of the. restrictions earlier generations of graphics hardware. Current colour displays are based on red green and blue primaries Colours are. described by directly specifying the levels of red green and blue to be combined It. is convenient to think of the RGB levels as lying in the interval 0 1 although in. practise they are more likely to be specified as an 8 bit integer in the range 0 255. The colour produced by a given RGB description is also affected by the response. characteristics of the display For most displays the principal characteristic of. interest is the display gamma The intensity I of the on screen primaries is. related to the specified level L for that primary by. It is possible for displays to have a distinct gamma value for each of their primaries. but in they are generally quite close in value, If colours are to be made to appear identically on different displays the display. gammas must be taken into account For most current displays the gamma value. is close to 2 2 and there has been some attempt to standardise colour description. so that they will display faithfully on this kind of display 8. RGB colour descriptions do not correspond naturally to the way in which we. think about colour Because of this a number of other equivalent colour descriptions. have been formulated These include HSV HSL HSB and others These new. spaces seek to transform the RGB coordinates to a more intuitive set In the case. of HSV these coordinates are hue saturation and value which loosely correspond. to dominant wavelength colour purity and brightness Although the coordinates. do have a more intuitive interpretation than RGB they are device dependent and. do not offer the ease of interpretation of the perceptually based CIE spaces. 5 Colour Harmony, The colour descriptions of the previous sections provide a way of precisely specifying. colours which are to appear in a figure or graph They do not however address. the issue of which colours should be used, Choosing a set of colours which work well together is a challenging task for. anyone who does not have an intuitive gift for colour Some general guidance on. colour choice is available in books on art and graphic design These books suggest. the use of complementary colours split complementaries triads and tetrads Most. of the advice is based on the use of a vaguely described colour wheel and does. not recognise the fact that there are many different colour wheels to choose from. The notable exception to this rule is to be found in the work of the noted 19th. century colourist Albert Munsell Munsell developed a colour notation system 13. 5 1 which he used in teaching The system is deeply rooted in how we perceive. colour but has a strong quantitative basis In addition Munsell gives well defined. quantitative rules which can be used to choose harmonious sets of colours Munsell s. work has always been appreciated in publishing and related graphic arts and it now. Proceedings of DSC 2003 6,R 5 5 BG 5 5 N5,R 6 5 BG 4 5 N5. Figure 2 Illustrations of colour balance after Munsell These figures are redrawn. Proceedings of DSC 2003 7, appears to be undergoing a rediscovery by those working in user interface design.
and visualisation see 9 12 and 7 for example, Munsell describes colour in terms of hue value and chroma hue corresponding. to dominant wavelength value to brightness and chroma to colourfulness Unlike. saturation which is a statement of colourfulness relative to the maximum possible. for a given hue and value chroma is an absolute measure of colourfulness the. maximum chroma possible for red is much greater than that for green Munsell. divided the circle of hues into 5 main hues R Y G B P for red yellow. green blue and purple He placed 5 intermediate hues YR GY BG PB and RP. between them and provided a way of specifying finer hue divisions on a decimal. scale He also divided the value range into 10 equal equal steps and provided a. similar quantification of chroma Under Munsell s system the colour specification. R 4 5 means a hue of red with a value of 4 and a chroma of 5 A specification of N. 5 refers to a mid range neutral grey, Munsell s ideas on colour harmony are rooted in the notion of colour balance. Munsell uses the term balance in a variety of ways but usually it means that set of. colours is chosen to be centred on a mid range or neutral value Figure 2 shows an. illustration of the Munsell concept of balance The picture in top part of the figure. uses the colours R 5 5 and BG 5 5 on an N5 background R 5 5 and BG 5 5 are. at opposite sides of the Munsell colour wheel and hence they balance at N5 Using. these two colours together with an N5 background produces a colour scheme which. balances at N5 The lower figure shows the picture as above but with the value of. the red raised by a step and the value of the blue green lowered by a step Again. N5 lies at the centre of these two colours and this again creates a balanced colour. Most current graphics systems provide very little assistance in making good colour choices Indeed most systems require that a user specify their colours in ways which are closely related to the hardware representation of the colours rather than to the way we most naturally think about colour This tends to encourage bad colour choices In this paper we ll examine some principles and

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