Martin Constable on Contrast

CONSTABLE ON CONTRAST PAGE 50/78 Simply put, the term Color as hue, saturation MARTIN and lightness contrast refers to I shall be addressing color as an CONSTABLE the evaluation of expression of the three values: hue, saturation and lightness. These values are ON difference. It is general not the only way to chart color, nor are term that can be used they the most accurate, but they are the CONTRAST most friendly to human understanding. almost anywhere: Lightness and saturation both exist as a round thing may dimensions of intensity that are measured according to ascending or descending be contrasted with a values (e.g. lighter, darker, greyer, more square thing, a male colorful etc.). Hue exists as a dimension of quality that is commonly mapped by may be contrasted terms (e.g. red, green, purple etc). The word term was first used in this with a female, our context by the perceptual psychologist expectations may be W. H. Nault. It was Nault who demonstrated that children generally contrasted with associate change in lightness and reality etc. saturation with a change in intensity, such as depth, degree etc., however change in hue they associate with a Contrast within color is change in quality, such as the difference important to painters for the simple between water, land, coral etc. reason that it constitutes their up and Differences in quality are down in the navigation of their palette. difficult to evaluate, which makes any Any such compass is important as color addressal of hue contrast a problematic is such a devilish realm to pilot. one. For example, can the average of As David Bachelor declared in his book blue and yellow be said to be green? Chromophobia: Is red greater than purple? It is a topic ‘Color is uncontainable... it effortlessly reveals the that deserves more room than this article limits of language and evades our best attempts to can afford and for this reason I shall impose a rational order on it’. avoid hue contrast and concentrate Contrast is an easy word to exclusively on lightness and saturation use, but very difficult to use precisely. contrast. For our purposes we may John Ruskin alluded to its importance assume that there are two types of when he wrote that, ‘Of course the contrast: local and global. character of everything is best manifested by Contrast’. However, the literature Local contrast on the topic is old, partial and wildly A local contrast is a contrast of one thing inconsistent. In the following pages with another. The atomic indivisible I shall address some of the many forms of human judgment is the pair-wise that this beast assumes. It shall be a comparison (e.g. ‘A’ is taller than ‘B’). non-technical summary of some of In the Rembrandt painting (Fig. 1) region what I have learned from three years C can be described as being lighter than of collaboration with engineers at region B. This pairwise comparison may Singapore’s Nanyang Technological be staggered to include more values; University whilst researching the hence region C is lighter than region topic of computational aesthetics. B but darker than region A. Expressed TURPS BANANA ISSUE FOURTEEN FEATURE PAGE 51/78 Figure 1 - The Ascension Of Christ - Rembrandt Van Rijn(detail)(1636) in this way the fundamentally relative Global contrast nature of local contrast is revealed. The global contrast of a painting is the In mathematics such evaluation contrast of all the values in that painting through staggered, relative comparison understood or expressed as a singular. lies at the heart of something called a In the case of the lightness and saturation sorting algorithm, using which large values of a painting this is done with data sets may be sorted into meaningful respect to its statistical extremes, i.e. its configurations. In the same way the highest, lowest and average intensity painter, in the manufacture of a painting, values. Different to the ongoing nature of will sort its values by the application of a local contrast appraisal, it is generally many staggered local contrast appraisals. related to the finished ‘fact’ of the In this manner a blob of color on painting: that which has been left behind an artist’s pallet may be considered after the painter has finished (or died). as existing in a state of suspended If the artist’s contrast toolbox animation, waiting to be related to other were an armory then the histogram blobs of color within a painting. This would be an atomic bomb. It was observation is nicely validated by the invented 120 years ago but it is only Language Philosopher Diana Soeiro who since the development of the digital declared that “Like mathematics, color is image that it has been commonly used only demonstrated when we use it”. to visualize the global intensity values of a picture. It can offer an elegant visual summery of the lightness and saturation CONSTABLE ON CONTRAST PAGE 52/78 Fig. 2 - Left: Bathing Bathsheba - Rembrandt Van Rijn 1654 Right: - The Convalescent - Gwen John 1918 Bottom: The histograms of these paintings. global contrast of a painting and it can therefore blind to how values are spatially fairly be assumed that there is no digital distributed. To illustrate this point, photographer alive today who does not I present in Figure 3 two images that know how to read one. Figure 2 illustrates share nearly identical lightness how nicely the histogram can visualize histograms yet are plainly different in the difference between two paintings. terms of contrast impact. This difference See how the ‘top and tail’ of the Gwen resides in the spatial distribution of their John painting values are dark grey and lightness values. light grey and how light the bulk of the values are, compared to the black to Global contrast and spatial white, bottom-loaded values of distribution the Rembrandt. The importance of this spatial The workings of a histogram are distribution becomes very apparent when simple: imagine a painting chopped up lightness and saturation are considered. into many even squares. These squares Hue, saturation, and lightness are are arranged into stacks according to generally co-dependent, but with a their average intensity values and these bias towards light. Should the hue and stacks are then further arranged in order saturation of an image be changed, of ascending intensity. A histogram is the lightness will be little affected. TURPS BANANA ISSUE FOURTEEN FEATURE PAGE 53/78 Fig. 3- Left: Portrait of Nicholas Ruts - Rembrandt Van Rijn 1631 Right: - The Good Samaritan - Eugène Delacroix 1849 Bottom: The histograms of these paintings. However, decrease the light till there is The two values will always lead different only blackness or increase it to the point lives, but it was Delacroix who was the of whiteness and hue will disappear and first to knowingly and aggressively unlink the saturation will flatline. In this way, them. He was one of the first artists to lightness can be said to play the role of take a serious interest in color theory and the master in the HSL triumvirate. may disputably be regarded as Western In most paintings the form of art histories’ first colorist. What is a the saturation will be generally similar to colorist? In my view it is someone who that of the lightness, which in turn will be recognizes that color (i.e. saturation and a close perceptual equivalent to the full hue) may have a life that is significantly color image. This is especially true of independent of lightness. Figure 5 shows pre-modern paintings. In Figure 4 a a painting by Delacroix together with its painting by Gainsborough is shown lightness and saturation maps. See how together with its lightness and saturation the intense points of saturation are values expressed as grey-scale maps. concentrated in a cluster, yet the See how braodly similar all three lightness values are evenly distributed. images are. See also how the lightness map is a close Of course, this linking of perceptual equivalent to the full color lightness and saturation is not absolute. image, yet the saturation map seems to CONSTABLE ON CONTRAST PAGE 54/78 Fig. 4 - A Peasant Girl with Dog and Jug - Thomas Gainsborough (1784). Middle: Saturation map Right: Lightness map refer to a different painting altogether. exclusively, associated with lightness. This unlinking was one of the Event-form spatial distribution: key turning points of color application in This manifests as infrequent and large- Western art history and was rigorously scale events in the spatial dimension and knowingly pursued by such artists as (i.e. ‘clumpy’) and a sudden dark to the impressionists, post impressionists, op light (i.e. ‘stepped’) appearance artists and color field painters. in intensity values. Event-form is An extreme example of its usage can predominately, though not exclusively, be found in the painting ‘Impression, associated with saturation. Sunrise’ by Monet (Fig. 6), in which the visual impression of the sun is carried by Defining Global Contrast the saturation, being almost invisible in A histogram may fairly be described as a the lightness map. re-presentation of a painting: This disengagement of a painting that has been chopped up and saturation from lightness may be under- re-assembled into a new form. It does stood as a difference in the contrast not define global contrast, not unless impression between the two, which is in you subscribe to the notion that a turn driven by the spatial distribution of dismembered dog is a definition of a dog. these values. Mathematics provides us with Even-form spatial distribution: two definitions that we can use to define This manifests as a steady state of small- global contrast (there are other definitions scale change in the spatial dimension that are not suitable for the purpose).

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