Light and Line

TONAL RENDERING TECHNIQUES

The tonal carbon-dust illustrations become, through the force of a consistent visual rhetoric, the ‘evidence’ in Grant’s Atlas of Anatomy (1943). The viewer comes to trust them as optimized yet faithful depictions of the human body. Designed specifically to be at the core of an atlas used in a cadaver lab, the tonal drawings represent what the student might see at dissection. Thanks to the skill of the illustrators and the tonal depth of the carbon-dust technique, these drawings provide a convincing depiction of the dissection morphology, without the liabilities of photography. We briefly enumerate some of the illustrative techniques the artists used to provide this ‘better than photorealism’ approach.

Contrast effects

The illustrators of Grant’s Atlas selectively amplified the luminance contrast along object edges, in order to enhance the illusion of depth. This technique exploits a powerful perceptual depth cue described only relatively recently: edge contrast at object boundaries, and its relative sharpness or blurriness, will strongly affect the perception of depth discontinuities. (Marshall et al., 1996)

A detail of a Dorothy Chubb carbon-dust illustration of the cranial nerves
which demonstrates selective enhancement of edge contrast

Texture gradients

Texture gradients are represented by patterned luminance changes that vary in scale and orientation with the angle of the surface relative to the observer (Ware, 2004). These surface patterns help to convey the complex three-dimensional morphology of the anatomical subjects. The Grant’s Atlas illustrators deployed texture orientation and scale changes across surfaces to reinforce the three-dimensional nature of the subject.

A detail of a Dorothy Chubb carbon-dust illustration of the cranial nerves
which demonstrates the use of texture gradients.

Cast shadows

Cast shadows, another powerful depth cue, are frequently used to convey shape as well as the spatial inter-relationships of objects. Note the use of an upper-left light source, a convention of medical illustration (and of the academic tradition in fine art).

A detail of a Dorothy Chubb carbon-dust illustration of the cranial nerves
which demonstrates the use of cast shadows.

LINEAR RENDERING TECHNIQUES

Grant’s artists used line drawings in two primary modes: as keys to tonal depictions of structure, and as schematic representations of structure–function relationships. Why do these line drawings play such a distinct role in the Atlas?

Linear depiction is a special case in visual perception. Real-world scenes do not resemble line drawings, yet human beings have little trouble interpreting them. Strong evidence supports the notion that the ability to understand line drawings is somehow inherent to visual and spatial perception, not learned (Hochberg and Brooks, 1962; Kennedy, 1993). There is even evidence that line drawings are interpreted more readily than realistically shaded scenes in many circumstances (Biederman and Bar, 1999; Biederman and Gerhardstein, 1993; Ryan and Schwartz, 1956). As the cognitive scientist Julian Hochberg (1972: 48) has noted: In line drawings, the artist has not invented a completely arbitrary language: instead, he has discovered a stimulus that is equivalent in some way to the features by which the visual system normally encodes the images of objects in the visual field.

Skilled illustrators use various techniques to extend the expressive range of line drawings.

Occlusion

Occlusion, where a nearer object overlaps a more distant object in the visual field, is the most powerful depth cue. The illustrators of Grant’s Atlas would selectively enhance the occlusion effect in line drawings by breaking the contours of more distant objects as they intersected the silhouette lines of nearer objects.

A detail of a Nancy Joy pen-and-ink illustration of the arterial supply of the head
and neck region which demonstrates occlusion.

Line weight

Line weight may be used to denote various kinds of contours, including silhouettes, depth discontinuities, and surface discontinuities. In establishing a hierarchy of line characteristics, the illustrator can assist the viewer’s reconstruction of the three-dimensional form (Newman et al., 2002), and selectively emphasize or de-emphasize specific structures. This is frequently deployed in the Grant’s Atlas images to make the linear depiction of structure more readable and coherent.

A detail of a Nancy Joy pen-and-ink illustration of the arterial supply of the head
and neck region which demonstrates variation in line weight.

Linear patterns

Varied patterns of thin-to-thick-to-thin lines are used to indicate shading and describe the form of the anatomical structure. The resulting pattern forms a type of linear texture gradient. This technique is termed ‘eye-lashing’ among medical illustrators.

A detail of a Dorothy Chubb illustration of the posterior cranial fossa
demonstrating the use of linear patterns to indicate form.

Luminance and line

In a number of instances, both tone and line are used in Grant’s Atlas for the same specimen or subject. How these illustrations are labelled provides clear evidence of the perceived strengths of each visual approach {Figs. 198, 199}. In these posterior views of the posterior cranial fossa, the tone illustration is labelled mostly with surface features of the cranium, while the line drawing is labelled to demonstrate important internal features, especially the small foramina that are functionally important transcranial routes for blood vessels and nerves.

Tone is never used in the atlas when:
• The image is schematic (i.e. scale varies across the illustration).
• The image is conceptual (e.g. representations of opposing surfaces ‘opened up’ and laid flat with a the graphic guide of a surrounding open-book motif).
• Multiple layers are represented simultaneously (images incorporating elements made see-through or transparent for didactic purposes).

Line illustrations could be used effectively to convey not only what the student might see, but also what they should know. In many circumstances, line drawings also provide visual scaffolds that would allow students to form the proper conceptual models of the structure–function relationships in the human body.


Kim Sawchuk, Nicholas Woolridge and Jodie Jenkinson excerpt adapted from “Illustrating medicine: line, luminance and the lessons from J.C.B. Grant’s Atlas of Anatomy (1943)”, Visual Communication 10 (3) 2011 – Special Issue: The Visual Essay, pp 442-464.

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Vol 10(3): DOI 10.1177/1470357211408816