If we can economically characterize good drawing, we can identify effective training techniques for the novice, and explore the underlying conceptual models of the expert.
The authors propose angle accuracy as a measure of excellence in perspective drawing. Using this metric, we establish a robust expert-novice difference: when drawing a complex still life in perspective, experienced artists are significantly more accurate than novices in angle representation. However, the experts are no better (or worse) than novices when estimating planar angles simply from observation. In other words, experts demonstrate a clear advantage in execution of angles while there is no difference between experts and novices for a measure tapping into their perception of those same angles.
This suggests that training and practice improves accuracy in drawing angles—a domain-specific skill. Drawing expertise does not demand or create more accurate angle judgment overall.
Novices were unusually accurate when participants drew a teapot, in many cases as accurate as the experts. This suggests that while most people can cope with the ordinary, the advantage of a drawing expert may rest in the ability to render even the extraordinary.
When participants made angle judgments from a still life, another important difference emerged. Experts were indistinguishable from novices when they estimated the angles that certain edges projected onto the picture plane (an egocentric measure). When they estimated the geographical slant (an allocentric measure) of edges in the still life, experts were more accurate than novices, a difference that approached significance. Drawing experts may use a three-dimensional internal model of the still life rather than simply ‘flat-mapping’ it onto the picture plane.