Contour (strong edges), Area (size) and Enclosedness (closure) as the primary determinants of figure identification, a model that gained wide support, soon becoming a core principle of the cartographic canon found in textbooks (including his own). He tied it directly to the idea of visual levels, the illusion that some elements on the map appear to float above the page, suggesting that figures are "above" their ground. This correlation has also gained widespread, if not universal, acceptance, even though there are common situations when figures appear below their ground (such as a river beneath a road network). Further research was largely grounded in Gestalt psychology and perception, which largely corroborated and expanded upon Dent's model. In summarizing the work to date, MacEachren added Orientation and Convexity to Dent's list, with the acknowledgment that these are relatively minor influences compared to the others. MacEachren discussed the concept of visual levels as "related," but not equal, to figure-ground contrast. Influences on figure–ground contrast Several visual patterns are believed to contribute to figure-ground contrast, such that features that exhibit these patterns are easier to recognize as figures. These have been largely adopted from Gestalt psychology. Differentiation or heterogeneity, the amount of difference in appearance between the intended figure and the intended ground. A strong figure looks significantly different than everything in the ground, while the elements of the ground look relatively similar to each other. This can be accomplished using almost any of the visual variables, such as color, size, and texture. Achieving difference is more important than using prescribed design techniques on the figure and ground; for example, a dark figure stands out on a light background just as much as a light figure stands out on a dark background. Closure, the appearance of a figure to be completely contained within the visual field, and thus entirely surrounded by ground. A figure with closure stands out because it looks like "a thing." The proclivity for closed objects is so strong in human visual processing that humans will perceive closure even when figures are only mostly closed. Centrality, in which features near the center of the visual field (the map) are easier to isolate as figures than features on the edge. The concept of centrality is important because the object located in the center of a map is most often assumed to be the figure. Other map elements can be centered in the