Two-dimensional shape perception is based on a salience map.

A salience map is a dynamic topographical map that combines information from individual feature maps into a real-number measure of conspicuity (salience). Originally, Koch and Ullman (1985) used salience to predict the priority of locations in visual search. The defining characteristic of a salience map is substance indifference--computations made on the map contents are independent of the features that produced the salience values because the features are not represented in the map. Salience maps have been proposed for computations other than search or processing priority: attention-based motion perception (Lu and Sperling, 1995), isoluminant red-green grating motion (Lu, Lesmes, Sperling centroid computation (Sun, Chubb, Wright, Sperling, 2018) and frontal-plane distance perception (Gan, Sun, Sperling, 2021). Here we demonstrate the well-known fact that simple shapes, such as letters and numbers, can be recognized not only when they are painted black or white but also when the are defined by outlines, or filled with different textures than the background, or filled with colors that are isoluminant with the background. Substance indifference means 2D shape perception is a computation performed in large part on a salience map. We further demonstrate that luminance is not necessary for accurate shape perception, that isoluminant text is easily readable, and so-called luminance artifacts are irrelevant, i.e., salience is sufficient for accurate 2D shape perception.