In art, which method is primarily used to give a three-dimensional quality to flat surfaces?

The method primarily used to give a three-dimensional quality to flat surfaces is shading. Shading involves the application of light and dark tones to an object in order to provide the illusion of depth, volume, and form. By varying the intensity and distribution of these tones, artists can simulate how light interacts with surfaces, creating highlights, shadows, and gradients that contribute significantly to the perception of three-dimensionality on a two-dimensional plane.

In contrast, while crosshatching is a technique that can create depth and texture through overlapping lines, it does not convey the same level of volumetric qualities as shading. Linear perspective is a technique that creates the illusion of space by depicting receding lines and horizon points, which helps establish depth but does not affect the three-dimensional rendering of individual objects as directly as shading does. Color theory focuses on the relationships between colors and their combinations rather than on the spatial or dimensional qualities of form. Therefore, shading is the most direct method for achieving a three-dimensional appearance in flat artworks.