How Light Defines Texture
Distinguishing between rough and smooth objects begins by understanding the way light bounces off surfaces of different textures. When light coming from a single direction strikes a rough object, it disperses in many directions, creating what is perceived as a soft-edged, or unfocused, highlight. The same light striking a smooth object will bounce off the object in a single direction, resulting in a hard-edged, or focused, highlight. On very smooth objects the highlight 'will be a distorted picture of the light source. This distortion will be similar to the defining patterns of light and dark we associate with the basic forms. For example, the light from a square window seen on a glossy cone-shaped object will be distorted into triangles. The round sun, seen in the highlight of a polished cylinder will appear as a stripe of light.
Side lighting best reveals the texture any object. This is important to remember when you want to give viewers the most textural information in a drawing. The texture of a rough object is most apparent in the area of transition between light and shadow; smooth surface texture is revealed by the presence of a sharply focused highlight.
On a smooth cone, the light from a square window becomes a distorted triangular picture of the window. On a smooth cylinder, the round sun becomes a stripe of bright light. The smoother a surface, the more we depend on reflections to define its texture.
The most information about the texture of a rough surface is found where light and shadow are in transition. In the highlight area and the shadow area, texture is obscured.