Figure 2.

(a) Body orientation is described using yaw (−180° to 180°) and pitch (0° to 180°). Pitch values above 90° correspond to a roll of 180° (i.e. body upside down). The cylinder sits back uppermost with yaw = 45°, pitch = 30° and roll = 0°. (b–e) Influence of body position and light distribution on the optimal pattern. The light distribution corresponds to June 21, noon, St Andrews, Scotland (56° 20′25·44″N, 2° 47′43·8″W), with a standard CIE sunny sky (sun elevation 60°) for b, c and d, and a standard CIE cloudy sky in e. Top row: optimal coloration for a cylindrical body with yaw = 0° (i.e. long axis towards north), roll 0° (back uppermost) and pitch is (b, e) 0°, (c) 30° and (d) 90°. For each, the body is observed by a viewer looking west–east. Bottom row: corresponding coloration along a dorsoventral transect of the body; 0° = top of the dorsum. The reflectance of the background is 0·175.