Hillshading is a way to determine the hypothetical illumination of a surface based on a given light source position in the sky. Positions on the terrain that are most exposed to the light source have high illumination or brightness values, whereas terrain positions that are hidden or protected from the light source have low to no illumination or brightness. Hillshade values range from 255 (high illumination) to 0 (no illumination).
The position of the light source or sun is specified by the user in degrees altitude above the horizon and azimuth. The hillshade or brightness value for a single terrain position may differ significantly for different positions of the light source, such as the movement of the sun during the day and for different seasons of the year.