GIMP Bump Map: Elevation and Azimuth Explained

The GIMP Bump Map filter creates an illusion of three-dimensional depth and surface texture on a flat layer using a grayscale heightmap. This simulated relief shading relies on a virtual light source defined by two spatial coordinates: azimuth and elevation. Together, these angles dictate the direction, length, and intensity of highlights and shadows across a surface, transforming brightness values into perceived peaks, slopes, and depressions.

Azimuth governs the horizontal compass direction from which the virtual light shines, measured in degrees from 0 to 360. In GIMP, an azimuth setting controls the orientation of shadows around the relief. For example, setting the azimuth to 90 degrees places the light source to the north (top), casting shadows downward, while 180 degrees positions the light to the west (left), casting shadows to the right. In relief shading and digital cartography, the light is traditionally set between 315 and 335 degrees (northwest). This specific direction prevents an optical illusion known as relief inversion, where peaks mistakenly appear as valleys.

Elevation dictates the vertical angle of the light source above the horizon, measured from 0 to 90 degrees. An elevation of 0 degrees places the light flat on the horizon, generating long, exaggerated shadows and high-contrast edges across even the subtlest height variations. Conversely, an elevation of 90 degrees places the light directly overhead at the zenith. An overhead light eliminates directional cast shadows, leaving only faint shading on steep slopes and flattening the overall relief. Setting elevation between 30 and 45 degrees generally produces the most natural balance of depth and surface clarity.

The relationship between azimuth and elevation determines the mathematical calculation of the surface normal vectors. The Bump Map algorithm reads the grayscale gradients in the heightmap to establish slope steepness, then computes how directly each point faces the light vector formed by the two angles. Adjusting the azimuth rotates the primary highlight-and-shadow axis across your design, while adjusting the elevation fine-tunes shadow falloff and relief depth. Mastering both controls allows for precise simulation of distinct materials, from harsh, chiseled stone to soft, subtly embossed paper.