GMT allows for artificial illumination and shading. What this means is that we imagine an artificial sun placed at infinity in some azimuth and elevation position illuminating our surface. The parts of the surface that slope toward the sun should brighten while those sides facing away should become darker; no shadows are cast as a result of topographic undulations.
While it is clear that the actual slopes of the surface and the orientation of the sun enter into these calculations, there is clearly an arbitrary element when the surface is not topographic relief but some other quantity. For instance, what does the slope toward the sun mean if we are plotting a grid of heat flow anomalies? While there are many ways to accomplish what we want, GMT offers a relatively simple way: We may calculate the gradient of the surface in the direction of the sun and normalize these values to fall in the range; +1 means maximum sun exposure and -1 means complete shade. Although we will not show it here, it should be added that GMT treats the intensities as a separate data set. Thus, while these values are often derived from the relief surface we want to image they could be separately observed quantities such as back-scatter information.
Colors in GMT are specified in the RGB system used for computer screens; it mixes red, green, and blue light to achieve other colors. The RGB system is a Cartesian coordinate system and produces a color cube. For reasons better explained in Appendix I in the Reference book it is difficult to darken and brighten a color based on its RGB values and an alternative coordinate system is used instead; here we use the HSV system. If you hold the color cube so that the black and white corners are along a vertical axis, then the other 6 corners project onto the horizontal plane to form a hexagon; the corners of this hexagon are the primary colors Red, Yellow, Green, Cyan, Blue, and Magenta. The CMY colors are the complimentary colors and are used when paints are mixed to produce a new color (this is how printers operate; they also add pure black (K) to avoid making gray from CMY). In this coordinate system the angle 0-360 is the hue (H); the Saturation and Value are harder to explain. Suffice it to say here that we intend to darken any pure color (on the cube facets) by keeping H fixed and adding black and brighten it by adding white; for interior points in the cube we will add or remove gray. This operation is efficiently done in the HSV coordinate system; hence all GMT shading operations involve translating from RGB to HSV, do the illumination effect, and transform back the modified RGB values.