16.1.4.4 Annotating Metrics

A variation on the contouring idea, in [98], Teschner proposes a method for displaying metrics, such as 2D tick marks, on an object using a 2D texture map containing the metrics. Texture coordinates are generated as a distance from object coordinates to a reference plane. For the 2D case, two reference planes are used. An example application for this technique is to create a 2D texture marked off with tick marks every kilometer in both the $s$ and $t$ directions and map this texture on to terrain data using a GL_ REPEAT texture coordinate wrap mode. An GL_ OBJECT_LINEAR texture coordinate generation mode is used, with the reference planes at $x = 0$ and $z = 0$ and a scale factor set such that a vertex coordinate which is 1km from the $x-y$ or $z-y$ plane produces a texture coordinate value equal to the distance between two tick marks in texture coordinate space.