A conditionally visible bite line may be demarcated on a shoe upper using one of a fluorescent material and an Infrared (IR) material. Such a conditionally visible bite line may be observable only under particular conditions, such as when illuminated by an ultraviolet light source or an IR light source, as appropriate. A light may be projected to intersect the conditionally visible bite line under conditions rendering the conditionally visible bite line detectable. The intersection(s) of the projected light and the conditionally visible bite line may be used to create a virtual bite line for use in generating a tool path to process the surface of a shoe upper bounded by the conditionally visible bite line.