Once suitable interpolation formulas have been deduced to obtain the contour lines in <PLOT>, attention must be given to some practical aspects of the contouring programs. When the contour maps were prepared with pen plotters, the overriding consideration was the extremely long time needed to raise and lower the pen, relative to the time required for horizontal or vertical movements. Thus it was advantageous to trace out an entire contour line, until it either closed on itself or reached the boundary of the contoured region. Electrostatic plotters or displays on video monitors do not impose such a penalty, so that all the contours desired for a given triangle can be computed and displayed before moving on to the next triangle.
Since the completion of one contour before commencing another could lead the pen through triangles that would eventually be scanned as possible starting points, a bitmap was constructed in which a flag indicates whether a triangle has previously been traversed or not. The same information prevents retracing a closed contour once the pen has returned to its starting point. Much space can be saved by programming the bitmap in assembly language, but confining <PLOT> exclusively to a high level language such as Fortran or ``C'' encourages wasting an entire word of computer memory per bit.
The use of simplices instead of the coordinate grid leads to several special cases for the interpolation formulas. By permuting the vertices of the simplices, a single subroutine for the interpolation suffices. Passage from one the vertices of one simplex to the next can be accomplished by reflection in the face of the simplex, once again leading to a uniform formula for the process. It is unfortunate that space cannot be filled by any kind of uniform simplices.