Retrieves the parallelogram coordinates of a point in the reference parallelogram, given a point in the projection plane (for use with selected TDPACK routines.)
Available in version 4.3.1 and later.
function tdprpi ( xyi2d  : float ) return_val  : float
A float array of 2 elements specifying the coordinates of a point in the projection plane.
A float array of two elements containing the parallelogram coordinates of that point in the reference parallelogram that projects into (xyi2d(0), xyi2d(1)).
This function is part of the low-level TDPACK package, which is a group of Fortran and C callable routines for projecting objects from a 3-dimensional coordinate system having U, V, and W axes to a 2-dimensional projection plane having X and Y axes and/or for drawing the projections of those objects. This can be referred to somewhat loosely as "drawing objects in three dimensions".
Please see the documentation on TDPRPI for a full description of this function.