void vuabs(xvu, yvu, zvu) D3PLT.C Level 1
 
float xvu The x position of viewpoint in workbox units.
float yvu The y position of viewpoint in workbox units.
float zvu The z position of viewpoint in workbox units.
 
To obtain a perspective view, you must specify the position (viewpoint) of the observer. The viewpoint must be outside the workbox. If it is very close, there will be a lot of perspective distortion. If it is far off, the perspective distortion will be eliminated. If also, xvu = yvu = zvu, the projection will be isometric.

The plot will not shrink as the distance of the view-point is increased unless sclfix() has been called. The viewpoint should be above the x-y plane or else the labels will be backwards. Do not use with view() or vuangl(). In order that the hidden line removal for 3-D surfaces can work properly, the viewpoint must be selected so that one corner of the x-y plane is closest to the viewpoint as is shown below.

To view the surface from below, negate the values produced by zfun(), or the values loaded in zmat, and view the 
surface from above. Be sure that your call to graf3d() reflects the negated data values.