As suggested by geekmaster I ported my demo to C, using gmlib 1.5a and newtrix 2.0 routines (and compiled with tccmake). I have been away the forum for a while and didn't read if there were more currrent updates from geekmaster.
It -- of course -- runs much faster now : while pressing the keyboard keys you can interact with the plotting in real time.
The 3D.c code is much more readable than the previous contrieved 3D.sh code. C being a "natural" for the job, I found the port less interesting but for the object structure which I kept identical forcing me into array indexing, type casting and rounding fun.
Happy to share it: