There were two graphic output options in the previewer. There were specialized graphic terminals using the GDDM protocol (this code was actually written by someone in Germany who sent me their changes). The original code that I wrote used Tektronics graphics protocols available in the terminal driver that connected to the mainframe via a protocol converter that enabled the use of cheap ASCII terminals instead of the standard dedicated IBM terminals.

Oh man, you were drawing pixel fonts on a 4014 storage tube? That must have taken forever.

It was an emulation of it on a PC. VT100+Tektronix was a common graphics option on terminals of the era and the PC terminal software provided that as its graphics choice. I had some optimizations like replacing any characters below a certain threshold size with a solid box based on the bitmap's bounding box. It was reasonably fast given the speed of the connection.