As Russell Borogove wrote, scribing projector displays were used.
The displayed image was generated with three different types of projectors in parallel. The light from all these projectors was added on the screen.
- One background projector displayed the static slide with the Earth or Moon map.
- Another projector to display the moving symbol for the spacecraft.
The slide was moved horizontally and vertically under numeric
computer control to move the spacecraft symbol to the right position
on the map.
- Up to five scribing projectors to display the spacecraft trajectory.
An opaque slide was used to scribe the line under computer
control by removing some of the opaque material. The scribed image
is displayed as it is plotted to the slide and thus to the screen.
The slides with the maps could be changed as neccessary during the several phases of the mission, Earth orbit, transfer trajectory, Moon orbit, transfer back to Earth and reentry to Earth until splash down. There were special maps for abort modes and recovery. But all these maps slides should be prepared before the mission and could not be modified during the mission.
Both the map and the scribing projectors had slide magazines for up to 40 different slides random selectable under computer control.
The slides were made with transparent lines on a black background. Filters in red, green, blue, magenta, yellow and cyan were used with projectors to compose an image with a magenta map, red spacecraft symbol and a yellow trajectory.
Different symbols could be selected for the CSM, CM and LM.
These projection displays should not be used in a very dark room like a cinema, the mission control room should be lighted to do paperwork. High power xenon arc lamps were used to get a bright image. The slides should be cooled with forced air from compressors and exhaust pipes were needed to get the heat out of the projector room.
Many thanks to Russell Borogove for giving me the right words "scribing projector display" for the search machine to find the image at arsTECHNICA.