I'm interested in controlling a VGA signal directly, using various kind of signals. A VGA signal is made up of five main components: red, green, blue, horizontal sync, vertical sync.
I've made a cable that has RCA connections for RGB, TS for horizontal and vertical sync and the other end terminates in a 15 pin VGA socket. Note that both the horizontal and vertical sync TS connectors have 68 ohm resistors built in.
The horizontal and vertical signals are generated via an Arduino. There are many examples around of code that will allow this. In this case, a 640 x 480 field is generated at 60Hz.