When changing to Manual AGC, what needs to be done to generate a useful image?

The basic way to get a good image when switching to Manual AGC mode is to switch from Auto to Once-Bright AGC mode first, then to Manual. This way the camera sets up a suitable brightness value for manual mode, and an image is visible immediately.

A further refinement (if desired) would be to ramp down the Contrast, Plateau, and Max gain settings first, then switch to Once-Bright, then Manual, and then ramp up Contrast and Max Gain to the desired manual settings. This can create a more gradual transition from Auto to Manual AGC settings, but requires external processing, programming, and possibly some experimentation to determine the values needed during the transition.