A simple yet effective method used in interactive projections consist of alternating between two or more visuals. This could be the result of pressing a button or entering an area that's being tracked with a webcam, etc,.
In this brief tutorial we will look at the Switch TOP, a very useful operator for interactive projects. It is easy to use: Link two MovieDeviceIn TOPs to a Switch TOP and end your network with a Null TOP - activate the operator's display viewer to better monitor any alterations .
In the parameters window for Switch TOP, use the INDEX slider to alternate between your visuals. Touchdesigner likes to start counting from 0, hence your first image will be indexed as "0" while your second image will be indexed as "1".
As mentioned before, we could use a button, sensor or tracking device to control the operator's INDEX slider.