Not sure I understood your question, but will try to answer below. If you have any more question please ask again!
The visual victims does not need to be close to any heated victims. The visual victims can be located anywhere.
You have to detect the visual victims with something that measure light / images. A camera or a bunch of light sensors would make it?
The letter does not need to be in the middle. But will probably be. But just as heated victims they can be in different places and even in the corners.
// Fredrik Löfgren