How to achieve visual field

After several days of thinking, I have been trying to do -- visual area analysis, but I have no idea.
    The way I think about it is 1: set the position of the light source
-->2: open the shadow of model and terrain --ShadowMap -->3: circle a certain region -- change the color of the shadow part inside the region to red.

    I want to use the above method to achieve visual area analysis, but in this process encountered several problems, 1: how to set the light source, 2: in the source code can not be found to change the shadow color...

   I do not know if I set up the visual analysis method is correct, I hope some friends can tell me how to proceed...please

The closest thing I can think of that’s already built in is the classification system:

Otherwise, check out the Cesium ion SDK I mentioned in your other thread.