Dynamic Lighting question

Hey, first time roll20 user here. been using it for a few days and so far my PC's and I LOVE it!!! Lots of cool features. Now I have a question about dynamic lighting. Lets say I didn't want the PC's to see past a rock. Is there a way to let them view the rock but not whats behind them depending on where they move their token? Like drawing a polygon line around the rock will block sight and light, but the PC's have no idea whats actually blocking their sight. Is there anyways to do what im asking? Thanks

Make sure you have something on your map layer that represents whatever you've drawn on the dynamic light layer. I use the copy feature after I draw something on one layer and paste it on the other. Also, on the dynamic layer I use a bright color like red.

How I did something similar is make an X with dynamic lighting polylines inside the boulder image itself (did it with trees also). This allows the boulder to be seen but causes a shifting shadow to stretch out behind the boulder.

Oops. I guess I didn't read Brian C's question well enough. I usually just draw a smaller inner dynamic lighting barrier, so that they can see more of the out edge of the object that is blocking sight, make an X with dynamic lighting polyline That seems like a cool trick Pat. I going to try that one. I think this would be a good use for the Foreground Map Layer suggestion. You could place a boulder image on a foreground layer so you could see the top of the boulder, but the opposite side would be black as per the line of site. edit: just added a quick mock-up of using a foreground layer to help with line of sight.

yeah thats exactly what i want

I learned that trick from Gauss, I think.