Sorry for the late response and thanks again!
It's basically the flood fill approach but I'm currently having a few troubles with the unit-unit collision detection, especially when there are many units they start to penetrate each other or even stockpile. However, my three approaches so far which stop that lead to a chunky unit movement, I guess this will take a while before it works smoothly and solid.

@sivan: Why did you go for a 1366^2 shadow map resolution (camera size/ texture size?)? Seems a little inappropriate to me.