How to draw illegal-spawn-area like in clash royale?

[FuTou]

What I mean is the transparent red area in which you cannot click to spawn a troop.

1. It is inside the Nav-mesh area
2. it prevents player from click here to spawn troop
3. When the tower goes down, the area automatically grow smaller. Like shown in the picture.

Thanks a lot!

[FuTou]

I know I can draw a picture like that, but how to do it by bluerpint or programming?

[Deathrey (DeathreyCG)]

Whole scene is grid based. So just enable/disable visibility of a plane, with transparent red material, that corresponds to the area, that has changed.