Watched this great video from UE4 Team at this exact moment Math for Artists | Live from HQ | Inside Unreal - YouTube they said that, this Fortnite effect was made using 2 levels of transform. First, it took the position of character feet in world space and transform it into the local space of the block. Then from local block space, it somehow transformed it into block UV space.
I wonder, how it was made? How to transform from local space into UV space, there is no Transform From Local to UV node in the material editor? Also, why you need to do this for this kind of effect when you can just calculate the distance from each pixel to character feet position and make a distance mask.