CSS / JS slider transition effect

Question

I recently came across a slider on this website - https://antoni.de/. I really like the transition effect but I can't for the life of me figure out how it's done. I get how the slider works but its the actual transition that is baffling me. I have no idea what this effect is called and I am not sure if it is done by CSS or JS? Would love to be able to imitate the same sort of transition effect. Does anyone know how this done and can point me in the right direction to building something similar?

This is an image of the transition effect:

Answer 1

I actually messaged the developer to find out how he made this. It is made using webGL!

Answer 2

Its made with canvas HTML5, you can see this in source code.