Quote - A 4096 * 4096 displacement map = brings  16,777,216 micro polygons at render time.

This is not entirely correct. It sets a maximum, but you will rarely get anywhere near that. 

Mirco polygons are only calulated to the pixel level, so you will end up with far less than that depending on the size of the render, how much of the UV you can see at once, etc. If a pixel at render time has 20k of those points, it isn't going to do 20k passes, of that pixel.

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