[PD] Gem - how to get a concave/convex plane

[oliver]

Hi, dear list !

I am looking for a Geo, that looks like a distorted plane, in particular 
a "concave" or "convex" plane as shown in this .jpg:

https://wolke.klingt.org/index.php/s/QO7Vb5UDNn42CG9

can anybody hint me to a way that this can be done in Gem ?

the "convex" version seems easier, it's more or less a scaled cylinder, 
but i can't ssem to achieve the "concave" version with this Geo.

Best

Oliver


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