How to calculate an angle between two lines - Conformal mapping

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I have a rounded-triangle in $z$-plane that conformally mapped into unit disk in $\zeta$-plane using the Schwarz-Cristoffell method.

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By definition, the conformal mapping will preserve the local angle (intersection between two lines) in the $z$-plane. In other words, the two plots should have a similar local angle between the lines.

If I have all the points to generate the two plots, how to prove that the local angle between the plots are the same?

enter image description here

Can someone help me on this?