shap3d52, rectangular hole in a rectangular plate.
The system has been made symmetric about x = 0 and y = 0. Then there are 2 electrodes, one in the octant between x = 0 and x = y and the other between x = y and y = 0.
For the first electrode the equations, say g1(x,y,z), for the inner edge are:
x = sx, y = sy*tan(phi), z = 0, where phi = 0 to pi/4.
The equations, say g2(x,y,z), for the outer circular are:
x = tx*cos(phi), y = ty*sin(phi), z = 0.
In the usual fashion we can combine these by using the general equation
g(x,y,z) = (1 - f^2)*g1 + f^2*g2,
where f goes from 0 to 1 and is squared to concentrate segments at the inner edge.
This reproduces g1 when f = 0, and g2 when f = 1.
Here sx = 2, sy = 0.5, tx = 10 ty = 5.
Analogous equations are used for the second electrode.