Pink, blue, green and yellow links create a parallelogram mechanism.
Length of the pink link is equal to center distance of the internal gear drive.
Z1: tooth number of blue gear (= 30).
Z2: tooth number of internal gear (= 60).
n1: pink input crank velocity
n3: yellow output crank velocity
Transmission ratio i = n1/n3 = Z1/(Z2-Z1) = 30/(60-30) = 1
Pink and yellow cranks (coaxial in the same side) rotate in opposite directions with the same velocity.