The pink slider reciprocates in a groove of the yellow arm. Its pink pin is always in contact with the blue guide inner profile.
Violet part is pivoted on the pink pin.
A surface of the violet part is always in contact with the guide outer profile (distance from the pink pin to the surface is equal to the guide width).
When turning the violet part the green sweeper creates outside surface of the planter (in glass), a right square frustum in this case.
The green sweeper has complicated motion: successive translation and rotation. This motion helps to create variable fillets on the frustum edges.