Complex sentence: Mary's long robe swirls around the pictorial space, obscuring her throne and eventually falling to the support at her feet, the three dimensionality of which gives the disorienting impression that the figures are part of a relief.
Split into simpler sentences: Mary's long robe swirls around the pictorial space, obscuring her throne and eventually falling at the support by her feet. The three dimensionality of the support gives the disorienting impression that the figures are part of a relief.