The trapezoid layer is created using information from both the cylinder and the first triangle layer. The coordinates used to create the bottom of the first triangle layer are the same coordinates for the top of the trapezoid layer, while the bottom part of the trapezoid layer connects to the top of the cylinder. The cylinder is sectioned off into twelve vertical strips leaving every other one blank in order to simulate the idea that a space represents a window. Thus, in order to finish the trapezoid and complete the layer, twelve equally-spaced vertices lying on the circle at the top of the cylinder are needed. The measurements from the side-elevation establish the height of these points. The radius is the number measured as the radius of the cylinder. So, using the same method that is used to create the triangle layers, one is able to compute the twelve desired points.

Optical Illusion & Projection in Domes: A Study of Guarino Guarini's Santissima Sindone