My students love doing the Vortex project. On a poster board, select a spot for the point - I encourage something off center, not too close to the edge for first timers. This results in the page being broken into wedges. The first Optical illusion occurs in that the lines seem to bend closest to the point.
Students select a wedge and paint a space in the selected color.
Within the same wedge an alternate color is painted in the next space. The next wedge is selected and the alternate color is painted in the shape next to the first shape painted so that the same color touches only at the corner of the shapes. It required a lot of editing too as he had to be careful of the line placement in developing the characters. I remember seeing a project done using convex and concave lines, but I have no idea how it was done.