In the Vega series of paintings by Vasarely, half spheres bulge out of the plane. We make an attempt to construct transformations that produce this effect. We take a close look at the two spheres in the painting Bull. We visualize parametric stereographic projection and show what parameter choices fit best for the two spheres in the painting. It remains an open issue if Vasarely had the same projection in mind. All the same, the interactive experimental tool can serve as a means to give insight into the mathematical idea behind the painting.