This is a portrait of Luca Pacioli with geometry equipment and an unknown figure in the background. An important feature to note is the suspended rhombicuboctahedron in the top left corner. This is the first appearance of the polyhedron (which has eight triangular faces and eighteen square faces) in a painting; notice that this painting was done before the publication of da Vinci’s geometric sketches. Since the rhombicuboctahedron is a rather complicated polyhedron, Barbari’s attempt at portraying it is imperfect; it is difficult to make out the individual faces and get a sense of the overall shape of the polyhedron (especially its backside). This painting is evidence of the fact that da Vinci’s work was no small feat. Although geometric shapes are abstract and idealized, a great deal of artistic mastery is still required to depict such figures in two dimensions.
The image source is Wikimedia Commons, and it is in the public domain in the United States because it was published (or registered with the U.S. Copyright Office) before January 1, 1924.