Convex or Concave Sphere? - Drawing Optical Illusion with Charcoal Pencils - Vamos

Is it convex or concave sphere? Drawing a convex or concave sphere. How to draw optical illusion on lined paper. Cool 3d trick art. Awesome magic realism. Artistic graphic with Vamos. • Material used: Paper: 250x175 mm, white coloured cardboard. Graphite