https://www.youtube.com/watch?v=JpR5EoBI0Z4 Zeichnen lernen, Akadmie Ruhr, Tutorials, perfekter Kreis in Perspektive, 4-Punkt-Technik
https://www.youtube.com/watch?v=2GpQMxcHOEU Creating Ellipses and Cylinders in Perspective
https://www.youtube.com/watch?v=z06dMQMgwe4 2 Point Perspective Ellipses
https://www.youtube.com/watch?v=yFjrSddZiv4 Drawabox: Perfect Circles and Cubes in 3D Space
https://www.youtube.com/watch?v=Hg16J_4tmPk How to draw isometric circles