Minecraft wiki entry for Curved_roofs, associated text: For reference, this image shows a set of pixel-perfect circles with diameters from 1 to 18 pixels. It was generated from a simple graphics program zoomed in to 800% and with the grid turned on. If you want to generate larger circles for yourself, or ellipses or other curves, Paintbrush (macOS), Microsoft Paint (Windows), and GIMP (free software available on most platforms including macOS, Windows and Linux) are all useful tools. However, if you need to generate more sophisticated shapes then you may begin to need more powerful tools than just a basic bitmap editing package.