OpenGL and SDL. Code
Milton is focused on painting on infinite canvas, using techniques similar to raster systems, but processing the image in vector form. The editor does not support editing individual pixels, but at the vector level it allows you to delve into any level of detail. Features such as layers, brushes, lines, blurs, etc. are supported. All results are saved automatically as changes are made with the possibility of unlimited rollback of changes (unlimited undo / redo, not interrupted by closing the program). Using the vector format allows you to store data in a very compact form. It is possible to export to raster formats JPEG and PNG.
The new release adds soft brushes, stylus pressure-based transparency, rotation operations, and responsive brush resizing relative to the canvas.
Source: opennet.ru