SDL pen event handling.
SDL provides an API for pressure-sensitive pen (stylus and/or eraser) handling, e.g., for input and drawing tablets or suitably equipped mobile / tablet devices.
To get started with pens, simply handle SDL_EVENT_PEN_* events. When a pen starts providing input, SDL will assign it a unique SDL_PenID, which will remain for the life of the process, as long as the pen stays connected.
Pens may provide more than simple touch input; they might have other axes, such as pressure, tilt, rotation, etc.