Draw with the mouse. Press space to fill shapes. UPDATE!: Shape recognition improved! It now fills on the X AND Y axis, so it can more accuarately tell what is a shape or not. To prevent it from filling things that are not closed shapes, it now erases anything with only one layer of pen, which is lighter than if you run the pen over it a second time. It takes about 3x longer, but is about 10x more precise
Basically, it scans each line it works on on the X axis. If it detects 2 areas or more on a line which are black, it will fill the area between them. It will do the same thing on the Y axis. Then it will make sure everything has been filled twice. If anything has not, it will remove it.