Canvas

Summary
Canvas is an app present in InternalUI builds which is used to test the interface used in iMessage's handwritten messages feature. Aside from its many options within the Settings panel, Canvas has an overly simple interface. Within the Settings panel, there are options for fine-tuning the stroke, including anti-aliasing. There are also different modes, views, prototyping options, and the ability to enable a specific background image, which can be imported from Photos.