Camera Lucida is a 5-star rated app for iOS devices. It was created in 2010 and has been constantly improved throughout the years and is up to version 12.0.
Camera Lucida helps you to draw - on paper or canvas - by using your iPhone or iPad. It is not a project, it doesn't help you to draw art on your screen. Instead, you point your device at your drawing surface and draw (on paper) by looking at your screen.
Many other app developers have tried to copy the app, but Camera Lucida says far ahead of the curve and is the only app that supports remote keyboards, mirrored and tilted setups, using all available cameras, automatic correction for perspective, etc. The videos posted here are mostly tutorials. I heard a rumor that an accordion video made it in by mistake.