Ask HN: How do drawing programs work?

I have a decent idea how the web works. From the DOM to the database, I have a fairly solid understanding of each. Now I want to learn something new.

Drawing programs. How do they work? How does MS Paint work? How does Photoshop work? How does Autocad work?

What kind of architecture do these systems have? What kind of systems and subsystems is needed for this to work? How different are drawing programs from MVC and network architecture?

Can anyone point me to some good resources?