Povray source is available (povray.org), if you'd like to look at the source for a nice ray tracer, or things like ogre 3d (http://www.ogre3d.org) etc have source available.
POVRay is a great source for learning. I haven't looked at it for years, so I don't know how it aged, but it basically taught me C and graphics programming.
huh, so Pov ray is still updating after all these decades. I used that in a college course a decade ago and it felt dated. But it's a small but fully featured tool, so you can't really go wrong with it as a learning jumpstart. Good starting step and you can jump into pbrt later to see how a "middle grade" ray tracer operates.
I'm a bit less sure about Ogre3d. Another very old repository, but it's been through quite a few revisions over the years. I don't know how clean the code would be for a beginner approaching it. But if you learn by reading and changing code, it sure would be the exact challenge you are seeking.
