I was wondering how Max Fleischer's classic cartoon shorts had 3D images for the background back in those days.

He actually devised a system where they built miniture sets and placed the cels in front of them...quite a few "Popeye's" and "Betty Boop" shorts from the mid-1930s contain them, including "Betty Boop and Grampy" (1935); "House Cleaning Blues"(1937); and all three color "Popeye" specials ("Popeye Meets Sinbad the Sailor"{1936}, "Popeye Meets Ali Baba and the Forty Theives"{1937), and "Popeye Meets Aladdin and his Wonderful Lamp" {1939}) contain excellent examples of the Fleisher's "setbacks" techique...predating Walt Disney's multiplane camera by several years!

(And, of course, Max Fleisher invented the rotoscope, the technique where you can trace over live-action film to make fluid animation, way back in 1915!!);)