It isn't due to transparency of cels. They were transparent back then too (you can buy ye olde cels from productions and see on youtube how it was done in ye ol

It isn't due to transparency of cels. They were transparent back then too (you can buy ye olde cels from productions and see on youtube how it was done in ye olde days). I did in-betweening and later layout for years, first by hand and later on computers. It's simply due to when you paint your cel, you can either paint it flat (like a bucket fill) or shade it when you paint. You get an inked one (photocopied) where ink is on one side, you flip it over and paint behind the ink lines. When you paint, you paint as anything else, thus you can shade it. However, keeping shading consistent and in motion between each frame (frame, not keyframe, since you're painting in-betweens as well) is mission impossible task. It's extremely intensive process which is 'dancing' around because you can't keep shading consistent. What you will get is what someone else has linked here: https://www.youtube.com/watch?v=vnZD6RBt9S0 https://www.youtube.com/watch?v=vnZD6RBt9S0 this was oil painted/shaded per frame.