I recently got around to scanning old animation dating back to college and before I started working in animation - I had saved the entire 2D animated artwork from a fully funded 7 minute collaboration with an ex-art college friend and although a lot of necessary short cuts were made in the animation style - (mostly animated on "4's" and linked with 4 frame dissolves using a technique inspired by Richard Williams' film version of Dickens "A Christmas Carol", painstaking in the extreme when shooting on film, in this case 35mm) one or two shots were fully animated, such as this crane shot, which I felt added a much needed element of dynamism to a film that was designed to be quite static.
"A Christmas Carol" was notable for many such animated "crane shots" & I was quite obsessed by them, especially when they involved architecture, something carried forward in films like the animated adaptation of Raymond Briggs "The Snowman" much later - both films had a very rich and detailed illustrative style that was very labour intensive to produce - I was working single-handedly.
Scanning animation cels into a digital environment poses several issues, especially in the case of this artwork - the cel trace line was made with wax pencils and therefore not a solid black that would make compositing more straightforward - placing a cel on a background resulted in a soft transition from the edge of the drawing to the image underneath, but here it was difficult for the image processing software (Photoshop) to achieve the same subtlety.
Initially I used plain white animation paper behind the cel on the flatbed of the Epson V600 scanner I was using, on the assumption that I could easily separate out the background since there was no "peak white" in the animated part - this didn't turn out to be the case and I was left with a thin one-pixel outline that looked terrible when the different cel levels were composited in After Effects.
I didn't have any chroma green card / paper so I ended up flood filling the blank,(ie transparent) area of each scan with green, mainly to avoid having to go through the tedious process of scanning again. Photoshop does a great job of a clean flood-fill and after some clean up of stray pixels and grime I was able to export to After Effects for the final comp.
In After Effects I used Keylight to key out the green and obtained a very good key with some level of transparency at the edges of the wax pencil drawing resulting in a good final composite.
Scanning can be a tedious process but at the same time it was a bit of a physical workout that is absent in digital animation, where you are sitting for long hours looking at a screen and just moving an arm or wrist to do anything - just no hot lights and heavy equipment to deal with :)