I think I may have erred in stating that compositing may have been a backformation of compositor. In thinking about it it seems more likely that it is a backformation from the adjective composite, because compositing means taking two images and making a composite of them.
As to the printing compositor, the OED has it as the third definition, after (believe it or not) one who is an arbiter and one who composes or compiles a literary work.
And interestiongly enough, there is composititious meaning of a composed or made-up sort.
I had scurried to the OED, which I am forced to keep at home, because I wondered if compositor in the typing sense might not be a combination of compose and deposit. Nope. Comes from the same root as the other compose words.
Note that composit means to place or put together. a type compositor definitely does that.
