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I have project which was implemented using processing. I want to add an user interface to this project. What can i use ?
you can just write your own interface.
you can also use a library for that.see library section on the website. Eg G4P comes with an editor you can use to draw an user interface (abbrevation is ui or gui)
and for the draw a gui tool see
it comes with a variety of control types like buttons. it depends of course what controls you need.