I'm interested in making a clone of a useful piece of CAD software, and am interested in possible implications of copying something. In other words, you're wor

I'm interested in making a clone of a useful piece of CAD software, and am interested in possible implications of copying something. In other words, you're worried about getting sued. This is always a possibility when you set out to create a "derivative work". You can create a new copyleft novel about a boy wizard without too much worry. But if you play off some of the characters in a Harry Potter novel or call it "The Free Harry Potter", you're asking for trouble. In software terms, you can write a new CAD sofware without too much worry. Even copying some existing artistic/UI choices isn't too much of an issue. But what will likely get you in trouble is attempting to make it compatible with some existing software and it's "proprietary" data formats and/or command structures. In other words, "reverse engineering" is what really tends to raise some legal red flags in software circles. It's kinda hard to sell this in court as anything other than an intentional attempt to piggyback off of and have an adverse financial impact on prior work. This is what lawyers live for --- redress for harm to others.