we are talking about commercial DVDs…?
in most territories, they have a technical copy protection, you're not allowed to override.
in some territories (Germany), you're not even allowed to NAME tools for overriding copyprotection… (but you are allowed to do a "personal copy"… weird, isn't it ;-)? )
in most territories, copyright laws don't allow to "change" (=edit) the content of any "opus"…
in most territories, you own the DVD, but not its content…
in many territories you're allwowed to do personal copies or override copy protection for "universal studies"… weird again…
but, it depends on where're you livin… Panama, Iran, Germany, Canada…
don't trust any legal advice on a forum