Here is a simple answer ( from USA copyright law):
Copyright is Automatic � but still needs to be asserted.
Copyright protection is automatic. The act of creating the work also creates the Copyright. There is no requirement to Register for Copyright, although in the US and most other countries it is important to show (assert) that copyright is claimed in a particular work. This is easily enough done by clearly marking any original work with the symbol � followed by the date from which copyright is claimed and the owner of the copyright.
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What this means is that regardless what you create, the moment you have created it it is protected by copyright and yes that includes porn. Now, this form of copyright is kind of "whimsical" because it only rests with you and in your own home/business. However as soon as you publish it in any way or form, the copyright protection automatically kicks in. Therefore, if anyobody copies it for any commercial purpose ( there are some exceptions for documentaary and non comercial copying) they are breaking a law and can be criminally charged. Before a copyright owner can file charges though he/she must register the copyright, because without the registration there is no case: See following excerpt:
The US Situation � Infringement Proceedings
The US also has special requirements concerning the infringement of any Copyright and it is advisable to Contact an IP expert about these. The most important thing to be aware of is that before an infringement suit may be filed in court it is necessary to Register the works of US origin with the Library of Congress www.copyright.gov
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To summarize: You have created it you are protected automatically, but if you want to sue somebody you must register the work. It does NOT matter if it is much later, the registration is just a formality acknowledging the copyright NOT creating it. It is of course essential to denote your work with a copyright symbol and the year when the work was created to claim it and so that the violator cannot claim ignorance.