Unless your directly influencing the output you own the copyright to the program not the output. Basically, if your code generates a random image you don't get copyright, which is useful as generating say every English haiku is actually possible. It's under ~(15,831 syllable candidates) ^ 17.

Here's a real case: The Policeman's Beard is Half Constructed, a book released in 1984. The prose and poetry was written by Racter, a computer program written by William Chamberlain. The illustrations were done by Joan Hall, and the introduction to the book was written by William Chamberlain.

So it's clear that Joan Hall owns the copyright to the illustrations, and William Chamberlain the introduction. But what of the rest of the book?