Highlights from: https://en.wikipedia.org/wiki/Miller_test

The test:

* Whether "the average person, applying contemporary community standards", would find that the work, taken as a whole, appeals to the prurient interest,

* Whether the work depicts or describes, in a patently offensive way, sexual conduct or excretory functions specifically defined by applicable state law,

* Whether the work, taken as a whole, lacks serious literary, artistic, political, or scientific value.

Also:

Critics of obscenity law argue that defining what is obscene is paradoxical, arbitrary, and subjective.

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I think it would be hard to not find generated child porn obscene by this test, unless you have a good lawyer, at which point there is plenty of wiggle room.