The Masses test, although never adopted adopted by the Supreme Court, held that speech directly advocating criminal activity is unprotected. It does not matter how likely, serious, or imminent the crime would be.
The clear and present danger test is not followed anymore, but it used to be the test for whether or not speech could constitutionally be prohibited.
The clear and present danger test says that the government may punish speech that is intended to produce, or of which the natural and probable effect is to create, a danger of a likely, imminent, and serious crime.
The Brandenburg test is the current test for what inflammatory speech Congress can prohibit. It requires advocacy directed to inciting or producing imminent lawless action and that is likely to incite or produce such action.
Under the Miller test, speech is obscenity and therefore unprotected if it:
Appeals to the prurient interest of the average person in the community,
Is patently offensive to the community as defined by statute, and
Lacks serious literary, artistic, political, or scientific value.
This includes child pornography as long as it involves actual children and not just drawings of children.
The purpose of the protection is to protect children from the harm in the creation of the pornography, and this is not implicated if it is drawn or computer-generated.
The Supreme Court theoretically could create new categories of unprotected speech if it was shown the founders intended such speech to be unprotected, but it is very hesitant to do so. (The Chaplinsky test is descriptive, not prescriptive.)