Best method to cut somebody's head off

[Kingda Ka]

We are planning a scene where a person has the top of their skull removed beneath a glass helmet and brains exposed. We already have the helmet - it's an actual helmet for brain injuries. The plan was to put a little green beanie on the actor's head beneath the helmet and remove it in post.

Can someone recommend the best software/approach to do this? And is it going to be possible to put the glass shine of the helmet back in? Or would it be better to CGI the entire top of the helmet?

 

[Sweetie]

In my opinion, practical effects makeup would be your best and cheapest bet.

Alternatively Rotoscoping and a combination of practical would be your next best bet. After Effects can easily handle that.

The green beanie thing is likely to get you into trouble.

What do you mean by glass shine? If it's what I'm thinking you're meaning, you could be getting into expensive CGI territory.

Anyway you choose, test, test, test.