Ansel is an NVidia technology which enables a photomode with shaders in pre-defined games. A game doesn't support Ansel out of the box, it has to be setup for that, which is usually done by using the Ansel SDK. The Ansel SDK is available to people who register for an account with NVidia, and I don't have such an account, so all I provide here is what I figured out myself from what NVidia provided online and what I've seen in game code.
Ansel works with call-backs, which means the game has to register a function with Ansel which is then called when Ansel needs