A lot of anime are based off of the plot of a manga and/or light novels. Sometimes an anime is an original work that gets a manga story afterwards, but that isn't always the case.
I remember watching Amagami, where you could obviously notice that it was based on a game, and some are even named after the games such as Final Fantasy.
So how common is it for a anime to be based of a game? I have only seen it once or twice but is it really that uncommon? Or are they just animated in a way that you won't directly notice that it comes from a game unless you already know so?