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Unity will start charging developers each time their game is installed
(www.engadget.com)
This is a most excellent place for technology news and articles.
Does a pizza contain an oven? No. Does a pizza contain tomatoes? Yes. Therefore tomatoes are an ingredient and an oven is not.
Does a game contain Unity? Yes. Therefore Unity is an ingredient.
The game ships with Unity which handles the rendering, physics, sound and a whole bunch more. Basically Unity is a pizza base, but it gives you a bunch of toppings too. The developer combines the base with the toppings and voila you've got a game. Not saying that last part isn't hard, but a business model where Unity, or any game engine for that matter, is charged proportionally to the amount of installs isn't a totally unreasonable business model.