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a 3d engine is basically a rendering device of 3d objects/scenes for a game or any interactive/realtime application. it's basically the part that is responsible for taking data like a pixel or an object, texture.. and rendering to the screen. it can use different ways to render, like using direct3d or opengl interface or make your own rendering functions. you dont need to build engines, you can download already made up engines that take care of rendering calls, collisions, all kind of dymanics for you from all kind of websites online, try gamasutra.com OR you can learn how to use/call direct3d or opengl using a coding platform like Vc++ and make an engine your self.
