Normal maps are commonly used in the games industry to create an illusion of depth that is not actually present in the geometry. This is done by transferring the information from a high resolution model to a low resolution model and baking it into a normal map which allows you to keep a low poly count, something that is absolutely necessary for real time applications like games.
In this tutorial Michael McKinley from Zombie Studios shows you how it's done by modeling an arched brick wall using some common polygon modeling tools like point snap, extrude, fill hole and append to poly tool. The surface information from this high resolution wall is then baked into a normal map using Maya's transfer surface abilities and assigned to a corresponding low poly model allowing you to display this illusion of high res geometry which is not actually present on the model itself.
Michael McKinley has many years of experience from working in the games industry and has been involved with many large games productions such as the SAW franchise. You can find out more about him on his website
If you're interested in learning more about modeling for games and keeping a low poly count you can check out our Low Poly Modeling for Games
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