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In 3D computer graphics, normal mapping is an application of the technique known as bump mapping. While bump mapping perturbs the existing normal (the way the surface is facing) of a model, normal mapping replaces the normal entirely. Like bump mapping, it is used to add details to shading without using more polygons. But where a bump map is usually calculated based on a single-channel (interpreted as grayscale) image, the source for the normals in normal mapping is usually a multichannel image (that is, channels for "red", "green" and "blue" as opposed to just a single color) derived from a set of more detailed versions of the objects.
Normal mapping is usually found in two varieties: object-space and tangent-space normal mapping. They differ in coordinate systems in which the normals are measured.
The idea of storing the exact normal, instead of a grayscale image as in bump mapping, was first presented in "Appearance Preserving Simplification", by Cohen et al.
How it works
To calculate the light on a surface, the vector for an incoming light source is dotted with the vector normal to that surface, and the result is the intensity of the light on that surface. Imagine a polygonal model of a sphere - you can only approximate the shape of the surface. By using an RGB bitmap textured accross the model, more detailed normal vector information can be encoded. Each color channel in the bitmap (red, green and blue) corresponds to a spacial dimension (X, Y and Z). These spacial dimensions are usually relative to the base normal of a particular polygon, generating a normal vector for each pixel in the normal map. This adds much more detail to the surface of a model, especially in conjunction with advanced lighting techniques.
Normal mapping in computer entertainment
Normal mapping was originally created for use almost exclusively in non-interactive 3D rendering. However, with the increasing processing power and sophistication of home PCs and gaming consoles, normal mapping has spread to the public consciousness through its use in several high-profile games, including: Deus Ex: Invisible War (Eidos Interactive), (Eidos Interactive), The Chronicles of Riddick: Escape from Butcher Bay (Vivendi Universal), Halo 2 (Microsoft), and (perhaps most well-known) Doom 3 (id Software) and Half-Life 2 (Valve Software). It is also used extensively in the upcoming third version of the Unreal engine (Epic Games). Normal mapping's increasing popularity amongst video-game designers is due to its combination of excellent graphical quality and decreased processing requirements versus other methods of producing similar effects. This decreased processing requirement translates into better performance and is made possible by distance-indexed detail scaling, a technique which decreases the detail of the normal map of a given texture (cf. mipmapping). Basically, this means that more distant surfaces require less complex lighting simulation. This in turn cuts the processing burden, while maintaining virtually the same level of detail as close-up textures.
Currently, normal mapping has been utilized successfully on the PC and Xbox console. Though theoretically it's also possible to utilize normal mapping on the Sony PlayStation 2 and Nintendo GameCube, it's unlikely that these platforms have the hardware power to utilize this technique with satisfactory results.
See also
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