Normal mapping
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Normal_map_example.png
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 and stored.
One of the most interesting use of this technique is to greatly enhance the appearance of a low poly model exploiting a normal map coming from a high resolution model. While this idea of taking geometric details from a high resolution model had been introduced in "Fitting Smooth Surfaces to Dense Polygon Meshes" by Krishnamurthy and Levoy, Proc. SIGGRAPH 1996, where this approach was used for creating displacement maps over nurbs, its application to more common triangle meshes came later. In 1998 two papers were presented with the idea of transferring details as normal maps from high to low poly meshes: "Appearance Preserving Simplification", by Cohen et al. SIGGRAPH 1998, and "A general method for recovering attribute values on simplified meshes" by Cignoni et al. IEEE Visualization '98. The former presented a particular constrained simplification algorithm that during the simplification process tracks how the lost details should be mapped over the simplified mesh. The latter presented a more simpler approach that decouples the high and low polygonal mesh and allows to recreate the lost details in a way not dependent on how the low model was created. This latter approach (with some minor variations) is still the one used by most of the currently available tools.
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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 across the model, more detailed normal vector information can be encoded. Each color channel in the bitmap (red, green and blue) corresponds to a spatial dimension (X, Y and Z). These spatial dimensions are usually relative to the base normal of a particular polygon, generating a normal vector for each pixel in the normal map (whereas with bump mapping, the normal vectors of the pixels are the same as the base normals). This adds much more detail to the surface of a model, especially in conjunction with advanced lighting techniques.
Normal mapping in computer entertainment
Interactive normal map rendering was originally only possible on PixelFlow, a parallel graphics machine built at the University of North Carolina at Chapel Hill. It was later possible to perform normal mapping on high-end SGI workstations using multi-pass rendering and frame buffer operations or on low end PC hardware with some tricks using paletted textures. 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: Far Cry (Crytek), Deus Ex: Invisible War (Eidos Interactive), Thief: Deadly Shadows (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
External links
- GIMP normalmap plugin (http://nifelheim.dyndns.org/%7ecocidius/normalmap/)
- Photoshop normalmap plugin (http://developer.nvidia.com/object/photoshop_dds_plugins.html)
- normal mapping tutorials (http://www.monitorstudios.com/bcloward/tutorials_normal_maps1.html), Ben Cloward
- Maya normal mapping plugin (http://www.drone.org/tutorials/displacement_maps.html), Olivier Renouard
- Normal Mapping with paletted textures (http://vcg.isti.cnr.it/activities/geometryegraphics/bumpmapping.html) using old OpenGL extensions.
Bibiliography
- Fitting Smooth Surfaces to Dense Polygon Meshes (http://www-graphics.stanford.edu/papers/surfacefitting/), Krishnamurthy and Levoy, SIGGRAPH 1996
- Appearance-Preserving Simplification (http://www.cs.unc.edu/~geom/APS/APS.pdf), Cohen et. al, SIGGRAPH 1998
- A general method for recovering attribute values on simplifed meshes (http://vcg.isti.cnr.it/publications/papers/rocchini.pdf), Cignoni et al, IEEE Visualization 1998
- Realistic, Hardware-accelerated Shading and Lighting (http://www.cs.ubc.ca/~heidrich/Papers/Siggraph.99.pdf), Heidrich and Seidel, SIGGRAPH 1999