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Concept
Lightmap
Summary
A lightmap is a data structure used in lightmapping, a form of surface caching in which the brightness of surfaces in a virtual scene is pre-calculated and stored in texture maps for later use. Lightmaps are most commonly applied to static objects in applications that use real-time 3D computer graphics, such as video games, in order to provide lighting effects such as global illumination at a relatively low computational cost.
John Carmack's Quake was the first computer game to use lightmaps to augment rendering. Before lightmaps were invented, realtime applications relied purely on Gouraud shading to interpolate vertex lighting for surfaces. This only allowed low frequency lighting information, and could create clipping artifacts close to the camera without perspective-correct interpolation. Discontinuity meshing was sometimes used especially with radiosity solutions to adaptively improve the resolution of vertex lighting information, however the additional cost in primitive setup for realtime rasterization was generally prohibitive. Quake's software rasterizer used surface caching to apply lighting calculations in texture space once when polygons initially appear within the viewing frustum (effectively creating temporary 'lit' versions of the currently visible textures as the viewer negotiated the scene).
As consumer 3d graphics hardware capable of multitexturing, light-mapping became more popular, and engines began to combine light-maps in real time as a secondary multiply-blend texture layer.
Lightmaps are composed of lumels (lumination elements), analogous to texels in texture mapping. Smaller lumels yield a higher lightmap, providing finer lighting detail at the price of reduced performance and increased memory usage. For example, a lightmap scale of 4 lumels per world unit would give a lower quality than a scale of 16 lumels per world unit. Thus, in using the technique, level designers and 3d artists often have to make a compromise between performance and quality; if high resolution lightmaps are used too frequently then the application may consume excessive system resources, negatively affecting performance.
Official source
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