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Alightmap is adata structure used inlightmapping, a form ofsurface caching in which the brightness of surfaces in a virtual scene ispre-calculated and stored intexture maps for later use. Lightmaps are most commonly applied to static objects in applications that usereal-time3D computer graphics, such asvideo games, in order to provide lighting effects such asglobal illumination at a relatively low computational cost.
John Carmack'sQuake was the first computer game to use lightmaps to augmentrendering.[1] Before lightmaps were invented, realtime applications relied purely onGouraud 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 withradiosity 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 usedsurface caching to apply lighting calculations in texture space once when polygons initially appear within theviewing frustum (effectively creating temporary 'lit' versions of the currently visible textures as the viewer negotiated the scene).
As consumer 3d graphics hardware capable ofmultitexturing, light-mapping became more popular, and engines began to combine light-maps in real time as a secondarymultiply-blend texture layer.
Lightmaps are composed oflumels[2] (lumination elements), analogous to texels intexture mapping. Smaller lumels yield a higherresolution 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 and3d 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. Lightmap resolution and scaling may also be limited by the amount of disk storage space, bandwidth/download time, or texture memory available to the application. Some implementations attempt to pack multiple lightmaps together in a process known asatlasing[3] to help circumvent these limitations.
Lightmap resolution and scale are two different things. The resolution is the area, in pixels, available for storing one or more surface's lightmaps. The number of individual surfaces that can fit on a lightmap is determined by the scale. Lower scale values mean higher quality and more space taken on a lightmap. Higher scale values mean lower quality and less space taken. A surface can have a lightmap that has the same area, so a 1:1 ratio, or smaller, so the lightmap is stretched to fit.
Lightmaps in games are usually colored texture maps. They are usually flat, without information about the light's direction, whilst somegame engines use multiple lightmaps to provide approximate directional information to combine with normal-maps. Lightmaps may also store separate precalculated components of lighting information for semi-dynamic lighting with shaders, such as ambient-occlusion & sunlight shadowing.
When creating lightmaps, any lighting model may be used, because the lighting is entirely precomputed and real-time performance is not always a necessity. A variety of techniques includingambient occlusion, direct lighting with sampled shadow edges, and fullradiosity[4] bounce light solutions are typically used. Modern 3D packages include specific plugins for applying light-map UV-coordinates, atlas-ing multiple surfaces into single texture sheets, and rendering the maps themselves. Alternatively game engine pipelines may include custom lightmap creation tools. An additional consideration is the use of compressedDXT textures which are subject to blocking artifacts – individual surfaces must not collide on 4x4 texel chunks for best results.
In all cases,soft shadows for static geometry are possible if simple occlusion tests (such as basicray-tracing) are used to determine which lumels are visible to the light. However, the actual softness of the shadows is determined by how the engine interpolates the lumel data across a surface, and can result in apixelated look if the lumels are too large. Seetexture filtering.
Lightmaps can also be calculated in real-time[5] for good quality colored lighting effects that are not prone to the defects of Gouraud shading, although shadow creation must still be done using another method such asstencil shadow volumes orshadow mapping, as real-time ray-tracing is still too slow to perform on modern hardware in most 3D engines.
Photon mapping can be used to calculate global illumination for light maps.
Invertex lighting, lighting information is computed per vertex and stored invertex color attributes. The two techniques may be combined, e.g. vertex color values stored for high detail meshes, whilst light maps are only used for coarser geometry.
Indiscontinuity mapping, the scene may be furthersubdivided andclipped along major changes in light and dark to better define shadows.