In computer graphics, a mipmap (mip being an acronym of the Latin phrase multum in parvo, meaning "much in little") is a pre-calculated, optimized sequence of images, each of which has an image resolution which is a factor of two smaller than the previous. Their use is known as mipmapping.
They are intended to increase rendering speed and reduce aliasing artifacts. A high-resolution mipmap image is used for high-density samples, such as for objects close to the camera; lower-resolution images are used as the object appears farther away. This is a more efficient way of downscaling a texture than sampling all texels in the original texture that would contribute to a screen pixel; it is faster to take a constant number of samples from the appropriately downfiltered textures. Since mipmaps, by definition, are pre-allocated, additional storage space is required to take advantage of them. They are also related to wavelet compression.
Mipmaps are widely used in 3D computer games, flight simulators, other 3D imaging systems for texture filtering, and 2D and 3D GIS software. Mipmap textures are used in 3D scenes to decrease the time required to render a scene. They also improve image quality by reducing aliasing and Moiré patterns that occur at large viewing distances, at the cost of 33% more memory per texture.
History
Mipmapping was invented by Lance Williams in 1983 and is described in his paper Pyramidal parametrics.
The origin of the term mipmap is an initialism of the Latin phrase multum in parvo ("much in little"), and map, modeled on bitmap.
- Improving image quality. Rendering from large textures where only small, discontiguous subsets of texels are used can easily produce Moiré patterns;
- Speeding up rendering times, either by reducing the number of texels sampled to render each pixel, or increasing the memory locality of the samples taken;
- Reducing stress on the GPU or CPU.
- Water surface reflections
Anisotropic filtering
When a texture is viewed at a steep angle, the filtering should not be uniform in each direction (it should be anisotropic rather than isotropic), and a compromise resolution is required. If a higher resolution is used, the cache coherence goes down, and the aliasing is increased in one direction, but the image tends to be clearer. If a lower resolution is used, the cache coherence is improved, but the image is overly blurry. This would be a tradeoff of MIP level of detail (LOD) for aliasing vs blurriness. However anisotropic filtering attempts to resolve this trade-off by sampling a non isotropic texture footprint for each pixel rather than merely adjusting the MIP LOD. This non isotropic texture sampling requires either a more sophisticated storage scheme or a summation of more texture fetches at higher frequencies.