Texture mapping is a term used in computer graphics to describe how 2D images are projected onto 3D models. The most common variant is the UV unwrap, which can be described as an inverse paper cutout, where the surfaces of a 3D model are cut apart so that it can be unfolded into a 2D coordinate space (UV space).
Semantic Texture mapping can multiply refer to (1) the task of unwrapping a 3D model (converting the surface of a 3D model into a 2D texture map), (2) applying a 2D texture map onto the surface of a 3D model, and (3) the 3D software algorithm that performs both tasks. A texture map refers to a 2D image ("texture") that adds visual detail to a 3D model. The image can be stored as a raster graphic. Textures can also store properties beyond color and are then correspondingly referred to by other names, such as normal map, roughness map, or emission map The coordinate space that converts from a 3D model's 3D space into a 2D space for sampling from the texture map is variously called UV space, UV coordinates, or texture space.
Algorithm The following is a simplified explanation of how an algorithm could work to render an image:
For each pixel, trace the coordinates of the screen into the 3D scene. If a 3D model is hit or, more precisely, the polygon of a 3D model hits the 2D UV coordinates, then The UV coordinates are used to read the color from the texture and apply it to the pixel.
History The original technique was pioneered by Edwin Catmull in 1974 as part of his doctoral thesis. Texture mapping originally referred to diffuse mapping, a method that simply mapped pixels from a texture to a 3D surface ("wrapping" the image around the object). In recent decades, the advent of multi-pass rendering, multitexturing, mipmaps, and more complex mappings such as height mapping, bump mapping, normal mapping, displacement mapping, reflection mapping, specular mapping, occlusion mapping, and many other variations on the technique (controlled by a materials system) have made it possible to simulate near-photorealism in real time by vastly reducing the number of polygons and lighting calculations needed to construct a realistic and functional 3D scene.
Texture maps
A texture map is an image applied ("mapped") to the surface of a shape or polygon. This may be a bitmap image or a procedural texture. They may be stored in common image file formats, referenced by 3D model formats or material definitions, and assembled into resource bundles. They may have one to three dimensions, although two dimensions are most common for visible surfaces. For use with modern hardware, texture map data may be stored in swizzled or tiled orderings to improve cache coherency. Rendering APIs typically manage texture map resources (which may be located in device memory) as buffers or surfaces, and may allow 'render to texture' for additional effects such as post processing or environment mapping. Texture maps usually contain RGB color data (either stored as direct color, compressed formats, or indexed color), and sometimes an additional channel for alpha blending (RGBA) especially for billboards and decal overlay textures. It is possible to use the alpha channel (which may be convenient to store in formats parsed by hardware) for other uses such as specularity. Multiple texture maps (or channels) may be combined for control over specularity, normals, displacement, or subsurface scattering, e.g. for skin rendering. Multiple texture images may be combined in texture atlases or array textures to reduce state changes for modern hardware. (They may be considered a modern evolution of tile map graphics). Modern hardware often supports cube map textures with multiple faces for environment mapping.
Creation Texture maps may be acquired by scanning or digital photography, designed in image manipulation software such as GIMP or Photoshop, or painted onto 3D surfaces directly in a 3D paint tool such as Mudbox or ZBrush.
Texture application This process is akin to applying patterned paper to a plain white box. Every vertex in a polygon is assigned a texture coordinate (which in the 2D case is also known as UV coordinates). This may be done through explicit assignment of vertex attributes, manually edited in a 3D modelling package through UV unwrapping tools. It is also possible to associate a procedural transformation from 3D space to texture space with the material. This might be accomplished via planar projection or, alternatively, cylindrical or spherical mapping. More complex mappings may consider the distance along a surface to minimize distortion. These coordinates are interpolated across the faces of polygons to sample the texture map during rendering. Textures may be repeated or mirrored to extend a finite rectangular bitmap over a larger area, or they may have a one-to-one unique "injective" mapping from every piece of a surface (which is important for render mapping and light mapping, also known as baking).
Texture space Texture mapping maps the model surface (or screen space during rasterization) into texture space; in this space, the texture map is visible in its undistorted form. UV unwrapping tools typically provide a view in texture space for manual editing of texture coordinates. Some rendering techniques such as subsurface scattering may be performed approximately by texture-space operations.
Multitexturing Multitexturing is the use of more than one texture at a time on a polygon. For instance, a light map texture may be used to light a surface as an alternative to recalculating that lighting every time the surface is rendered. Microtextures or detail textures are used to add higher frequency details, and dirt maps add weathering and variation; this can greatly reduce the apparent periodicity of repeating textures. Modern graphics may use more than 10 layers, which are combined using shaders, for greater fidelity. Another multitexture technique is bump mapping, which allows a texture to directly control the facing direction of a surface for the purposes of its lighting calculations; it can give a very good appearance of a complex surface (such as tree bark or rough concrete) that takes on lighting detail in addition to the usual detailed coloring. Bump mapping has become popular in video games, as graphics hardware has become powerful enough to accommodate it in real-time.
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