View source for Reflection mapping

{{Short description|Technique in computer graphics to represent reflective surfaces}}
[[Image:Spoon fi.jpg|upright=1.2|thumb|An environment texture mapped onto models of spoons, to give the illusion that they are reflecting the world around them]]

In [[computer graphics]], '''reflection mapping''' or '''environment mapping'''<ref>{{Cite web|url=http://www.pearsonhighered.com/samplechapter/0321194969.pdf|title = Higher Education &#124; Pearson}}</ref><ref>{{Cite web | title=Directory {{!}} Computer Science and Engineering | url=http://web.cse.ohio-state.edu/~whmin/courses/cse5542-2013-spring/17-env.pdf | access-date=2025-02-18 | website=web.cse.ohio-state.edu}}</ref><ref>{{Cite web| title=Bump and Environment Mapping | url=http://www.ics.uci.edu/~majumder/VC/classes/BEmap.pdf | archive-url=https://web.archive.org/web/20120129012326/http://www.ics.uci.edu/~majumder/VC/classes/BEmap.pdf | archive-date=2012-01-29}}</ref> is an efficient [[image-based lighting]] technique for approximating the appearance of a [[Specular reflection|reflective]] surface by means of a precomputed [[Texture mapping|texture]]. The texture is used to store the [[Digital image|image]] of the distant environment surrounding the rendered object.

Several ways of storing the surrounding environment have been employed. The first technique was [[sphere mapping]], in which a single texture contains the image of the surroundings as reflected on a [[Curved mirror|spherical mirror]]. It has been almost entirely surpassed by [[cube mapping]], in which the environment is projected onto the six faces of a cube and stored as six square textures or [[Net (polyhedron)|unfolded]] into six square regions of a single texture. Other projections that have some superior mathematical or computational properties include the [[paraboloid]] mapping, the [[Pyramid (geometry)|pyramid]] mapping, the [[octahedron]] mapping, and the [[HEALPix]] mapping.

Reflection mapping is one of several approaches to [[Reflection (computer graphics)|reflection rendering]], alongside e.g. [[screen space reflections]] or [[Ray tracing (graphics)|ray tracing]] which computes the exact reflection by tracing a ray of light and following its [[optical path]]. The reflection color used in the shading computation at a [[pixel]] is determined by calculating the reflection vector at the point on the object and mapping it to the [[texel (graphics)|texel]] in the environment map. This technique often produces results that are superficially similar to those generated by raytracing, but is less computationally expensive since the radiance value of the reflection comes from calculating the angles of [[Angle of incidence (optics)|incidence]] and reflection, followed by a texture lookup, rather than followed by tracing a ray against the scene geometry and computing the radiance of the ray, simplifying the [[GPU]] workload.

However, in most circumstances a mapped reflection is only an approximation of the real reflection. Environment mapping relies on two assumptions that are seldom satisfied:
# All radiance incident upon the object being [[Shading|shaded]] comes from an infinite distance. When this is not the case the reflection of nearby geometry appears in the wrong place on the reflected object. When this is the case, no [[parallax]] is seen in the reflection.
# The object being shaded is [[Convex set|convex]], such that it contains no self-interreflections. When this is not the case the object does not appear in the reflection; only the environment does.

Environment mapping is generally the fastest method of rendering a reflective surface. To further increase the speed of rendering, the renderer may calculate the position of the reflected ray at each vertex. Then, the position is interpolated across polygons to which the vertex is attached. This eliminates the need for recalculating every pixel's reflection direction.

If [[normal mapping]] is used, each polygon has many face normals (the direction a given point on a polygon is facing), which can be used in tandem with an environment map to produce a more realistic reflection. In this case, the angle of reflection at a given point on a polygon will take the normal map into consideration. This technique is used to make an otherwise flat surface appear textured, for example corrugated metal, or [[brushed metal|brushed]] aluminium.

== Types ==

===Sphere mapping===
<!-- Deleted image removed: [[Image:Bubble spheremap.jpg|thumb|Spherical environment mapping by its very design creates a singularity where reflected pixels are calculated from texels on the edge of the map.]] -->
<!-- Deleted image removed: [[Image:Bubble spheremapmap.jpg|thumb|The singularity corresponds to texels on the edge of the [[azimuthal projection]].]] -->
[[Sphere mapping]] represents the [[sphere]] of incident illumination as though it were seen in the reflection of a reflective sphere through an [[orthographic projection (geometry)|orthographic]] camera. The texture image can be created by approximating this ideal setup, or using a [[fisheye lens]] or via [[pre-rendering|prerendering]] a scene with a spherical mapping.

The spherical mapping suffers from limitations that detract from the realism of resulting renderings. Because spherical maps are stored as [[azimuthal projection]]s of the environments they represent, an abrupt point of singularity (a "[[black hole]]" effect) is visible in the reflection on the object where texel colors at or near the edge of the map are distorted due to inadequate resolution to represent the points accurately. The spherical mapping also wastes pixels that are in the square but not in the sphere.

The artifacts of the spherical mapping are so severe that it is effective only for viewpoints near that of the virtual orthographic camera.

===Cube mapping===
[[Image:cube mapped reflection example.jpg|thumb|A diagram depicting an apparent reflection being provided by cube-mapped reflection. The map is actually projected onto the surface from the point of view of the observer. Highlights which in raytracing would be provided by tracing the ray and determining the angle made with the normal, can be "fudged", if they are manually painted into the texture field (or if they already appear there depending on how the texture map was obtained), from where they will be projected onto the mapped object along with the rest of the texture detail.]]
[[Image:Cube mapped reflection example 2.JPG|thumb|Example of a three-dimensional model using cube-mapped reflection]]

[[Cube mapping]] and other polyhedron mappings address the severe distortion of sphere maps. If cube maps are made and filtered correctly, they have no visible seams, and can be used independent of the viewpoint of the often-virtual camera acquiring the map. Cube and other polyhedron maps have since superseded sphere maps in most computer graphics applications, with the exception of acquiring [[image-based lighting]]. Image-based lighting can be done with parallax-corrected cube maps.<ref>{{Cite web|url=http://seblagarde.wordpress.com/2012/09/29/image-based-lighting-approaches-and-parallax-corrected-cubemap/|title = Image-based Lighting approaches and parallax-corrected cubemap|date = 29 September 2012}}</ref>

Generally, cube mapping uses the same [[skybox (video games)|skybox]] that is used in outdoor renderings. Cube-mapped reflection is done by determining the [[vector (geometric)|vector]] that the object is being viewed at. This ''camera ray'' is reflected about the [[surface normal]] of where the camera vector intersects the object. This results in the ''reflected ray'' which is then passed to the [[cube map]] to get the [[Texel (graphics)|texel]] which provides the radiance value used in the lighting calculation. This creates the effect that the object is reflective.

===HEALPix mapping===
[[HEALPix]] environment mapping is similar to the other polyhedron mappings, but can be hierarchical, thus providing a unified framework for generating polyhedra that better approximate the sphere. This allows lower distortion at the cost of increased computation.<ref name="HEALPix">Tien-Tsin Wong, Liang Wan, Chi-Sing Leung, and Ping-Man Lam. [http://appsrv.cse.cuhk.edu.hk/~lwan/paper/sphquadmap/sphquadmap.htm  Real-time Environment Mapping with Equal Solid-Angle Spherical Quad-Map] {{Webarchive|url=https://web.archive.org/web/20071023212156/http://appsrv.cse.cuhk.edu.hk/~lwan/paper/sphquadmap/sphquadmap.htm |date=2007-10-23 }}, Shader X4: Lighting & Rendering, Charles River Media, 2006.</ref>

== History ==
In 1974, [[Edwin Catmull]] created an algorithm for "rendering images of bivariate surface patches"<ref name=":0">{{Cite journal |last=Blinn |first=James F. |last2=Newell |first2=Martin E. |date=October 1976 |title=Texture and reflection in computer generated images |url=https://dl.acm.org/doi/10.1145/360349.360353 |journal=Communications of the ACM |language=en |volume=19 |issue=10 |pages=542–547 |doi=10.1145/360349.360353 |issn=0001-0782}}</ref><ref>Catmull, E.A. Computer display of curved surfaces. Proc. Conf. on Comptr. Graphics, Pattern Recognition, and Data Structure, May 1975, pp. 11-17 (IEEE Cat. No. 75CH0981-1C).</ref> which worked directly with their mathematical definition. Further refinements were researched and documented by [[Bui Tuong Phong|Bui-Tuong Phong]] in 1975, and later [[James Blinn]] and [[Martin Newell (computer scientist)|Martin Newell]], who developed environment mapping in 1976; these developments which refined Catmull's original algorithms led them to conclude that "these generalizations result in improved techniques for generating patterns and texture".<ref name=":0" /><ref>{{Cite web|url=http://www.comphist.org/computing_history/new_page_6.htm|title=Computer Graphics|access-date=2007-01-09|archive-date=2021-02-24|archive-url=https://web.archive.org/web/20210224203858/http://www.comphist.org/computing_history/new_page_6.htm|url-status=dead}}</ref><ref>{{Cite web|url=http://www.debevec.org/ReflectionMapping/|title = Reflection Mapping History}}</ref>

Gene Miller experimented with spherical environment mapping in 1982 at [[Mathematical Applications Group|MAGI]].

Wolfgang Heidrich introduced Paraboloid Mapping in 1998.<ref>Heidrich, W., and H.-P. Seidel. "View-Independent Environment Maps". Eurographics Workshop on Graphics Hardware 1998, pp. 39–45.</ref>

Emil Praun introduced Octahedron Mapping in 2003.<ref>Emil Praun and Hugues Hoppe. "Spherical parametrization and remeshing". ACM Transactions on Graphics, 22(3):340–349, 2003.</ref>

Mauro Steigleder introduced Pyramid Mapping in 2005.<ref>Mauro Steigleder. "Pencil Light Transport". A thesis presented to the University of Waterloo, 2005.</ref>

Tien-Tsin Wong, et al. introduced the existing [[HEALPix]] mapping for rendering in 2006.<ref name="HEALPix"/>

== See also ==
* [[Skybox (video games)]]

== References ==
{{reflist|2}}

== External links ==
* [http://www.pauldebevec.com/ReflectionMapping/ The Story of Reflection mapping] by [[Paul Debevec]]
* [http://www.nvidia.com/object/feature_cube.html NVIDIA's paper] Cube Environment Mapping
* {{usurped|1=[https://archive.today/20140702043618/http://sunandblackcat.com/tipFullView.php?l=eng&topicid=16 Approximation of reflective and transparent objects with environmental maps]}}
{{Texture mapping techniques}}

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[[Category:3D rendering]]
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