Editing Ray tracing (graphics) (section)

===Reversed direction of traversal of scene by the rays===
The process of shooting rays from the eye to the light source to render an image is sometimes called ''backwards ray tracing'', since it is the opposite direction photons actually travel. However, there is confusion with this terminology. Early ray tracing was always done from the eye, and early researchers such as James Arvo used the term ''backwards ray tracing'' to mean shooting rays from the lights and gathering the results. Therefore, it is clearer to distinguish ''eye-based'' versus ''light-based'' ray tracing.

While the direct illumination is generally best sampled using eye-based ray tracing, certain indirect effects can benefit from rays generated from the lights. [[Caustic (optics)|Caustics]] are bright patterns caused by the focusing of light off a wide reflective region onto a narrow area of (near-)diffuse surface. An algorithm that casts rays directly from lights onto reflective objects, tracing their paths to the eye, will better sample this phenomenon. This integration of eye-based and light-based rays is often expressed as bidirectional path tracing, in which paths are traced from both the eye and lights, and the paths subsequently joined by a connecting ray after some length.<ref>{{cite journal | url = http://www.graphics.cornell.edu/~eric/Portugal.html | title = Bi-Directional Path Tracing | author = Eric P. Lafortune and Yves D. Willems | journal = Proceedings of Compugraphics '93 | date = December 1993 | pages = 145–153}}</ref><ref>{{cite web | url = https://old.cescg.org/CESCG98/PDornbach/paper.pdf | title =  Implementation of bidirectional ray tracing algorithm | author = Péter Dornbach | access-date = 2008-06-11 |date=1998 }}</ref>

[[Photon mapping]] is another method that uses both light-based and eye-based ray tracing; in an initial pass, energetic photons are traced along rays from the light source so as to compute an estimate of radiant flux as a function of 3-dimensional space (the eponymous photon map itself). In a subsequent pass, rays are traced from the eye into the scene to determine the visible surfaces, and the photon map is used to estimate the illumination at the visible surface points.<ref>[http://graphics.ucsd.edu/~henrik/papers/photon_map/global_illumination_using_photon_maps_egwr96.pdf Global Illumination using Photon Maps] {{webarchive|url=https://web.archive.org/web/20080808140048/http://graphics.ucsd.edu/~henrik/papers/photon_map/global_illumination_using_photon_maps_egwr96.pdf |date=2008-08-08 }}</ref><ref>{{cite web| url = http://web.cs.wpi.edu/~emmanuel/courses/cs563/write_ups/zackw/photon_mapping/PhotonMapping.html| title = Photon Mapping - Zack Waters<!-- Bot generated title -->}}</ref> The advantage of photon mapping versus bidirectional path tracing is the ability to achieve significant reuse of photons, reducing computation, at the cost of statistical bias.

An additional problem occurs when light must pass through a very narrow aperture to illuminate the scene (consider a darkened room, with a door slightly ajar leading to a brightly lit room), or a scene in which most points do not have direct line-of-sight to any light source (such as with ceiling-directed light fixtures or [[torchiere]]s). In such cases, only a very small subset of paths will transport energy; [[Metropolis light transport]] is a method which begins with a random search of the path space, and when energetic paths are found, reuses this information by exploring the nearby space of rays.<ref>{{cite book |first1=Eric |last1=Veach |first2=Leonidas J. |last2=Guibas |chapter=Metropolis Light Transport |title=SIGGRAPH '97: Proceedings of the 24th annual conference on Computer graphics and interactive techniques |year=1997 |pages=65–76 |doi=10.1145/258734.258775 |isbn=0897918967 |s2cid=1832504 }}</ref>

[[File:PathOfRays.svg|thumb|Image showing recursively generated rays from the "eye" (and through an image plane) to a light source after encountering two [[diffuse surface]]s]]
To the right is an image showing a simple example of a path of rays recursively generated from the camera (or eye) to the light source using the above algorithm. A diffuse surface reflects light in all directions.

First, a ray is created at an eyepoint and traced through a pixel and into the scene, where it hits a diffuse surface. From that surface the algorithm recursively generates a reflection ray, which is traced through the scene, where it hits another diffuse surface. Finally, another reflection ray is generated and traced through the scene, where it hits the light source and is absorbed. The color of the pixel now depends on the colors of the first and second diffuse surface and the color of the light emitted from the light source. For example, if the light source emitted white light and the two diffuse surfaces were blue, then the resulting color of the pixel is blue.