Phong Shading
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3D computer graphics 3D computer graphics, or “3D graphics,” sometimes called CGI, 3D-CGI or three-dimensional computer graphics are graphics that use a three-dimensional representation of geometric data (often Cartesian) that is stored in the computer for th ...
, Phong shading, Phong interpolation, or normal-vector interpolation shading is an interpolation technique for surface
shading Shading refers to the depiction of depth perception in 3D models (within the field of 3D computer graphics) or illustrations (in visual art) by varying the level of darkness. Shading tries to approximate local behavior of light on the object's ...
invented by computer graphics pioneer
Bui Tuong Phong Bui Tuong Phong (December 14, 1942 – July 1975) was a Vietnamese-born computer graphics researcher and pioneer. He invented the widely used Phong shading algorithm and Phong reflection model. Life Phong was born in Hanoi, then French I ...
. Phong shading interpolates
surface normal In geometry, a normal is an object such as a line, ray, or vector that is perpendicular to a given object. For example, the normal line to a plane curve at a given point is the (infinite) line perpendicular to the tangent line to the curve at ...
s across
rasterized In computer graphics, rasterisation (British English) or rasterization (American English) is the task of taking an Digital image, image described in a vector graphics format (shapes) and converting it into a raster image (a series of pixels, dots ...
polygons and computes
pixel In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device. In most digital display devices, pixels are the smal ...
colors based on the interpolated normals and a reflection model. ''Phong shading'' may also refer to the specific combination of Phong interpolation and the
Phong reflection model The Phong reflection model (also called Phong illumination or Phong lighting) is an empirical model of the local illumination of points on a surface designed by the computer graphics researcher Bui Tuong Phong. In 3D computer graphics, it is somet ...
.


History

Phong shading and the
Phong reflection model The Phong reflection model (also called Phong illumination or Phong lighting) is an empirical model of the local illumination of points on a surface designed by the computer graphics researcher Bui Tuong Phong. In 3D computer graphics, it is somet ...
were developed at the
University of Utah The University of Utah (U of U, UofU, or simply The U) is a public research university in Salt Lake City, Utah. It is the flagship institution of the Utah System of Higher Education. The university was established in 1850 as the University of De ...
by
Bui Tuong Phong Bui Tuong Phong (December 14, 1942 – July 1975) was a Vietnamese-born computer graphics researcher and pioneer. He invented the widely used Phong shading algorithm and Phong reflection model. Life Phong was born in Hanoi, then French I ...
, who published them in his 1973 Ph.D. dissertation and a 1975 paper.Bui Tuong Phong, "Illumination for Computer Generated Pictures," Comm. ACM, Vol 18(6):311-317, June 1975. Phong's methods were considered radical at the time of their introduction, but have since become the de facto baseline shading method for many rendering applications. Phong's methods have proven popular due to their generally efficient use of computation time per rendered pixel.


Phong interpolation

Phong shading improves upon
Gouraud shading Gouraud shading, named after Henri Gouraud, is an interpolation method used in computer graphics to produce continuous shading of surfaces represented by polygon meshes. In practice, Gouraud shading is most often used to achieve continuous l ...
and provides a better approximation of the shading of a smooth surface. Phong shading assumes a smoothly varying surface normal vector. The Phong interpolation method works better than Gouraud shading when applied to a reflection model with small
specular highlight A specular highlight is the bright spot of light that appears on shiny objects when illuminated (for example, see image on right). Specular highlights are important in 3D computer graphics, as they provide a strong visual cue for the shape of a ...
s such as the Phong reflection model. The most serious problem with Gouraud shading occurs when specular highlights are found in the middle of a large polygon. Since these specular highlights are absent from the polygon's vertices and Gouraud shading interpolates based on the vertex colors, the specular highlight will be missing from the polygon's interior. This problem is fixed by Phong shading. Unlike Gouraud shading, which interpolates colors across polygons, in Phong shading, a normal vector is linearly interpolated across the surface of the polygon from the polygon's vertex normals. The surface normal is interpolated and normalized at each pixel and then used in a reflection model, e.g. the
Phong reflection model The Phong reflection model (also called Phong illumination or Phong lighting) is an empirical model of the local illumination of points on a surface designed by the computer graphics researcher Bui Tuong Phong. In 3D computer graphics, it is somet ...
, to obtain the final pixel color. Phong shading is more computationally expensive than Gouraud shading since the reflection model must be computed at each pixel instead of at each vertex. In modern graphics hardware, variants of this algorithm are implemented using pixel or fragment shaders.


Phong reflection model

''Phong shading'' may also refer to the specific combination of Phong interpolation and the
Phong reflection model The Phong reflection model (also called Phong illumination or Phong lighting) is an empirical model of the local illumination of points on a surface designed by the computer graphics researcher Bui Tuong Phong. In 3D computer graphics, it is somet ...
, which is an empirical model of local illumination. It describes the way a surface reflects light as a combination of the
diffuse reflection Diffuse reflection is the reflection (physics), reflection of light or other radiation, waves or particles from a surface such that a ray (optics), ray incident on the surface is scattering, scattered at many angles rather than at just one angle ...
of rough surfaces with the
specular reflection Specular reflection, or regular reflection, is the mirror-like reflection of waves, such as light, from a surface. The law of reflection states that a reflected ray of light emerges from the reflecting surface at the same angle to the surf ...
of shiny surfaces. It is based on
Bui Tuong Phong Bui Tuong Phong (December 14, 1942 – July 1975) was a Vietnamese-born computer graphics researcher and pioneer. He invented the widely used Phong shading algorithm and Phong reflection model. Life Phong was born in Hanoi, then French I ...
's informal observation that shiny surfaces have small intense
specular highlight A specular highlight is the bright spot of light that appears on shiny objects when illuminated (for example, see image on right). Specular highlights are important in 3D computer graphics, as they provide a strong visual cue for the shape of a ...
s, while dull surfaces have large highlights that fall off more gradually. The reflection model also includes an ''ambient'' term to account for the small amount of light that is scattered about the entire scene. See also *
List of common shading algorithms {{Short description, none This article lists common shading algorithms used in computer graphics. Interpolation techniques These techniques can be combined with any illumination model: * Flat shading * Gouraud shading * Phong shading Illuminatio ...
* Blinn–Phong shading model – Phong reflection model modified to trade precision with computing efficiency *
Flat shading Shading refers to the depiction of depth perception in 3D models (within the field of 3D computer graphics) or illustrations (in visual art) by varying the level of darkness. Shading tries to approximate local behavior of light on the object's ...
– shading of polygons with a single color *
Gouraud shading Gouraud shading, named after Henri Gouraud, is an interpolation method used in computer graphics to produce continuous shading of surfaces represented by polygon meshes. In practice, Gouraud shading is most often used to achieve continuous l ...
– shading of polygons by interpolating colors that are computed at vertices *
Phong reflection model The Phong reflection model (also called Phong illumination or Phong lighting) is an empirical model of the local illumination of points on a surface designed by the computer graphics researcher Bui Tuong Phong. In 3D computer graphics, it is somet ...
– reflection model often used with Phong shading *
Specular highlight A specular highlight is the bright spot of light that appears on shiny objects when illuminated (for example, see image on right). Specular highlights are important in 3D computer graphics, as they provide a strong visual cue for the shape of a ...
– other specular lighting equations


References

{{DEFAULTSORT:Phong Shading Computer graphics algorithms Shading Vietnamese inventions