Editing Shading (section)

=====Other approaches=====
Both [[Gouraud shading]] and [[Phong shading]] can be implemented using [[bilinear interpolation]]. Bishop and Weimer <ref>Gary Bishop and David M. Weimer. 1986. Fast Phong shading. ''SIGGRAPH Comput. Graph''. 20, 4 (August 1986), 103–106.</ref> proposed to use a [[Taylor series]] expansion of the resulting expression from applying an [[illumination model]] and bilinear interpolation of the normals. Hence, second-degree [[polynomial interpolation]]  was used. This type of biquadratic interpolation was further elaborated by Barrera et al.,<ref>T. Barrera, A. Hast, E. Bengtsson. [http://www.cb.uu.se/~aht/articles/a03.pdf Fast Near Phong-Quality Software Shading]. ''WSCG'06'', pp. 109–116. 2006</ref> where one second-order polynomial was used to interpolate the diffuse light of the [[Phong reflection model]] and another second-order polynomial was used for the specular light.

Spherical linear interpolation ([[Slerp]]) was used by Kuij and Blake<ref>Kuijk, A. A. M. and E. H. Blake, [http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.109.2663&rep=rep1&type=pdf Faster Phong shading via angular interpolation]. ''Computer Graphics Forum'' 8(4):315–324. 1989 ([[PDF]])</ref> for computing both the normal over the polygon, as well as the vector in the direction to the light source. A similar approach was proposed by Hast,<ref>A. Hast. [http://www.cb.uu.se/~aht/articles/quat.pdf Shading by Quaternion Interpolation]. ''WSCG'05''. pp. 53–56. 2005.</ref> which uses [[quaternion]] interpolation of the normals with the advantage that the normal will always have unit length and the computationally heavy normalization is avoided.