Editing Alpha compositing (section)

== History ==
The concept of an alpha channel was introduced by [[Alvy Ray Smith]] and [[Ed Catmull]] in the late 1970s at the [[New York Institute of Technology Computer Graphics Lab]]. Bruce A. Wallace derived the same straight '''over''' operator based on a physical [[reflectance]]/[[transmittance]] model in 1981.<ref name=Wallace>{{cite conference |last=Wallace |first=Bruce A. |book-title=Proceedings of the 8th annual conference on Computer graphics and interactive techniques - SIGGRAPH '81 |title=Merging and transformation of raster images for cartoon animation |date=1981 |location=New York City, New York |publisher=ACM Press |volume=15 |issue=3 |pages=[https://archive.org/details/siggraph81confer15sigg/page/253 253–262] |citeseerx=10.1.1.141.7875 |doi=10.1145/800224.806813 |isbn=0-89791-045-1 |s2cid=1147910 |url-access=registration |url=https://archive.org/details/siggraph81confer15sigg/page/253 |doi-access=free }}</ref> A 1984 paper by [[Thomas Porter (Pixar)|Thomas Porter]] and [[Tom Duff]] introduced premultiplied alpha using a geometrical approach.<ref name=Duff>{{Cite conference|last1=Porter|first1=Thomas|author-link=Thomas Porter (Pixar)|last2=Duff|first2=Tom|author-link2=Tom Duff|book-title=Proceedings of the 11th annual conference on Computer graphics and interactive techniques - SIGGRAPH '84|title=Compositing digital images|date=July 1984|url=http://graphics.pixar.com/library/Compositing/paper.pdf|url-status=live |language=en|location=New York City, New York|publisher=ACM Press|volume=18|issue=3|pages=253–259|doi=10.1145/800031.808606|isbn=9780897911382|s2cid=18663039|archive-url=https://web.archive.org/web/20110429041428/http://graphics.pixar.com/library/Compositing/paper.pdf|archive-date=2011-04-29|access-date=2019-03-11}}</ref>

The use of the term ''alpha'' is explained by Smith as follows: "We called it that because of the [[Convex combination|classic linear interpolation formula]] <math>\alpha A + (1-\alpha)B</math> that uses the Greek letter <math>\alpha</math> (alpha) to control the amount of interpolation between, in this case, two images A and B".<ref>{{cite web|author=Alvy Ray Smith|date=1995-08-15|title=Alpha and the History of Digital Compositing|url=http://alvyray.com/Memos/CG/Microsoft/7_alpha.pdf|work=alvyray.com|page=6|archiveurl=https://web.archive.org/web/20211025162251/http://alvyray.com/Memos/CG/Microsoft/7_alpha.pdf|archivedate=2021-10-25}}</ref> That is, when compositing image A atop image B, the value of <math>\alpha</math> in the formula is taken directly from A's alpha channel.