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In page Alpha compositing:
"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[citation needed]. Bruce A. Wallace derived the same straight over operator based on a physical reflectance/transmittance model in 1981.[1] A 1984 paper by Thomas Porter and Tom Duff introduced premultiplied alpha using a geometrical approach.[2]
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