[stoeckley]

I don't know the history, but there is supposedly a fixed set of Porter Duff blend modes, specific algorithms. For example, this C++ library:

> Blend2D supports all Porter & Duff compositing operators

https://blend2d.com/

Also, the link shared by one of the sibling comments can be searched for "Porter" to see how they are referenced:

https://drafts.fxtf.org/compositing/