I don't missed it, I ignored it on purpose, for two reasons:

One, I did not understood it well enough to try to explain even the basics.

Two, I could not find a binary or source I could compile easily to make a example image.

Also you can put a "three" there: it is not widely used, while even the recent xBR has a couple of plug-in implementations on emulators for example (and has binaries)

EDIT: Lack of binary is also why I did not made sections for 2xSal, SuperEagle and so on... If people help me finding that kind of stuff or creating the needed images, I will gladly update the article.

This looks like a FOSS implementation of Depixelizing Pixel Art - https://github.com/jerith/depixel

That's probably my favorite research paper ever. The web link in the PDF is offline, but it looks like they moved everything to the Microsoft site -- http://research.microsoft.com/en-us/um/people/kopf/pixelart/. I just spent a good 15 minutes mousing over the image comparisons again