New ask Hacker News story: Ask HN: What free software do you prefer for image upscaling?

Ask HN: What free software do you prefer for image upscaling?
3 by hnarn | 0 comments on Hacker News.
There are plenty of proprietary online resources for image upscaling, and they seem to do a decent job, especially for paintings or drawn content: examples include bigjpg.com, icons8.com etc. I'm curious what these services are using as a back-end, are there any free alternatives that they might be using, or are most of them proprietary?