ImageMagick, an image processing library developed with multiple versions to satisfy different programme languages.
http://www.imagemagick.org/ is the official website of ImageMagick. Online image processing is available.
http://www.imagemagick.org/discourse-server/viewtopic.php?t=23046
This thread pointed out ImageMagick has developed a number of image scaling functions, such as SuperEagle, Super2xSaI, Hqx, xBR, etc.