Sup files are subtitle files that contain pictures of text. Televisions can show these pictures and not have to worry about text encoding or fonts. The downside to this is that the text in the pictures can't be changed, and that computers usually can't display the subtitles.
This tool uses optical character recognition (OCR) to read the text in the pictures, and converts the file to srt. You need to select the language of your sup file for the OCR to work correctly. If you're not sure what language your sup file is, just give English a try and see if it works.
This tool supports three different sup formats: bluray, hddvd and normal dvd. I have made the code for reading these formats open source, you can help improve it.