Capturing subtitles

Even if this question may have already been answered under a different context, better get my own thread before someone accuses me of stealing a thread.

There's a film I already downloaded that I can't find any subtitles for on the web.

As I already have a DVD I made from my VHS copy of that film, I was thinking of finding a way to capture the subtitles that tape had.

Apparently the subtitle-capturing programs I have seen use some form of OCR, don't they? If they do then it's only a matter of feeding the video images of the film and subtitles for it to get them. Is that so?