I'm not sure what engine it uses for OCR, but I recently had to convert some image-based subtitles to text-based `.srt` format, and had quite good success using

I'm not sure what engine it uses for OCR, but I recently had to convert some image-based subtitles to text-based `.srt` format, and had quite good success using a tool named `subtitlecomposer` to do the conversion. During the initial import of the original image-based subtitles, the software stops on every character it does not immediately recognize and asks you what symbol it's looking at. It then builds up a "symbols" map by doing this over and over again, each time it does not recognize the image of a letter, number, or other symbol. This map file can then be renamed to match other subtitle files, in order to process other subtitles in the same font/typestyle. Eventually the symbol map ends up so complete that entire subtitles get converted automagically on import, without ever needing to ask the user any further questions.