PGS, VOBSUB, SUP, SUB/IDX: image-based vs text-based subtitles explained

If you have ever ripped a disc and wondered why you can edit some subtitle files in a text editor but not others, the answer comes down to one distinction: image-based vs text-based subtitles. This guide explains the formats you’ll run into — PGS, VOBSUB, .sup, .sub/.idx, SRT, ASS, WebVTT — and which ones need OCR.

The core difference

• Text-based subtitles store the actual words. A computer already knows what they say, so you can open, edit, translate, and re-time them directly.
• Image-based subtitles store a picture of each line, drawn over the video. The words only exist as pixels, so getting editable text requires OCR (optical character recognition) to read the images first.

That single difference decides whether a subtitle track is instantly editable or needs a conversion step.

Image-based formats (need OCR)

PGS (Blu-ray) — .sup

PGS stands for Presentation Graphic Stream, the subtitle format on Blu-ray discs. Each subtitle is a bitmap shown over the film. When ripped, a PGS track often lands as a standalone .sup file, or rides inside an MKV as a PGS track. Because it’s images, it needs OCR to become text.

VOBSUB (DVD) — .sub / .idx

VOBSUB is the DVD equivalent: bitmap “sub-pictures” stored in a .sub file, with a small .idx file holding the timing and metadata. Same idea as PGS, just from DVD — and like PGS, it needs OCR.

Text-based formats (no OCR needed)

• SRT (SubRip) — the simplest and most widely supported: plain text lines with start/end timestamps. This is the format most editors and players expect.
• ASS/SSA (SubStation Alpha) — text with rich styling (fonts, positioning, karaoke). Common in anime fansubs.
• WebVTT — the web captioning format, used by HTML5 video.

All three are already text, so they can be pulled straight out of an MKV with no recognition step.

So which do you have — and what next?

• Blu-ray / PGS / .sup → it’s images; convert it with OCR. See how to convert PGS subtitles to SRT.
• DVD / VOBSUB / .sub+.idx → also images; see VOBSUB to SRT.
• SRT / ASS / WebVTT → already text; see extract text subtitles from an MKV, free.
• Not sure? Start with how to extract subtitles from an MKV — it detects which kind you have.

PGSExtract handles both worlds: image tracks via OCR in 100+ languages, and text tracks pulled straight out for free, all in your browser. Read the docs for the full pipeline.

Try it free — free preview, no account needed.