Text versus Image Subtitles: Why PGS Runs into Trouble on the Way into MP4
There are two fundamentally different ways to store subtitles. A text subtitle keeps lines of words plus the times they appear, and the playback side draws them with whatever font its device has. An image subtitle keeps pictures that were already laid out, outlined and coloured at authoring time — PGS on Blu-ray and DVD subtitles work this way — and the player only pastes the right picture over the frame at the right moment. That difference decides whether a subtitle can live inside an MP4: MP4 subtitle tracks are built around text, so an image subtitle has nowhere to go as it is. That is why this kind of subtitle needs a step before it can be stored: the pictures are read, and the lines they show are written out as text.
This is the most common explanation for "the subtitles were there, and after conversion they are not": the file is not broken and the conversion did not fail — the tool in use simply had no track that a picture can live in. Where the pictures are read back into text instead, which is what CineX does, you get a subtitle track rather than nothing, and that track can contain recognition mistakes. For the overall order of checks, see what happens to subtitles after a CineX conversion.
1. Text subtitles store words and times
A text subtitle is timed plain text. It carries content and timing; size, font and placement are decided at playback. That makes it small, switchable and re-layable on another device, and it also means different devices can show the same file slightly differently.
SRT and ASS both belong here and are what you most often meet in local files. ASS carries more styling information than SRT, so on conversion its text is kept while the styling parameters are handled into a more general form rather than passed through unchanged.
A text subtitle is words plus time; how it finally looks is up to whatever is playing it.
2. Image subtitles store pictures
Image subtitles were typeset during disc authoring. What ends up in the file is a sequence of images, so the player never needs to understand the words — it just overlays the right frame. That is why they look identical on every device, and also why they cannot be edited, cannot be resized, and fit poorly into containers built around text.
PGS is the image format used on Blu-ray, and DVD subtitles work the same way. When they appear inside an MKV, the container is not the problem; the subtitle being an image is.
3. Why PGS has trouble becoming part of an MP4
MP4 subtitle tracks are organised as text: one line of content per time range. An image subtitle is a sequence of pictures, and there is no matching way to store that in such a track. In theory only three routes remain: carry the pictures in some other track form (the combinations that accept this are limited), read the text back out of the pictures (the result cannot match the original layout), or leave the subtitle out during conversion.
CineX takes the second of these. So if what you care about is a source that only has image subtitles, you want to know before converting what you will actually get back: recognised text, which can contain mistakes, rather than the original pictures and their layout.
An image subtitle only reaches MP4 after it has been read as text; where one goes missing, that is a mismatch between subtitle type and container, not an error during conversion.
4. How to tell which kind you have
Look at the subtitle entries in the file's media information. A few practical clues:
- Names such as SRT or ASS point at text; names such as PGS, VobSub or DVD point at images.
- One MKV often carries several subtitle tracks, some text and some image, at the same time.
- Two tracks with the same language tag are not guaranteed to hold the same content — one may be simplified Chinese and another traditional or a different language.
Audio is judged the same way: the advice in container versus codec — read what is inside rather than the extension — applies to subtitles too.
5. Three things worth doing
First, check the subtitle type before converting; it decides whether you should expect subtitles in the result. Second, if a source only has image subtitles, the conversion reads them and writes a text track — recognition can be wrong, so give that track a pass of your own. Third, renaming the extension changes nothing about the subtitle type: a picture subtitle does not become text because the filename says so.
6. What CineX actually does with subtitles
The subtitle inputs verified on real devices with actual samples are SRT and ASS, including bilingual and multi-language tracks; those stay in the output MP4 as text subtitles. Image-based Blu-ray subtitles (PGS) are scanned and recognised as text, and that text is written into the finished file as a subtitle track; recognition can make mistakes. Real-device readings exist for PGS only, so no other image-subtitle form is described as verified.
What survives as-is is text. An image subtitle has no matching track in the output until it has been read, and that step is stated up front instead of being something you discover afterwards.
Frequently asked questions
The source had subtitles. Why does the converted file not?
Most likely they were image subtitles: stored as pictures, which an MP4 subtitle track cannot hold as they are, so they have to be read as text before they can reach the finished file. Text subtitles such as SRT and ASS keep their content directly.
Can an image subtitle be turned into a usable one?
By the recognition step: the pictures are read back into text and written as a subtitle track. The result cannot match the original layout, so the track is worth checking yourself. If you would rather not check anything, use material that carries a text subtitle, or play the original file.
Does ASS keep its styling?
The text is kept; the styling parameters are not passed through unchanged. The extra font and position information in ASS is handled into a more general form on output.
Are all subtitle tracks kept?
Multiple text tracks, including bilingual and multi-language ones, are what the verified samples cover. If an image subtitle sits in the same file, that track goes through recognition instead.
Axiom One LLC — CineX. Figures current as of 24 September 2026.