ffmpeg may fail if a valid subtitle file if used but the extension is set to its format

[djalilhebal]

The following works. This is the main logic of the API (v0.0.1): Using the subtitle file's extension as input format.

ffmpeg -f srt unnamed output.vtt

This should work

# Moving forward, we will refer to this resource as talk-1-en
ffmpeg -i https://www.ted.com/talks/subtitles/id/1/lang/en output.vtt

curl -o 1.json https://www.ted.com/talks/subtitles/id/1/lang/en
ffmpeg -i 1.json output.vtt

But the current approach would result in something like the following and fail:

ffmpeg -f json -i 1.json output.vtt

talk-1-en is:

• A JSON file.
• The server specifies its type as Content-Type: application/json; charset=utf-8.
• FFprobe (and FFmpeg) recognize that it is actually of the format tedcaptions, whether the input is from the URL, 1.json, or an extensionless file.

---

We could simply not specify the format and let ffmpeg guess its type from the input, but some malformed(?) files like sintel-fr will fail to be processed. If I understand it correctly, sintel-fr has an incorrect magic number. Specifying the filename helps ffmpeg (and VLC) recognize the file type and process it.

Maybe it's not something we should bother with?

[djalilhebal]

We may only know the filename and ext, but we can't assume its format.

Goal: Make fluent-ffmpeg set the input filename even though it's just a stream.

• [x] Use Node's URL.createObjectURL

  • It is experimental and the obj is only available to the current thread.
  • See URL | Node.js v21.2.0 Documentation

• [x] Node in-memory files? Or Node named pipes? Apparently, Node does not support either of them natively.

• [ ] Vercel supports using the FS or temp files? Yes, seems doable. "you can use the /tmp folder available with serverless functions." See How to Use Files in Serverless Functions on Vercel.

Idea: Since we are working with "real" files, why not calculate the checksum (e.g. SHA-1) of the input file and use it as ETag for the response? You know, caching and stuff.

Much ado about nothing?