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feat(subtitle): read scc closed captions and convert them before muxing

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SCC goes straight to --sub-format when it is SRT, WebVTT or TTML, else to SRT; pycaption reads the CC1 channel, and an SCC with no CC1 captions warns. Raise the pycaption floor to 2.2.24 for its SCC reader fixes.
This commit is contained in:
imSp4rky
2026-11-13 14:09:16 -07:00
parent 7f91a1d36b
commit 94dfb5d677
8 changed files with 118 additions and 10 deletions
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docs/guide/subtitles.md
+11 -2
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@@ -28,6 +28,7 @@ shown in track listings. The **Extension** column is the file extension used on
| `MicroDVD` | SUB | `sub` | MicroDVD |
| `MPL2` | MPL2 | `mpl2` | MPL2 |
| `TMP` | TMP | `tmp` | TMP |
| `SCC` | SCC | `scc` | Scenarist Closed Captions (CEA-608), read only |
| `fTTML` | STPP | `stpp` | MPEG-DASH box-encapsulated TTML (IMSC1) |
| `fVTT` | WVTT | `wvtt` | MPEG-DASH box-encapsulated WebVTT |
@@ -65,6 +66,13 @@ format-appropriate clean-up to make a single, valid file:
output format, unshackle automatically converts any plain TTML track to WebVTT before
muxing (the closest lossless equivalent). Boxed `fTTML` is likewise unwrapped to TTML
first.
- **SCC for muxing**: Matroska cannot carry SCC, and no later step can read it. Before
muxing, unshackle converts an SCC track straight to the `--sub-format` you request
when that is SRT, WebVTT or TTML, and to SRT in all other cases (no `--sub-format`,
`original`, or another format, which unshackle then makes from the SRT). Choose
`--sub-format vtt` to keep the caption rows: WebVTT keeps a top-of-screen caption at
the top, SRT does not. unshackle reads only the CC1 channel. An SCC file with no CC1
captions gives an empty track and a warning.
You normally do not need to think about any of this. It gives a clean file.
@@ -302,8 +310,9 @@ styling that survives. `—` means that the backend cannot convert that pair.
Reading the table:
- **Keep the original**: leaving `--sub-format` unset (or set to `original`) never
round-trips the file, so every style survives. Only convert when your player cannot
- **Keep the original**: leaving `--sub-format` unset (or set to `original`) does not
round-trip the file, so every style survives. TTML and SCC are the exceptions, as noted
above. Only convert when your player cannot
read the source format.
- **`subtitleedit`** (SubtitleEdit / `seconv`): the best choice for anything → SRT. It
is the only backend that carries colour, and it embeds `{\an8}` tags to preserve