Matroska Attachments
Matroska attachments embed supporting files such as fonts and cover images alongside the timed media tracks.
A subtitle sign that fitted neatly in one player can wrap or shift in another even when its text and position commands are identical. The missing information may be the font file, carried as a Matroska attachment.
An attachment is an embedded supporting resource rather than a timed media stream. Matroska supplies its filename, media type, identifying information, and bytes. The renderer or library decides how to use that resource.
| Attached resource | Consumer |
|---|---|
| TrueType/OpenType font | Subtitle renderer needing its faces and metrics |
| Cover image | Software using embedded artwork |
| Other file | A consumer implementing that particular resource use |
From a named typeface to positioned text
A styled subtitle requests a font family, style, size, and positioning. The renderer finds an appropriate font face and uses its glyph shapes and metrics to calculate text layout. The attachment's filename is not necessarily the same as the family name embedded inside the font.
For an invented layout example, a line has 110 pixels of available width. One font's glyph advances total 96 pixels; a fallback's total 120. The first can remain on one line while the second needs wrapping or a different layout. That change comes from font metrics, not from a different subtitle sentence.
Embedding the intended font gives a renderer a way to reproduce the requested face without relying on a separate system installation. The result still follows the renderer's layout and supported style commands.
Fonts at the app or server subtitle renderer
In a route where the Emby app renders subtitles, the subtitle data and usable supporting resources need to reach that rendering stage. In a server burn-in route, the server's renderer needs them before it combines the result with the pictures. The attachment is needed at the consumer, not merely somewhere earlier in the system.
A remux selecting video, sound, and subtitles can omit attachments if they are not selected or supported. The video codec and subtitle text can survive while the intended font disappears. Comparing only the timed track list will miss that change.
Cover artwork likewise lives at a different layer from the library's stored poster choices. An image attached to the movie and an image chosen for a library role can both exist without being the same record.
To explain a changed subtitle, compare the requested family/style, available attachments, fallback chosen, and the location of rendering. It follows the information that actually determines the layout.