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MSC2530: Body field as media caption #2530

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Merged
merged 11 commits into from
Feb 19, 2024
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MSC2530: Body field as media caption #2530

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a72bf5e
Proposal to use body field as media caption
tulir May 7, 2020
6691780
Add paragraph about relation-based captions being difficult for bridges
tulir May 7, 2020
2cf7abb
Clarify how to treat body when filename is not present
tulir May 8, 2020
a7047f8
Refactor proposal text
tulir Oct 19, 2022
aebe61a
Fix heading size
tulir Oct 19, 2022
76d5b54
Add problem statement
tulir Jan 23, 2024
675198d
Add links to and quotes from current spec
tulir Jan 23, 2024
5b3d74e
Adjust wording and quote m.audio body spec
tulir Jan 23, 2024
cca27fd
Clarify that m.location and m.sticker are out of scope for this proposal
tulir Jan 23, 2024
ec23050
Add examples and summary of changes
tulir Feb 6, 2024
cbcd0dc
Fix JSON syntax in example
tulir Feb 6, 2024
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43 changes: 43 additions & 0 deletions proposals/2530-body-as-caption.md
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# Body field as media caption

This is an alternative to [MSC2529](https://github.com/matrix-org/matrix-doc/pull/2529).

## Proposal

This proposal defines a new field `filename` and allows the `format` and
`formatted_body` fields (from `m.text`) for all media msgtypes (`m.image`,
`m.audio`, `m.video`, `m.file`).

If the `filename` field is present in a media message, clients should treat
`body` as a caption instead of a file name. The `formatted_body` field should
also be supported and work the same way as they do in `m.text` messages.

## Comparison to MSC2529

While this approach isn't backwards-compatible like MSC2529, it's simpler to
implement as it doesn't require handling relations.

Implementing relation-based captions would be especially difficult for bridges.
It would require either waiting a predefined amount of time for the caption to
come through, or editing the message on the target platform (if edits are
supported).

Additionally, some clients already render the body (as a file name), so the
new-style caption would already be visible in those cases.

The format proposed by MSC2529 would also make it technically possible to use
other message types without changing the format of the events, which is not
possible with this proposal.

## Potential issues

The spec for `m.file` already defines a `filename` field, but the spec also
says `body` should contain the file name. This means old `m.file` messages
could be misinterpreted to contain a caption, even though it's just the file
name. To avoid this problem, we could require that `body` and `filename` must
be different for `body` to be interpreted as a caption.
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Another thing to note here is that body is required, so the filename is probably a reasonable fallback for body in the event that no description is given.


Like MSC2529, this would be obsoleted by [extensible events](https://github.com/matrix-org/matrix-doc/pull/1767).
In my opinion, unless extensible events are planned to be pushed through in the
near future, we shouldn't block basic features like captions on extensible
events hopefully eventually happening.