Webp with alpha channel creates jpg thumbnails with no alpha.

[shangjiaxuan]

Description

As stated in title. Webp pictures with transparency uploaded are generated jpg thumbnails without transparency. This shows glitchy thumbnails when the client renders it. The location issuing this thumbnail generation have not been identified (dynamic thumbnails have not been configured, so it should be synapse default).

Element android client shows the shown glitchy image in thumbnail, while desktop shows the full image, other clients have different result (fluffychat on android shows the full image). If the thumbnails are in jpg because Element requested it in jpg, then this is a problem with the client, and will move this issue to Element.

Original image (text may be inappropriate for work)
Image info given by ffmpeg:

$ ffmpeg -i '_ _glowing star__20230924231343.webp'
<!--ffmpeg compilation info redacted -->
Input #0, webp_pipe, from '_ _glowing star__20230924231343.webp':
  Duration: N/A, bitrate: N/A
  Stream #0:0: Video: webp, yuva420p(tv, bt470bg/unknown/unknown), 512x443, 25 fps, 25 tbr, 25 tbn, 25 tbc

Image info on postgres database:

synapse=# select * from local_media_repository where media_id = 'vBHhuRaunxOHxqmmnehZTjpT';
         media_id         | media_type | media_length |  created_ts   |             upload_name              |          user_id          | quarantined_by | url_cache | last_access_ts | safe_from_quarantine
--------------------------+------------+--------------+---------------+--------------------------------------+---------------------------+----------------+-----------+----------------+----------------------
 vBHhuRaunxOHxqmmnehZTjpT | image/webp |        36654 | 1696332462446 | _ _glowing star__20230924231343.webp | <!-- redacted -->|                |           |  1696336266112 | f
(1 row)

Thumbnail info on postgres database:

synapse=# select * from local_media_repository_thumbnails where media_id = 'vBHhuRaunxOHxqmmnehZTjpT';
         media_id         | thumbnail_width | thumbnail_height | thumbnail_type | thumbnail_method | thumbnail_length
--------------------------+-----------------+------------------+----------------+------------------+------------------
 vBHhuRaunxOHxqmmnehZTjpT |             277 |              240 | image/jpeg     | scale            |            20974
 vBHhuRaunxOHxqmmnehZTjpT |              32 |               32 | image/jpeg     | crop             |             1267
 vBHhuRaunxOHxqmmnehZTjpT |             512 |              443 | image/jpeg     | scale            |            32720
 vBHhuRaunxOHxqmmnehZTjpT |              96 |               96 | image/jpeg     | crop             |             5349
(4 rows)

Glitchy thumbnail shown on Element Android (same as original image, text may be inappropriate for work)

Steps to reproduce

• Upload a webp image with transparency to chatroom
• See the thumbnail on Element chat context.

Homeserver

self-hosted: matrix.nepfan.club (manual account registration only)

Synapse Version

{"server_version":"1.64.0","python_version":"3.9.2"}

Installation Method

Debian packages from packages.matrix.org

Database

PostgreSQL

Workers

Single process

Platform

Debian GNU/Linux 11 (bullseye), running on virtual machine.

Configuration

No response

Relevant log output

Not crashing, only usage level inconvenience.

Anything else that would be useful to know?

No response

[clokep]

Duplicate of #11840.