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## Summary
This fixes transparent images showing a black background. Image
thumbnails were always JPEG, which can't store transparency, so a
transparent PNG or GIF upload, such as an agent avatar, rendered on
black wherever a client showed its thumbnail.
The relay now writes a lossy WebP thumbnail at `{sha256}.thumb.webp`,
which keeps transparency, alongside the existing `{sha256}.thumb.jpg`:
- **Encoding:** the WebP comes from libwebp through the `webp` crate,
built from vendored C source, so the relay needs no system library. A
GIF uses its first frame.
- **Upload responses:** the upload descriptor's `thumb`, and its
per-tenant rewrite, point to the WebP.
- **JPEG kept:** installed clients derive `.thumb.jpg` themselves, so
the relay keeps writing it until clients move to WebP with a JPEG
fallback. Existing images still have only their JPEG thumbnail.
- **Serving:** `/media/` serves both variants, each with its own content
type. Other `.thumb.*` extensions still return 404.
- **imeta:** a `thumb` may name either variant, and the relay checks
that the named object exists.
- **Storage sweep:** the bucket taxonomy counts both variants as the
image's thumbnail bytes instead of classifying `.thumb.webp` as unknown.
| 320px thumbnail | JPEG | PNG | lossy WebP (q80) |
|---|---|---|---|
| Transparent avatar (starter-team `fizz.png`) | 11.3 KB, black
background | 65.1 KB | 13.8 KB |
| Opaque avatar (`sprout-agent.png`) | 23.6 KB | 156.5 KB | 12.3 KB |
| Screenshot (`channel-thread.png`) | 13.4 KB | 62.5 KB | 6.4 KB |
Encoding a WebP thumbnail takes 3 to 20 ms.
### Related issue
Supersedes #8772, which served a PNG at the `.thumb.jpg` path.
### Testing
Red/green against a local relay on `:3000`, built from the branch under
test, with Postgres, Redis and MinIO from `docker compose`:
```bash
cargo test -p buzz-test-client --test e2e_media test_transparent_png_thumbnail_is_webp_with_alpha -- --ignored
cargo test -p buzz-test-client --test e2e_media_extended test_ws_imeta_webp_thumb -- --ignored
```
- **On `main`:** both tests fail. The descriptor's `thumb` is
`.thumb.jpg`, and the WebSocket send is rejected with `invalid: imeta
thumb must be a local .thumb.jpg path`.
- **On this branch:** both tests pass:
- Uploading a half-transparent PNG returns a `thumb` ending
`.thumb.webp`.
- HEAD and GET on that URL return `Content-Type: image/webp`.
- The decoded thumbnail keeps alpha 0 on the transparent half and alpha
255 on the opaque half.
- `.thumb.jpg` still returns `image/jpeg`.
- A kind 9 message whose imeta `thumb` names the `.thumb.webp` is
accepted.
## Screenshots
The starter-team `fizz.png` avatar uploaded to a relay built from this
branch. Both thumbnails are shown as round avatars beside the original
PNG. `.thumb.jpg` fills the transparent area with a solid disc, and
`.thumb.webp` keeps the transparency.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
---------
Signed-off-by: John Matthew Tennant <jtennant@squareup.com>
Co-authored-by: Simone <0828be588d5c9c4d092c5b1cf01761d5aef63d6b104807fd8e8284d517dd1a99@buzz.block.builderlab.xyz>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
