MSC2448: Using BlurHash as a Placeholder for Matrix Media

proposals/2448-blurhash-for-media.md

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+# MSC2448: Using BlurHash as a Placeholder for Matrix Media
+
+[BlurHash](https://blurha.sh) is a compact representation of a placeholder
+for an image (or a frame of video). Currently in Matrix, clients must
+display a placeholder image in the message timeline while a piece of media is
+loading. Some clients, such as Element, simply display an empty space.
+
+While thumbnails exist to combat this to some degree, they still need to be
+downloaded from a homeserver, which is not instantaneous.
+
+Instead, a BlurHash can be sent inside the `m.room.message` event, which upon
+receipt other clients can render for a pretty preview while the actual
+thumbnail downloads. They also do not contain any `"` characters, making them
+simple to stick inside existing JSON blobs.
+
+To be clear: A BlurHash does not replace a thumbnail - it will be shown
+before the thumbnail is downloaded.
+
+## Proposal
+
+### m.room.message
+
+An optional field is added in `m.room.message`'s `content.info` dictionary
+with the key `blurhash`. It is a BlurHash of the original piece of media.
+Clients could then render this using [one of the available BlurHash
+implementations](https://github.com/woltapp/blurhash).
+
+This would be optionally displayed while the thumbnail of the media is loaded
+in parallel.
+
+Example `m.room.message` content:
+
+```json
+{
+  "body": "image.png",
+  "info": {
+    "size": 149234,
+    "mimetype": "image/png",
+    "thumbnail_info": {
+      "w": 301,
+      "h": 193,
+      "mimetype": "image/png",
+      "size": 72958
+    },
+    "w": 301,
+    "h": 193,
+    "thumbnail_url": "mxc://example.org/abcdefg",
+    "blurhash": "JadR*.7kCMdnj"
+  },
+  "msgtype": "m.image",
+  "url": "mxc://example.org/abcde",
+}
+```
+
+Note that a BlurHash representation is really only applicable to media, and
+as such should only be used in conjunction with the following
+`m.room.message` msgtypes:
+
+* `m.image`
+* `m.video`
+
+### m.sticker
+
+An optional field is added to `m.sticker`'s `content.info` dictionary with
+the key `blurhash`. Its value is a BlurHash of the sticker media.
+
+Example `m.sticker` content:
+
+```json
+{
+  "body": "Landing",
+  "info": {
+    "h": 200,
+    "mimetype": "image/png",
+    "size": 73602,
+    "thumbnail_info": {
+      "h": 200,
+      "mimetype": "image/png",
+      "size": 73602,
+      "w": 140
+    },
+    "thumbnail_url": "mxc://matrix.org/sHhqkFCvSkFwtmvtETOtKnLP",
+    "w": 140,
+    "blurhash": "JadR*.7kCMdnj"
+  },
+  "url": "mxc://matrix.org/sHhqkFCvSkFwtmvtETOtKnLP"
+}
+```
+
+### m.room.avatar
+
+Room avatars having BlurHashes available will be especially useful when
+viewing a server's Public Rooms directory.
+
+An optional field is added to `m.room.avatar`'s `content.info` dictionary with the
+key `blurhash`. Its value is a BlurHash of the media that is pointed to by
+`url`.
+
+Example `m.room.avatar` content:
+
+```json
+{
+  "url": "mxc://amorgan.xyz/a59ee02f180677d83d1b57d366127f8e1afdd4ed",
+  "info": {
+    "blurhash": "JadR*.7kCMdnj"
+  }
+}
+```
+
+### m.room.member
+
+Much like room avatars, user avatars can have BlurHashes as well. There is a
+little more required to implement this, but the outcome of no longer having
+missing avatars upon opening a room is worthwhile.
+
+An optional field is added to `m.room.member`'s `content` dictionary with
+the key `blurhash`. Its value is a BlurHash of the media that is pointed
+to by `avatar_url`.
+
+Note that `blurhash` MUST be omitted if `avatar_url` is not present.
+
+Example `m.room.member` event content:
+
+```json
+{
+  "avatar_url": "mxc://example.org/SEsfnsuifSDFSSEF",
+  "displayname": "Alice Margatroid",
+  "membership": "join",
+  "blurhash": "JadR*.7kCMdnj"
+}
+```
+
+### Profile endpoints
+
+Endpoints that return profile information, and thus MXC URLs to user avatars, are
+extended to optionally include BlurHashes as well.
+
+[`GET /_matrix/client/r0/profile/{userId}`](https://matrix.org/docs/spec/client_server/r0.6.1#get-matrix-client-r0-profile-userid) has an optional field added with
+the key `blurhash`. Its value is a BlurHash of the media that is pointed to
+by `avatar_url`. `blurhash` MUST be omitted if `avatar_url` is not present.
+The same applies to [`GET /_matrix/client/r0/profile/{userId}/avatar_url`](https://matrix.org/docs/spec/client_server/r0.6.1#get-matrix-client-r0-profile-userid-avatar-url), and to the federation endpoint [`GET /_matrix/federation/v1/query/profile`](https://matrix.org/docs/spec/server_server/r0.1.4#get-matrix-federation-v1-query-profile).
+
+[`PUT /_matrix/client/r0/profile/{userId}/avatar_url`](https://matrix.org/docs/spec/client_server/r0.6.1#put-matrix-client-r0-profile-userid-avatar-url) has an optional field added
+to the request body with the key `blurhash`. Its value is a BlurHash of the media that is pointed to by the value of the `avatar_url` field in the same request.
+
+### URL previews
+
+An optional attribute is added to the OpenGraph data returned by a call
+to
+[`GET /_matrix/media/r0/preview_url`](https://matrix.org/docs/spec/client_server/r0.6.1#get-matrix-media-r0-preview-url)
+called `matrix:image:blurhash`. The value
+of this attribute is the blurhash representation of the media specified
+by `og:image`. Note that we place this value under the `matrix` namespace as
+the [OpenGraph protocol](https://ogp.me/) does not (yet) have a spec'd field
+for BlurHashes.
+
+Note that `matrix:image:blurhash` MUST be omitted if `og:image` is not present
+in the response.
+
+Example response to `GET /_matrix/media/r0/preview_url`:
+
+```json
+{
+  "og:title": "Matrix Blog Post",
+  "og:description": "This is a really cool blog post from matrix.org",
+  "og:image": "mxc://example.com/ascERGshawAWawugaAcauga",
+  "og:image:type": "image/png",
+  "og:image:height": 48,
+  "og:image:width": 48,
+  "matrix:image:size": 102400,
+  "matrix:image:blurhash": "oyp8ky2BWn7VHEL"
+}
+```
+
+### Inline images
+
+An optional attribute is added to `<img>` tags in messages:
+`data-mx-blurhash`, where the value of the attribute is the blurhash
+representation of the inline image.
+
+This would be optionally displayed while the inline image itself is loaded in
+parallel.
+
+Example `m.room.message.formatted_body`:
+
+```
+"formatted_body": This is awesome <img alt=\"flutterjoy\" title=\"flutterjoy\" height=\"32\" src=\"mxc://matrix.example.org/abc\" data-mx-blurhash=\"LEHV6nWB2yk8pyo\" />
+```
+
+## Calculating a blurhash on the server
+
+BlurHashes are inserted into events by the client, however some clients may not
+be able to implement the BlurHash library for whatever reason. In this case, it
+would be nice to allow the media repository to calculate the BlurHash of a piece
+of media for the client, similar to how thumbnails are calculated by media
+repositories today.
+
+A new boolean query parameter, `generate_blurhash`, is added to
+[`/_matrix/media/r0/upload`](https://matrix.org/docs/spec/client_server/r0.6.0#post-matrix-media-r0-upload)
+which allows clients to ask the server to generate a BlurHash for them. This
+may be useful for clients that are designed to run on very low-power hardware,
+or otherwise cannot generate a BlurHash itself.
+If set to `true`, the server SHOULD generate a BlurHash of the uploaded media
+and return it as the value of the `blurhash` key in the response.
+
+If the server cannot generate a BlurHash of the media - perhaps because it is
+not a file that a BlurHash can be derived from or it is too expensive to process
+a very large piece of media - then the server SHOULD NOT return a `blurhash` key
+in the response. Additionally, if `generate_blurhash` is not `true`, then the
+server SHOULD NOT return a `blurhash` key in the response.
+
+Fundamentally, this means that clients SHOULD NOT assume that a server will always
+return a BlurHash in the response to `/_matrix/media/r0/upload`, even if they have
+set the `generate_blurhash` query parameter to `true` in the request.
+
+An example request from a client that would like the server to generate a
+blurhash would look like:
+
+```
+POST /_matrix/media/r0/upload?generate_blurhash=true&filename=My+Family+Photo.jpeg HTTP/1.1
+Content-Type: Content-Type: image/jpeg
+
+<bytes>
+```
+
+Example response:
+
+```
+{
+  "content_uri": "mxc://example.com/abcde123",
+  "blurhash": "LKO2?U%2Tw=w]~RB"
+}
+```
+
+We explicitly make this behaviour opt-in as it is assumed that the majority of
+clients that end up supporting BlurHashes will be capable of generating them
+locally. Thus the less load we can put on the homeserver, (by not making
+blurhash generation the default), the better.
+
+Note that media servers will not be able to return a BlurHash string for
+encrypted media; that must be left to the client.
+
+The server could additionally return the BlurHash string for an image when
+given an MXC URL. This would be through something like the Media Information
+API (specified in
+[MSC2380](https://github.com/matrix-org/matrix-doc/pull/2380)), or similar.
+
+## Visualisation
+
+Viewing an image message that is loading:
+
+![blurhashed preview](images/2448-blurhash.png)
+
+Once the image loads:
+
+![the image has loaded](images/2448-loaded-image.png)
+
+For reference, the current state of things in Element is:
+
+![boo, sad](images/2448-current-state.png)
+
+## Alternatives
+
+We could include a base64 thumbnail of the image in the event, but blurhash
+produces much more efficient textual representations.
+
+## Backwards compatibility
+
+Older clients would ignore the new `blurhash` parameter.
+
+Newer clients would only show it if it exists.
+
+Users who have not specified `blurhash` in their `m.room.member` event yet may
+stand out from users who have while both are loading. This is entirely up to
+clients to handle, though a suggestion may be to "fake" a blurhash by
+blurring some placeholder image (perhaps something with variation between
+users like Element's coloured backgrounds with letters in them, or [an
+identicon](https://en.wikipedia.org/wiki/Identicon) derived from the user's
+ID) until the user's actual avatar loads.
+
+## Unstable prefixes
+
+Implementations wishing to add this before this MSC is merged can do so with
+the following:
+
+* The `blurhash` key in any events, request or response bodies should be
+replaced with `xyz.amorgan.blurhash`.
+
+* `/_matrix/media/r0/upload`'s new `generate_blurhash` query parameter
+should instead be `xyz.amorgan.generate_blurhash`. And instead of the
+key `blurhash`, the endpoint should return `xyz.amorgan.blurhash`.
+
+* The `data-mx-blurhash` attribute in `<img>` tags should be replaced with
+`data-xyz-amorgan-blurhash`.
+
+## Security considerations
+
+BlurHash entries in encrypted events, be it as part of the `info` property,
+or `<img>` tags, should be encrypted along with the rest of the event
+content.
+
+A [discussion in
+#matrix-spec](https://matrix.to/#/!NasysSDfxKxZBzJJoE:matrix.org/$Cfa0dtF3DenIUAbC5aeg3Xo10gAF54mAJLZ6VzvYNfo?via=matrix.org&via=amorgan.xyz&via=pixie.town)
+questioned whether massive BlurHashes may be a potential DoS vector for
+clients. The discussion found that only a maximum of 100 x 100 components can
+be defined by a BlurHash. This may be in the higher range for low-resource
+(or unoptimised) clients, and clients are free to refuse to render a BlurHash
+with a large component count, but it shouldn't be a cause for concern.
+
+Invalid BlurHashes should not be rendered.
+
+## Links
+
+BlurHash's algorithm description can be found
+[here](https://github.com/woltapp/blurhash/blob/master/Algorithm.md), which
+also includes the full output character set.