Image Description

The Image type is defined in ActivityStreams Vocabulary.

In this support table, we only consider how the image description, commonly called AltText is handled. Image descriptions are important from an accessibility standpoint, see WCAG 2.2. Text Alternatives.

It seems that certain implementations, e.g. misskey, store the image description on a per image URL basis and not for every instance of an image reference.

Examples

Example 1

activity

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "type": "Create",
  "actor": "http://pasture-one-actor/actor",
  "to": [
    "http://sharkey/users/ac4f96i4gb150003",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/uaLjGPcFENQ",
  "published": "2025-09-01T08:30:59Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://sharkey/users/ac4f96i4gb150003"
    ],
    "id": "http://pasture-one-actor/object/jjfBJ1TehlU",
    "published": "2025-09-01T08:30:59Z",
    "content": "no description",
    "attachment": {
      "type": "Document",
      "url": "http://pasture-one-actor/images/100.png"
    }
  }
}

sharkey

{
  "id": "ac4fc8mwttj5001l",
  "uri": "http://pasture-one-actor/object/jjfBJ1TehlU",
  "url": "http://pasture-one-actor/object/jjfBJ1TehlU",
  "account": {
    "id": "ac4fasvattj50004",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "fqn": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "created_at": "2025-09-01T08:29:51.910Z",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 44,
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "uri": "http://pasture-one-actor/actor",
    "avatar": "https://dev.joinsharkey.org/static-assets/avatar.png",
    "avatar_static": "https://dev.joinsharkey.org/static-assets/avatar.png",
    "header": "https://dev.joinsharkey.org/static-assets/transparent.png",
    "header_static": "https://dev.joinsharkey.org/static-assets/transparent.png",
    "emojis": [],
    "moved": null,
    "fields": [],
    "bot": false,
    "discoverable": true,
    "noindex": false,
    "group": null,
    "suspended": false,
    "limited": false
  },
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "reblog": null,
  "content": "<p>no description</p>",
  "content_type": "text/x.misskeymarkdown",
  "text": "no description",
  "created_at": "2025-09-01T08:30:59.000Z",
  "edited_at": null,
  "emojis": [],
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "reblogged": false,
  "favourited": false,
  "muted": false,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "media_attachments": [
    {
      "id": "ac4fc8vfttj5001k",
      "type": "image",
      "url": "http://pasture-one-actor/images/100.png",
      "remote_url": "http://pasture-one-actor/images/100.png",
      "preview_url": "http://pasture-one-actor/images/100.png",
      "text_url": "http://pasture-one-actor/images/100.png",
      "meta": {
        "width": 40,
        "height": 40,
        "original": {
          "width": 40,
          "height": 40,
          "size": "40x40",
          "aspect": 1,
          "frame_rate": "undefined"
        },
        "size": "40x40",
        "aspect": 1
      },
      "description": null,
      "blurhash": "e4T9Q;~nfQ~nfQ~nj[fQj[fQfQfQfQfQfQ~nj[fQj[fQfQfQfQfQfQ"
    }
  ],
  "mentions": [],
  "tags": [],
  "card": null,
  "poll": null,
  "application": null,
  "language": null,
  "pinned": false,
  "bookmarked": false,
  "quote": null,
  "reactions": []
}

Example 2

activity

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "type": "Create",
  "actor": "http://pasture-one-actor/actor",
  "to": [
    "http://sharkey/users/ac4f96i4gb150003",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/DdHF6kF-Gag",
  "published": "2025-09-01T08:30:59Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://sharkey/users/ac4f96i4gb150003"
    ],
    "id": "http://pasture-one-actor/object/enapcdq2BC0",
    "published": "2025-09-01T08:30:59Z",
    "content": "text",
    "attachment": {
      "type": "Document",
      "url": "http://pasture-one-actor/images/100.png",
      "name": "name",
      "summary": "summary",
      "content": "content"
    }
  }
}

sharkey

{
  "id": "ac4fc8mwttj5001n",
  "uri": "http://pasture-one-actor/object/enapcdq2BC0",
  "url": "http://pasture-one-actor/object/enapcdq2BC0",
  "account": {
    "id": "ac4fasvattj50004",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "fqn": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "created_at": "2025-09-01T08:29:51.910Z",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 45,
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "uri": "http://pasture-one-actor/actor",
    "avatar": "https://dev.joinsharkey.org/static-assets/avatar.png",
    "avatar_static": "https://dev.joinsharkey.org/static-assets/avatar.png",
    "header": "https://dev.joinsharkey.org/static-assets/transparent.png",
    "header_static": "https://dev.joinsharkey.org/static-assets/transparent.png",
    "emojis": [],
    "moved": null,
    "fields": [],
    "bot": false,
    "discoverable": true,
    "noindex": false,
    "group": null,
    "suspended": false,
    "limited": false
  },
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "reblog": null,
  "content": "<p>text</p>",
  "content_type": "text/x.misskeymarkdown",
  "text": "text",
  "created_at": "2025-09-01T08:30:59.000Z",
  "edited_at": null,
  "emojis": [],
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "reblogged": false,
  "favourited": false,
  "muted": false,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "media_attachments": [
    {
      "id": "ac4fc9e6ttj5001m",
      "type": "image",
      "url": "http://pasture-one-actor/images/100.png",
      "remote_url": "http://pasture-one-actor/images/100.png",
      "preview_url": "http://pasture-one-actor/images/100.png",
      "text_url": "http://pasture-one-actor/images/100.png",
      "meta": {
        "width": 40,
        "height": 40,
        "original": {
          "width": 40,
          "height": 40,
          "size": "40x40",
          "aspect": 1,
          "frame_rate": "undefined"
        },
        "size": "40x40",
        "aspect": 1
      },
      "description": "name",
      "blurhash": "e2HWa3?ofQ?ofQ?oj@fQj@fQfQfQfQfQfQ?oj@fQj@fQfQfQfQfQfQ"
    }
  ],
  "mentions": [],
  "tags": [],
  "card": null,
  "poll": null,
  "application": null,
  "language": null,
  "pinned": false,
  "bookmarked": false,
  "quote": null,
  "reactions": []
}

Example 3

activity

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "type": "Create",
  "actor": "http://pasture-one-actor/actor",
  "to": [
    "http://sharkey/users/ac4f96i4gb150003",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/TCwvI_TMBpU",
  "published": "2025-09-01T08:31:00Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://sharkey/users/ac4f96i4gb150003"
    ],
    "id": "http://pasture-one-actor/object/oxvzHrPm_gk",
    "published": "2025-09-01T08:31:00Z",
    "content": "text",
    "attachment": [
      {
        "type": "Document",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png",
        "name": "name",
        "imageType": "image/jpeg"
      }
    ]
  }
}

sharkey

{
  "id": "ac4fc9eottj5001p",
  "uri": "http://pasture-one-actor/object/oxvzHrPm_gk",
  "url": "http://pasture-one-actor/object/oxvzHrPm_gk",
  "account": {
    "id": "ac4fasvattj50004",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "fqn": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "created_at": "2025-09-01T08:29:51.910Z",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 46,
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "uri": "http://pasture-one-actor/actor",
    "avatar": "https://dev.joinsharkey.org/static-assets/avatar.png",
    "avatar_static": "https://dev.joinsharkey.org/static-assets/avatar.png",
    "header": "https://dev.joinsharkey.org/static-assets/transparent.png",
    "header_static": "https://dev.joinsharkey.org/static-assets/transparent.png",
    "emojis": [],
    "moved": null,
    "fields": [],
    "bot": false,
    "discoverable": true,
    "noindex": false,
    "group": null,
    "suspended": false,
    "limited": false
  },
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "reblog": null,
  "content": "<p>text</p>",
  "content_type": "text/x.misskeymarkdown",
  "text": "text",
  "created_at": "2025-09-01T08:31:00.000Z",
  "edited_at": null,
  "emojis": [],
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "reblogged": false,
  "favourited": false,
  "muted": false,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "media_attachments": [
    {
      "id": "ac4fca2httj5001o",
      "type": "image",
      "url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "remote_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "preview_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "text_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "meta": {
        "width": 1548,
        "height": 1548,
        "original": {
          "width": 1548,
          "height": 1548,
          "size": "1548x1548",
          "aspect": 1,
          "frame_rate": "undefined"
        },
        "size": "1548x1548",
        "aspect": 1
      },
      "description": "name",
      "blurhash": "eQE_?c.}4.YJM^}fi{EVw[xu9JK[M~xI-T9+TJRPX2$$E]NFo}j1tO"
    }
  ],
  "mentions": [],
  "tags": [],
  "card": null,
  "poll": null,
  "application": null,
  "language": null,
  "pinned": false,
  "bookmarked": false,
  "quote": null,
  "reactions": []
}

Example 4

activity

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "type": "Create",
  "actor": "http://pasture-one-actor/actor",
  "to": [
    "http://sharkey/users/ac4f96i4gb150003",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/C6ib8dEVTdc",
  "published": "2025-09-01T08:31:01Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://sharkey/users/ac4f96i4gb150003"
    ],
    "id": "http://pasture-one-actor/object/_kZWuJQt1Os",
    "published": "2025-09-01T08:31:01Z",
    "content": "no comment in attachment",
    "attachment": [
      {
        "type": "Document",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png"
      }
    ]
  }
}

sharkey

{
  "id": "ac4fca6gttj5001q",
  "uri": "http://pasture-one-actor/object/_kZWuJQt1Os",
  "url": "http://pasture-one-actor/object/_kZWuJQt1Os",
  "account": {
    "id": "ac4fasvattj50004",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "fqn": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "created_at": "2025-09-01T08:29:51.910Z",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 47,
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "uri": "http://pasture-one-actor/actor",
    "avatar": "https://dev.joinsharkey.org/static-assets/avatar.png",
    "avatar_static": "https://dev.joinsharkey.org/static-assets/avatar.png",
    "header": "https://dev.joinsharkey.org/static-assets/transparent.png",
    "header_static": "https://dev.joinsharkey.org/static-assets/transparent.png",
    "emojis": [],
    "moved": null,
    "fields": [],
    "bot": false,
    "discoverable": true,
    "noindex": false,
    "group": null,
    "suspended": false,
    "limited": false
  },
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "reblog": null,
  "content": "<p>no comment in attachment</p>",
  "content_type": "text/x.misskeymarkdown",
  "text": "no comment in attachment",
  "created_at": "2025-09-01T08:31:01.000Z",
  "edited_at": null,
  "emojis": [],
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "reblogged": false,
  "favourited": false,
  "muted": false,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "media_attachments": [
    {
      "id": "ac4fca2httj5001o",
      "type": "image",
      "url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "remote_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "preview_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "text_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "meta": {
        "width": 1548,
        "height": 1548,
        "original": {
          "width": 1548,
          "height": 1548,
          "size": "1548x1548",
          "aspect": 1,
          "frame_rate": "undefined"
        },
        "size": "1548x1548",
        "aspect": 1
      },
      "description": "name",
      "blurhash": "eQE_?c.}4.YJM^}fi{EVw[xu9JK[M~xI-T9+TJRPX2$$E]NFo}j1tO"
    }
  ],
  "mentions": [],
  "tags": [],
  "card": null,
  "poll": null,
  "application": null,
  "language": null,
  "pinned": false,
  "bookmarked": false,
  "quote": null,
  "reactions": []
}