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. firefish, 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": [
    "https://www.w3.org/ns/activitystreams#Public",
    "http://mastodon/users/hippo"
  ],
  "id": "http://pasture-one-actor/actor/ZbOvLHZT1wY",
  "published": "2025-07-26T14:00:19Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mastodon/users/hippo"
    ],
    "id": "http://pasture-one-actor/actor/3Tlo-g3kgt8",
    "published": "2025-07-26T14:00:19Z",
    "content": "no description",
    "attachment": {
      "type": "Document",
      "url": "http://pasture-one-actor/images/100.png"
    }
  }
}

mastodon

{
  "id": "114919893865585905",
  "created_at": "2025-07-26T14:00:19.000Z",
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/actor/3Tlo-g3kgt8",
  "url": "http://pasture-one-actor/actor/3Tlo-g3kgt8",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "edited_at": null,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "content": "no description",
  "filtered": [],
  "reblog": null,
  "account": {
    "id": "114919887140144768",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "discoverable": false,
    "indexable": false,
    "group": false,
    "created_at": "2025-07-26T00:00:00.000Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "uri": "http://pasture-one-actor/actor",
    "avatar": "http://mastodon/avatars/original/missing.png",
    "avatar_static": "http://mastodon/avatars/original/missing.png",
    "header": "http://mastodon/headers/original/missing.png",
    "header_static": "http://mastodon/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 34,
    "last_status_at": "2025-07-26",
    "hide_collections": true,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "114919893859258067",
      "type": "image",
      "url": "http://localhost:2970/system/cache/media_attachments/files/114/919/893/859/258/067/original/2956aff3b6a48e9f.png",
      "preview_url": "http://localhost:2970/system/cache/media_attachments/files/114/919/893/859/258/067/small/2956aff3b6a48e9f.png",
      "remote_url": "http://pasture-one-actor/images/100.png",
      "preview_remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 40,
          "height": 40,
          "size": "40x40",
          "aspect": 1.0
        },
        "small": {
          "width": 40,
          "height": 40,
          "size": "40x40",
          "aspect": 1.0
        }
      },
      "description": null,
      "blurhash": "U0JM|,}KfQ}K}KjufQjufQfQfQfQ}KjufQju"
    }
  ],
  "mentions": [],
  "tags": [],
  "emojis": [],
  "quote": null,
  "card": null,
  "poll": null
}

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": [
    "https://www.w3.org/ns/activitystreams#Public",
    "http://mastodon/users/hippo"
  ],
  "id": "http://pasture-one-actor/actor/6FcY2ZnTNMA",
  "published": "2025-07-26T14:00:20Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mastodon/users/hippo"
    ],
    "id": "http://pasture-one-actor/actor/635CxySB4Tw",
    "published": "2025-07-26T14:00:20Z",
    "content": "text",
    "attachment": {
      "type": "Document",
      "url": "http://pasture-one-actor/images/100.png",
      "name": "name",
      "summary": "summary",
      "content": "content"
    }
  }
}

mastodon

{
  "id": "114919893929223387",
  "created_at": "2025-07-26T14:00:20.000Z",
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/actor/635CxySB4Tw",
  "url": "http://pasture-one-actor/actor/635CxySB4Tw",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "edited_at": null,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "content": "text",
  "filtered": [],
  "reblog": null,
  "account": {
    "id": "114919887140144768",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "discoverable": false,
    "indexable": false,
    "group": false,
    "created_at": "2025-07-26T00:00:00.000Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "uri": "http://pasture-one-actor/actor",
    "avatar": "http://mastodon/avatars/original/missing.png",
    "avatar_static": "http://mastodon/avatars/original/missing.png",
    "header": "http://mastodon/headers/original/missing.png",
    "header_static": "http://mastodon/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 35,
    "last_status_at": "2025-07-26",
    "hide_collections": true,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "114919893923759102",
      "type": "image",
      "url": "http://localhost:2970/system/cache/media_attachments/files/114/919/893/923/759/102/original/4d0857ae17615aa1.png",
      "preview_url": "http://localhost:2970/system/cache/media_attachments/files/114/919/893/923/759/102/small/4d0857ae17615aa1.png",
      "remote_url": "http://pasture-one-actor/images/100.png",
      "preview_remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 40,
          "height": 40,
          "size": "40x40",
          "aspect": 1.0
        },
        "small": {
          "width": 40,
          "height": 40,
          "size": "40x40",
          "aspect": 1.0
        }
      },
      "description": "summary",
      "blurhash": "U2SHI^|*fQ|*|*jufQjufQfQfQfQ|*jufQju"
    }
  ],
  "mentions": [],
  "tags": [],
  "emojis": [],
  "quote": null,
  "card": null,
  "poll": null
}

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": [
    "https://www.w3.org/ns/activitystreams#Public",
    "http://mastodon/users/hippo"
  ],
  "id": "http://pasture-one-actor/actor/K84lYvCKxKY",
  "published": "2025-07-26T14:00:21Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mastodon/users/hippo"
    ],
    "id": "http://pasture-one-actor/actor/YnOzABLA3_0",
    "published": "2025-07-26T14:00:21Z",
    "content": "text",
    "attachment": [
      {
        "type": "Document",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png",
        "name": "name",
        "imageType": "image/jpeg"
      }
    ]
  }
}

mastodon

{
  "id": "114919894002453168",
  "created_at": "2025-07-26T14:00:21.000Z",
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/actor/YnOzABLA3_0",
  "url": "http://pasture-one-actor/actor/YnOzABLA3_0",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "edited_at": null,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "content": "text",
  "filtered": [],
  "reblog": null,
  "account": {
    "id": "114919887140144768",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "discoverable": false,
    "indexable": false,
    "group": false,
    "created_at": "2025-07-26T00:00:00.000Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "uri": "http://pasture-one-actor/actor",
    "avatar": "http://mastodon/avatars/original/missing.png",
    "avatar_static": "http://mastodon/avatars/original/missing.png",
    "header": "http://mastodon/headers/original/missing.png",
    "header_static": "http://mastodon/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 36,
    "last_status_at": "2025-07-26",
    "hide_collections": true,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "114919893989248860",
      "type": "image",
      "url": "http://localhost:2970/system/cache/media_attachments/files/114/919/893/989/248/860/original/58a167d2cb4b3435.png",
      "preview_url": "http://localhost:2970/system/cache/media_attachments/files/114/919/893/989/248/860/small/58a167d2cb4b3435.png",
      "remote_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "preview_remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 1548,
          "height": 1548,
          "size": "1548x1548",
          "aspect": 1.0
        },
        "small": {
          "width": 480,
          "height": 480,
          "size": "480x480",
          "aspect": 1.0
        }
      },
      "description": "name",
      "blurhash": "UOEVB2.}4.T=}fjEEVw[4=K[RUw~9,TJV?bW"
    }
  ],
  "mentions": [],
  "tags": [],
  "emojis": [],
  "quote": null,
  "card": null,
  "poll": null
}

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": [
    "https://www.w3.org/ns/activitystreams#Public",
    "http://mastodon/users/hippo"
  ],
  "id": "http://pasture-one-actor/actor/OxGYtU3RPaA",
  "published": "2025-07-26T14:00:22Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mastodon/users/hippo"
    ],
    "id": "http://pasture-one-actor/actor/W8xw1-VYJTc",
    "published": "2025-07-26T14:00:22Z",
    "content": "no comment in attachment",
    "attachment": [
      {
        "type": "Document",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png"
      }
    ]
  }
}

mastodon

{
  "id": "114919894067969212",
  "created_at": "2025-07-26T14:00:22.000Z",
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/actor/W8xw1-VYJTc",
  "url": "http://pasture-one-actor/actor/W8xw1-VYJTc",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "edited_at": null,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "content": "no comment in attachment",
  "filtered": [],
  "reblog": null,
  "account": {
    "id": "114919887140144768",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "discoverable": false,
    "indexable": false,
    "group": false,
    "created_at": "2025-07-26T00:00:00.000Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "uri": "http://pasture-one-actor/actor",
    "avatar": "http://mastodon/avatars/original/missing.png",
    "avatar_static": "http://mastodon/avatars/original/missing.png",
    "header": "http://mastodon/headers/original/missing.png",
    "header_static": "http://mastodon/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 37,
    "last_status_at": "2025-07-26",
    "hide_collections": true,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "114919894054472256",
      "type": "image",
      "url": "http://localhost:2970/system/cache/media_attachments/files/114/919/894/054/472/256/original/28572385ded12436.png",
      "preview_url": "http://localhost:2970/system/cache/media_attachments/files/114/919/894/054/472/256/small/28572385ded12436.png",
      "remote_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "preview_remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 1548,
          "height": 1548,
          "size": "1548x1548",
          "aspect": 1.0
        },
        "small": {
          "width": 480,
          "height": 480,
          "size": "480x480",
          "aspect": 1.0
        }
      },
      "description": null,
      "blurhash": "UOEVB2.}4.T=}fjEEVw[4=K[RUw~9,TJV?bW"
    }
  ],
  "mentions": [],
  "tags": [],
  "emojis": [],
  "quote": null,
  "card": null,
  "poll": null
}