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": [
    "http://mastodon/users/hippo",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/Di14PyezDDU",
  "published": "2025-08-15T10:29:23Z",
  "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/object/sctE7vS7J4E",
    "published": "2025-08-15T10:29:23Z",
    "content": "no description",
    "attachment": {
      "type": "Document",
      "url": "http://pasture-one-actor/images/100.png"
    }
  }
}

mastodon

{
  "id": "115032310676303571",
  "created_at": "2025-08-15T10:29:23.000Z",
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/sctE7vS7J4E",
  "url": "http://pasture-one-actor/object/sctE7vS7J4E",
  "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": "115032302291156066",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "discoverable": false,
    "indexable": false,
    "group": false,
    "created_at": "2025-08-15T00:00:00.000Z",
    "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": 33,
    "last_status_at": "2025-08-15",
    "hide_collections": true,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "115032310669008779",
      "type": "image",
      "url": "http://localhost:2970/system/cache/media_attachments/files/115/032/310/669/008/779/original/3be46e4361bd6937.png",
      "preview_url": "http://localhost:2970/system/cache/media_attachments/files/115/032/310/669/008/779/small/3be46e4361bd6937.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": "U1OmLI}vfQ}v}vjufQjufQfQfQfQ}vjufQju"
    }
  ],
  "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": [
    "http://mastodon/users/hippo",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/55sVrj-M5qs",
  "published": "2025-08-15T10:29:24Z",
  "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/object/e35-BeaO6Ng",
    "published": "2025-08-15T10:29:24Z",
    "content": "text",
    "attachment": {
      "type": "Document",
      "url": "http://pasture-one-actor/images/100.png",
      "name": "name",
      "summary": "summary",
      "content": "content"
    }
  }
}

mastodon

{
  "id": "115032310739278531",
  "created_at": "2025-08-15T10:29:24.000Z",
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/e35-BeaO6Ng",
  "url": "http://pasture-one-actor/object/e35-BeaO6Ng",
  "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": "115032302291156066",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "discoverable": false,
    "indexable": false,
    "group": false,
    "created_at": "2025-08-15T00:00:00.000Z",
    "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": "2025-08-15",
    "hide_collections": true,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "115032310733224702",
      "type": "image",
      "url": "http://localhost:2970/system/cache/media_attachments/files/115/032/310/733/224/702/original/2afc80ec2fa749be.png",
      "preview_url": "http://localhost:2970/system/cache/media_attachments/files/115/032/310/733/224/702/small/2afc80ec2fa749be.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": "U1KoJE_KfQ_K_Kj[fQj[fQfQfQfQ_Kj[fQj["
    }
  ],
  "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": [
    "http://mastodon/users/hippo",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/JyES5WBZMLQ",
  "published": "2025-08-15T10:29:25Z",
  "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/object/8CW2r_2RE40",
    "published": "2025-08-15T10:29:25Z",
    "content": "text",
    "attachment": [
      {
        "type": "Document",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png",
        "name": "name",
        "imageType": "image/jpeg"
      }
    ]
  }
}

mastodon

{
  "id": "115032310801039932",
  "created_at": "2025-08-15T10:29:25.000Z",
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/8CW2r_2RE40",
  "url": "http://pasture-one-actor/object/8CW2r_2RE40",
  "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": "115032302291156066",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "discoverable": false,
    "indexable": false,
    "group": false,
    "created_at": "2025-08-15T00:00:00.000Z",
    "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": "2025-08-15",
    "hide_collections": true,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "115032310798550362",
      "type": "unknown",
      "url": "http://mastodon/media_proxy/115032310798550362/original",
      "preview_url": "http://mastodon/media_proxy/115032310798550362/small",
      "remote_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "preview_remote_url": null,
      "text_url": null,
      "meta": null,
      "description": "name",
      "blurhash": null
    }
  ],
  "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": [
    "http://mastodon/users/hippo",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "http://pasture-one-actor/activity/HUg_X5J_y8g",
  "published": "2025-08-15T10:29:26Z",
  "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/object/C5kTfe3EDLA",
    "published": "2025-08-15T10:29:26Z",
    "content": "no comment in attachment",
    "attachment": [
      {
        "type": "Document",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png"
      }
    ]
  }
}

mastodon

{
  "id": "115032310888095475",
  "created_at": "2025-08-15T10:29:26.000Z",
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/C5kTfe3EDLA",
  "url": "http://pasture-one-actor/object/C5kTfe3EDLA",
  "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": "115032302291156066",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "discoverable": false,
    "indexable": false,
    "group": false,
    "created_at": "2025-08-15T00:00:00.000Z",
    "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": "2025-08-15",
    "hide_collections": true,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "115032310885795698",
      "type": "unknown",
      "url": "http://mastodon/media_proxy/115032310885795698/original",
      "preview_url": "http://mastodon/media_proxy/115032310885795698/small",
      "remote_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "preview_remote_url": null,
      "text_url": null,
      "meta": null,
      "description": null,
      "blurhash": null
    }
  ],
  "mentions": [],
  "tags": [],
  "emojis": [],
  "quote": null,
  "card": null,
  "poll": null
}