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://mitra/users/admin"
  ],
  "id": "http://pasture-one-actor/actor/0b_otZaXqnk",
  "published": "2025-07-31T11:59:26Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mitra/users/admin"
    ],
    "id": "http://pasture-one-actor/actor/Jg_V7816obU",
    "published": "2025-07-31T11:59:26Z",
    "content": "no description",
    "attachment": {
      "type": "Document",
      "url": "http://pasture-one-actor/images/100.png"
    }
  }
}

mitra

{
  "id": "0198605a-0177-0c5d-6c3d-025ce0c25b73",
  "uri": "http://pasture-one-actor/actor/Jg_V7816obU",
  "url": "http://pasture-one-actor/actor/Jg_V7816obU",
  "created_at": "2025-07-31T11:59:26.000Z",
  "edited_at": null,
  "account": {
    "id": "01986058-7cab-434e-fbce-443b1a7a54af",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "actor_id": "http://pasture-one-actor/actor",
    "url": "http://pasture-one-actor/actor",
    "display_name": "Test Actor",
    "created_at": "2025-07-31T11:57:48.587Z",
    "note": "",
    "avatar": "http://mitra/api/v1/accounts/identicon?input=http://pasture-one-actor/actor",
    "header": "http://mitra/api/v1/accounts/identicon",
    "locked": false,
    "mention_policy": "none",
    "bot": false,
    "identity_proofs": [],
    "payment_options": [],
    "fields": [],
    "emojis": [],
    "followers_count": 0,
    "following_count": 0,
    "subscribers_count": 0,
    "statuses_count": 31,
    "source": null,
    "role": null,
    "authentication_methods": null,
    "client_config": null
  },
  "content": "no description",
  "language": null,
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "reblog": null,
  "visibility": "public",
  "sensitive": false,
  "spoiler_text": "",
  "pinned": false,
  "replies_count": 0,
  "favourites_count": 0,
  "reblogs_count": 0,
  "poll": null,
  "media_attachments": [
    {
      "id": "0198605a-0174-cbde-50a1-825418cea9c8",
      "type": "image",
      "url": "http://mitra/media/c43f0e35134ab5f0e759fdbb2e20e3028d72d205fb76c7c91897cde938503ba2.png",
      "preview_url": "http://mitra/media/c43f0e35134ab5f0e759fdbb2e20e3028d72d205fb76c7c91897cde938503ba2.png",
      "description": null
    }
  ],
  "quote": null,
  "mentions": [
    {
      "id": "01986057-420a-112f-c797-87067c04b328",
      "username": "admin",
      "acct": "admin",
      "url": "http://mitra/users/admin"
    }
  ],
  "tags": [],
  "emojis": [],
  "favourited": false,
  "reblogged": false,
  "bookmarked": false,
  "pleroma": {
    "emoji_reactions": [],
    "in_reply_to_account_acct": null,
    "parent_visible": true,
    "quote": null,
    "quote_visible": true
  },
  "hidden": false,
  "ipfs_cid": null,
  "links": []
}

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://mitra/users/admin"
  ],
  "id": "http://pasture-one-actor/actor/C5pFZP-eFVY",
  "published": "2025-07-31T11:59:28Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mitra/users/admin"
    ],
    "id": "http://pasture-one-actor/actor/KtuUWkbg9OE",
    "published": "2025-07-31T11:59:28Z",
    "content": "text",
    "attachment": {
      "type": "Document",
      "url": "http://pasture-one-actor/images/100.png",
      "name": "name",
      "summary": "summary",
      "content": "content"
    }
  }
}

mitra

{
  "id": "0198605a-0770-60af-2a84-97f32f38c089",
  "uri": "http://pasture-one-actor/actor/KtuUWkbg9OE",
  "url": "http://pasture-one-actor/actor/KtuUWkbg9OE",
  "created_at": "2025-07-31T11:59:28.000Z",
  "edited_at": null,
  "account": {
    "id": "01986058-7cab-434e-fbce-443b1a7a54af",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "actor_id": "http://pasture-one-actor/actor",
    "url": "http://pasture-one-actor/actor",
    "display_name": "Test Actor",
    "created_at": "2025-07-31T11:57:48.587Z",
    "note": "",
    "avatar": "http://mitra/api/v1/accounts/identicon?input=http://pasture-one-actor/actor",
    "header": "http://mitra/api/v1/accounts/identicon",
    "locked": false,
    "mention_policy": "none",
    "bot": false,
    "identity_proofs": [],
    "payment_options": [],
    "fields": [],
    "emojis": [],
    "followers_count": 0,
    "following_count": 0,
    "subscribers_count": 0,
    "statuses_count": 32,
    "source": null,
    "role": null,
    "authentication_methods": null,
    "client_config": null
  },
  "content": "text",
  "language": null,
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "reblog": null,
  "visibility": "public",
  "sensitive": false,
  "spoiler_text": "",
  "pinned": false,
  "replies_count": 0,
  "favourites_count": 0,
  "reblogs_count": 0,
  "poll": null,
  "media_attachments": [
    {
      "id": "0198605a-0769-4852-6e47-e977411e2d31",
      "type": "image",
      "url": "http://mitra/media/da93d5690cd6e687c4cc341a051b077d6919db98428f7ad70e6c3404c06c7d1d.png",
      "preview_url": "http://mitra/media/da93d5690cd6e687c4cc341a051b077d6919db98428f7ad70e6c3404c06c7d1d.png",
      "description": "name"
    }
  ],
  "quote": null,
  "mentions": [
    {
      "id": "01986057-420a-112f-c797-87067c04b328",
      "username": "admin",
      "acct": "admin",
      "url": "http://mitra/users/admin"
    }
  ],
  "tags": [],
  "emojis": [],
  "favourited": false,
  "reblogged": false,
  "bookmarked": false,
  "pleroma": {
    "emoji_reactions": [],
    "in_reply_to_account_acct": null,
    "parent_visible": true,
    "quote": null,
    "quote_visible": true
  },
  "hidden": false,
  "ipfs_cid": null,
  "links": []
}

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://mitra/users/admin"
  ],
  "id": "http://pasture-one-actor/actor/9a62JLU2qhE",
  "published": "2025-07-31T11:59:29Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mitra/users/admin"
    ],
    "id": "http://pasture-one-actor/actor/5MaSA2blkDk",
    "published": "2025-07-31T11:59:29Z",
    "content": "text",
    "attachment": [
      {
        "type": "Document",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png",
        "name": "name",
        "imageType": "image/jpeg"
      }
    ]
  }
}

mitra

{
  "id": "0198605a-0d36-0032-9e28-b719e7544549",
  "uri": "http://pasture-one-actor/actor/5MaSA2blkDk",
  "url": "http://pasture-one-actor/actor/5MaSA2blkDk",
  "created_at": "2025-07-31T11:59:29.000Z",
  "edited_at": null,
  "account": {
    "id": "01986058-7cab-434e-fbce-443b1a7a54af",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "actor_id": "http://pasture-one-actor/actor",
    "url": "http://pasture-one-actor/actor",
    "display_name": "Test Actor",
    "created_at": "2025-07-31T11:57:48.587Z",
    "note": "",
    "avatar": "http://mitra/api/v1/accounts/identicon?input=http://pasture-one-actor/actor",
    "header": "http://mitra/api/v1/accounts/identicon",
    "locked": false,
    "mention_policy": "none",
    "bot": false,
    "identity_proofs": [],
    "payment_options": [],
    "fields": [],
    "emojis": [],
    "followers_count": 0,
    "following_count": 0,
    "subscribers_count": 0,
    "statuses_count": 33,
    "source": null,
    "role": null,
    "authentication_methods": null,
    "client_config": null
  },
  "content": "text",
  "language": null,
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "reblog": null,
  "visibility": "public",
  "sensitive": false,
  "spoiler_text": "",
  "pinned": false,
  "replies_count": 0,
  "favourites_count": 0,
  "reblogs_count": 0,
  "poll": null,
  "media_attachments": [
    {
      "id": "0198605a-0d32-ed43-db37-9265fcfa44a3",
      "type": "image",
      "url": "http://mitra/media/2d7a61d31393a44edf2a30160cdd70b3582a4f97d5b550f8141dfee1641b4ae7.png",
      "preview_url": "http://mitra/media/2d7a61d31393a44edf2a30160cdd70b3582a4f97d5b550f8141dfee1641b4ae7.png",
      "description": "name"
    }
  ],
  "quote": null,
  "mentions": [
    {
      "id": "01986057-420a-112f-c797-87067c04b328",
      "username": "admin",
      "acct": "admin",
      "url": "http://mitra/users/admin"
    }
  ],
  "tags": [],
  "emojis": [],
  "favourited": false,
  "reblogged": false,
  "bookmarked": false,
  "pleroma": {
    "emoji_reactions": [],
    "in_reply_to_account_acct": null,
    "parent_visible": true,
    "quote": null,
    "quote_visible": true
  },
  "hidden": false,
  "ipfs_cid": null,
  "links": []
}

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://mitra/users/admin"
  ],
  "id": "http://pasture-one-actor/actor/68Cac8yROhk",
  "published": "2025-07-31T11:59:31Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://mitra/users/admin"
    ],
    "id": "http://pasture-one-actor/actor/OMGM7NOzylo",
    "published": "2025-07-31T11:59:31Z",
    "content": "no comment in attachment",
    "attachment": [
      {
        "type": "Document",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png"
      }
    ]
  }
}

mitra

{
  "id": "0198605a-131f-c050-d0ff-44cd8c11f824",
  "uri": "http://pasture-one-actor/actor/OMGM7NOzylo",
  "url": "http://pasture-one-actor/actor/OMGM7NOzylo",
  "created_at": "2025-07-31T11:59:31.000Z",
  "edited_at": null,
  "account": {
    "id": "01986058-7cab-434e-fbce-443b1a7a54af",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "actor_id": "http://pasture-one-actor/actor",
    "url": "http://pasture-one-actor/actor",
    "display_name": "Test Actor",
    "created_at": "2025-07-31T11:57:48.587Z",
    "note": "",
    "avatar": "http://mitra/api/v1/accounts/identicon?input=http://pasture-one-actor/actor",
    "header": "http://mitra/api/v1/accounts/identicon",
    "locked": false,
    "mention_policy": "none",
    "bot": false,
    "identity_proofs": [],
    "payment_options": [],
    "fields": [],
    "emojis": [],
    "followers_count": 0,
    "following_count": 0,
    "subscribers_count": 0,
    "statuses_count": 34,
    "source": null,
    "role": null,
    "authentication_methods": null,
    "client_config": null
  },
  "content": "no comment in attachment",
  "language": null,
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "reblog": null,
  "visibility": "public",
  "sensitive": false,
  "spoiler_text": "",
  "pinned": false,
  "replies_count": 0,
  "favourites_count": 0,
  "reblogs_count": 0,
  "poll": null,
  "media_attachments": [
    {
      "id": "0198605a-131c-2787-7201-19768fab0c33",
      "type": "image",
      "url": "http://mitra/media/2d7a61d31393a44edf2a30160cdd70b3582a4f97d5b550f8141dfee1641b4ae7.png",
      "preview_url": "http://mitra/media/2d7a61d31393a44edf2a30160cdd70b3582a4f97d5b550f8141dfee1641b4ae7.png",
      "description": null
    }
  ],
  "quote": null,
  "mentions": [
    {
      "id": "01986057-420a-112f-c797-87067c04b328",
      "username": "admin",
      "acct": "admin",
      "url": "http://mitra/users/admin"
    }
  ],
  "tags": [],
  "emojis": [],
  "favourited": false,
  "reblogged": false,
  "bookmarked": false,
  "pleroma": {
    "emoji_reactions": [],
    "in_reply_to_account_acct": null,
    "parent_visible": true,
    "quote": null,
    "quote_visible": true
  },
  "hidden": false,
  "ipfs_cid": null,
  "links": []
}