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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": [
    "https://www.w3.org/ns/activitystreams#Public",
    "http://hollo/@john"
  ],
  "id": "http://pasture-one-actor/activity/vZ0gfDxGKeE",
  "published": "2025-10-07T06:44:53Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://hollo/@john"
    ],
    "id": "http://pasture-one-actor/object/4_oMk8bYdQ4",
    "published": "2025-10-07T06:44:53Z",
    "content": "no description",
    "attachment": {
      "type": "Image",
      "url": "http://pasture-one-actor/images/29d99462-48a5-49e8-9062-a6b78b51b4b0.png"
    },
    "tag": [
      {
        "type": "Mention",
        "href": "http://hollo/@john"
      }
    ]
  }
}
hollo
{
  "id": "0199bd6a-6e88-769b-99fd-254b85793f61",
  "created_at": "2025-10-07T06:44:53.000Z",
  "in_reply_to_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/4_oMk8bYdQ4",
  "url": "http://pasture-one-actor/object/4_oMk8bYdQ4",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "pinned": false,
  "content": "no description",
  "reblog": null,
  "quote_id": null,
  "quote": null,
  "application": null,
  "account": {
    "id": "0199bd68-24e0-7001-bb0b-82b5d77020a3",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "created_at": "2025-10-07T06:42:23.014Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "avatar": "http://hollo/image/avatars/original/missing.png",
    "avatar_static": "http://hollo/image/avatars/original/missing.png",
    "header": "http://hollo/image/headers/original/missing.png",
    "header_static": "http://hollo/image/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 0,
    "moved": null,
    "last_status_at": null,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "0199bd6a-71e5-7bc7-be5b-900f944bb752",
      "type": "image",
      "url": "http://pasture-one-actor/images/29d99462-48a5-49e8-9062-a6b78b51b4b0.png",
      "preview_url": "http://pasture-one-actor/images/29d99462-48a5-49e8-9062-a6b78b51b4b0.png",
      "remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "small": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "focus": {
          "x": 0,
          "y": 0
        }
      },
      "description": null,
      "blurhash": null
    }
  ],
  "mentions": [],
  "tags": [],
  "card": null,
  "emojis": [],
  "emoji_reactions": [],
  "poll": null,
  "filtered": 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://hollo/@john"
  ],
  "id": "http://pasture-one-actor/activity/clZvc6RKIBU",
  "published": "2025-10-07T06:44:54Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://hollo/@john"
    ],
    "id": "http://pasture-one-actor/object/BM8mQ99pDxw",
    "published": "2025-10-07T06:44:54Z",
    "content": "name, summary and content",
    "attachment": {
      "type": "Image",
      "url": "http://pasture-one-actor/images/0b09f5dd-ed08-4974-b36f-a719a7c88907.png",
      "name": "name",
      "summary": "summary",
      "content": "content"
    },
    "tag": [
      {
        "type": "Mention",
        "href": "http://hollo/@john"
      }
    ]
  }
}
hollo
{
  "id": "0199bd6a-7270-780e-b81c-286982ccc538",
  "created_at": "2025-10-07T06:44:54.000Z",
  "in_reply_to_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/BM8mQ99pDxw",
  "url": "http://pasture-one-actor/object/BM8mQ99pDxw",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "pinned": false,
  "content": "name, summary and content",
  "reblog": null,
  "quote_id": null,
  "quote": null,
  "application": null,
  "account": {
    "id": "0199bd68-24e0-7001-bb0b-82b5d77020a3",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "created_at": "2025-10-07T06:42:23.014Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "avatar": "http://hollo/image/avatars/original/missing.png",
    "avatar_static": "http://hollo/image/avatars/original/missing.png",
    "header": "http://hollo/image/headers/original/missing.png",
    "header_static": "http://hollo/image/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 0,
    "moved": null,
    "last_status_at": null,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "0199bd6a-743d-7176-95ae-0bcb542e33a2",
      "type": "image",
      "url": "http://pasture-one-actor/images/0b09f5dd-ed08-4974-b36f-a719a7c88907.png",
      "preview_url": "http://pasture-one-actor/images/0b09f5dd-ed08-4974-b36f-a719a7c88907.png",
      "remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "small": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "focus": {
          "x": 0,
          "y": 0
        }
      },
      "description": "summary",
      "blurhash": null
    }
  ],
  "mentions": [],
  "tags": [],
  "card": null,
  "emojis": [],
  "emoji_reactions": [],
  "poll": null,
  "filtered": 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://hollo/@john"
  ],
  "id": "http://pasture-one-actor/activity/7IVxGw70KEM",
  "published": "2025-10-07T06:44:55Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://hollo/@john"
    ],
    "id": "http://pasture-one-actor/object/LmN1AIMENe8",
    "published": "2025-10-07T06:44:55Z",
    "content": "only name",
    "attachment": [
      {
        "type": "Image",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png",
        "name": "name",
        "imageType": "image/jpeg"
      }
    ],
    "tag": [
      {
        "type": "Mention",
        "href": "http://hollo/@john"
      }
    ]
  }
}
hollo
{
  "id": "0199bd6a-7658-7656-869a-258e338a4d01",
  "created_at": "2025-10-07T06:44:55.000Z",
  "in_reply_to_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/LmN1AIMENe8",
  "url": "http://pasture-one-actor/object/LmN1AIMENe8",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "pinned": false,
  "content": "only name",
  "reblog": null,
  "quote_id": null,
  "quote": null,
  "application": null,
  "account": {
    "id": "0199bd68-24e0-7001-bb0b-82b5d77020a3",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "created_at": "2025-10-07T06:42:23.014Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "avatar": "http://hollo/image/avatars/original/missing.png",
    "avatar_static": "http://hollo/image/avatars/original/missing.png",
    "header": "http://hollo/image/headers/original/missing.png",
    "header_static": "http://hollo/image/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 0,
    "moved": null,
    "last_status_at": null,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "0199bd6a-76b2-70de-99aa-747c93b4bd9d",
      "type": "image",
      "url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "preview_url": "http://pasture-one-actor/assets/FediverseLogo.png",
      "remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "small": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "focus": {
          "x": 0,
          "y": 0
        }
      },
      "description": "name",
      "blurhash": null
    }
  ],
  "mentions": [],
  "tags": [],
  "card": null,
  "emojis": [],
  "emoji_reactions": [],
  "poll": null,
  "filtered": 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://hollo/@john"
  ],
  "id": "http://pasture-one-actor/activity/UOiqSTWMNsA",
  "published": "2025-10-07T06:44:56Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://hollo/@john"
    ],
    "id": "http://pasture-one-actor/object/c2NlYrLVXRM",
    "published": "2025-10-07T06:44:56Z",
    "content": "only summary",
    "attachment": {
      "type": "Image",
      "url": "http://pasture-one-actor/images/6cdf309d-f96a-446d-864c-5cb2af9c2847.png",
      "name": "name",
      "summary": "summary",
      "content": "content"
    },
    "tag": [
      {
        "type": "Mention",
        "href": "http://hollo/@john"
      }
    ]
  }
}
hollo
{
  "id": "0199bd6a-7a40-7937-8918-b29a22fd9bfd",
  "created_at": "2025-10-07T06:44:56.000Z",
  "in_reply_to_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/c2NlYrLVXRM",
  "url": "http://pasture-one-actor/object/c2NlYrLVXRM",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "pinned": false,
  "content": "only summary",
  "reblog": null,
  "quote_id": null,
  "quote": null,
  "application": null,
  "account": {
    "id": "0199bd68-24e0-7001-bb0b-82b5d77020a3",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "created_at": "2025-10-07T06:42:23.014Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "avatar": "http://hollo/image/avatars/original/missing.png",
    "avatar_static": "http://hollo/image/avatars/original/missing.png",
    "header": "http://hollo/image/headers/original/missing.png",
    "header_static": "http://hollo/image/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 0,
    "moved": null,
    "last_status_at": null,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "0199bd6a-7b47-76f7-97b8-8e18496ede1a",
      "type": "image",
      "url": "http://pasture-one-actor/images/6cdf309d-f96a-446d-864c-5cb2af9c2847.png",
      "preview_url": "http://pasture-one-actor/images/6cdf309d-f96a-446d-864c-5cb2af9c2847.png",
      "remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "small": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "focus": {
          "x": 0,
          "y": 0
        }
      },
      "description": "summary",
      "blurhash": null
    }
  ],
  "mentions": [],
  "tags": [],
  "card": null,
  "emojis": [],
  "emoji_reactions": [],
  "poll": null,
  "filtered": null
}

Example 5

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://hollo/@john"
  ],
  "id": "http://pasture-one-actor/activity/iWcu-wEi5xs",
  "published": "2025-10-07T06:44:56Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://hollo/@john"
    ],
    "id": "http://pasture-one-actor/object/MoCIOGH2FyQ",
    "published": "2025-10-07T06:44:56Z",
    "content": "only content",
    "attachment": {
      "type": "Image",
      "url": "http://pasture-one-actor/images/4276cfb5-7f18-440e-9b05-9ae0c1cede85.png",
      "content": "content"
    },
    "tag": [
      {
        "type": "Mention",
        "href": "http://hollo/@john"
      }
    ]
  }
}
hollo
{
  "id": "0199bd6a-7a40-7937-8918-b29ade5f389d",
  "created_at": "2025-10-07T06:44:56.000Z",
  "in_reply_to_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/MoCIOGH2FyQ",
  "url": "http://pasture-one-actor/object/MoCIOGH2FyQ",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "pinned": false,
  "content": "only content",
  "reblog": null,
  "quote_id": null,
  "quote": null,
  "application": null,
  "account": {
    "id": "0199bd68-24e0-7001-bb0b-82b5d77020a3",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
    "created_at": "2025-10-07T06:42:23.014Z",
    "note": "",
    "url": "http://pasture-one-actor/actor",
    "avatar": "http://hollo/image/avatars/original/missing.png",
    "avatar_static": "http://hollo/image/avatars/original/missing.png",
    "header": "http://hollo/image/headers/original/missing.png",
    "header_static": "http://hollo/image/headers/original/missing.png",
    "followers_count": 0,
    "following_count": 0,
    "statuses_count": 0,
    "moved": null,
    "last_status_at": null,
    "emojis": [],
    "fields": []
  },
  "media_attachments": [
    {
      "id": "0199bd6a-7dea-73ff-b86f-97df315eefbe",
      "type": "image",
      "url": "http://pasture-one-actor/images/4276cfb5-7f18-440e-9b05-9ae0c1cede85.png",
      "preview_url": "http://pasture-one-actor/images/4276cfb5-7f18-440e-9b05-9ae0c1cede85.png",
      "remote_url": null,
      "text_url": null,
      "meta": {
        "original": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "small": {
          "width": 512,
          "height": 512,
          "size": "512x512",
          "aspect": 1
        },
        "focus": {
          "x": 0,
          "y": 0
        }
      },
      "description": null,
      "blurhash": null
    }
  ],
  "mentions": [],
  "tags": [],
  "card": null,
  "emojis": [],
  "emoji_reactions": [],
  "poll": null,
  "filtered": null
}

Example 6

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://hollo/@john"
  ],
  "id": "http://pasture-one-actor/activity/FGzLbn5JakA",
  "published": "2025-10-07T06:44:57Z",
  "object": {
    "type": "Note",
    "attributedTo": "http://pasture-one-actor/actor",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://hollo/@john"
    ],
    "id": "http://pasture-one-actor/object/TktBcNB6FFw",
    "published": "2025-10-07T06:44:57Z",
    "content": "no comment in attachment; reused image",
    "attachment": [
      {
        "type": "Image",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png"
      }
    ],
    "tag": [
      {
        "type": "Mention",
        "href": "http://hollo/@john"
      }
    ]
  }
}
hollo
{
  "id": "0199bd6a-7e28-7839-958f-934b256d34ff",
  "created_at": "2025-10-07T06:44:57.000Z",
  "in_reply_to_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "http://pasture-one-actor/object/TktBcNB6FFw",
  "url": "http://pasture-one-actor/object/TktBcNB6FFw",
  "replies_count": 0,
  "reblogs_count": 0,
  "favourites_count": 0,
  "favourited": false,
  "reblogged": false,
  "muted": false,
  "bookmarked": false,
  "pinned": false,
  "content": "no comment in attachment; reused image",
  "reblog": null,
  "quote_id": null,
  "quote": null,
  "application": null,
  "account": {
    "id": "0199bd68-24e0-7001-bb0b-82b5d77020a3",
    "username": "actor",
    "acct": "actor@pasture-one-actor",
    "display_name": "Test Actor",
    "locked": false,
    "bot": false,
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