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.

Objects

Object 1

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "attachment": {
    "type": "Document",
    "url": "http://pasture-one-actor/images/100.png"
  },
  "attributedTo": "http://actor.example",
  "content": "no description",
  "id": "http://actor.example/object/-2K6glRDmTE",
  "published": "2025-08-15T08:43:29Z",
  "to": [
    "as:Public"
  ],
  "type": "Note"
}

Object 2

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "attachment": {
    "content": "content",
    "name": "name",
    "summary": "summary",
    "type": "Document",
    "url": "http://pasture-one-actor/images/100.png"
  },
  "attributedTo": "http://actor.example",
  "content": "text",
  "id": "http://actor.example/object/pXdnlt7eMRE",
  "published": "2025-08-15T08:43:29Z",
  "to": [
    "as:Public"
  ],
  "type": "Note"
}

Object 3

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "attachment": [
    {
      "imageType": "image/jpeg",
      "name": "name",
      "type": "Document",
      "url": "http://pasture-one-actor/assets/FediverseLogo.png"
    }
  ],
  "attributedTo": "http://actor.example",
  "content": "text",
  "id": "http://actor.example/object/83SZiI45raU",
  "published": "2025-08-15T08:43:29Z",
  "to": [
    "as:Public"
  ],
  "type": "Note"
}

Object 4

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "attachment": [
    {
      "type": "Document",
      "url": "http://pasture-one-actor/assets/FediverseLogo.png"
    }
  ],
  "attributedTo": "http://actor.example",
  "content": "no comment in attachment",
  "id": "http://actor.example/object/pp0Q98FmYUU",
  "published": "2025-08-15T08:43:29Z",
  "to": [
    "as:Public"
  ],
  "type": "Note"
}

Activities

Activity 1

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "actor": "http://actor.example",
  "id": "http://actor.example/activity/_kMwrzEthM8",
  "object": {
    "attachment": {
      "type": "Document",
      "url": "http://pasture-one-actor/images/100.png"
    },
    "attributedTo": "http://actor.example",
    "content": "no description",
    "id": "http://actor.example/object/cpSFZ12_4XU",
    "published": "2025-08-15T08:43:29Z",
    "to": [
      "as:Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-08-15T08:43:29Z",
  "to": [
    "as:Public",
    "http://remote.example/"
  ],
  "type": "Create"
}

Activity 2

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "actor": "http://actor.example",
  "id": "http://actor.example/activity/jcJxBDQSGoY",
  "object": {
    "attachment": {
      "content": "content",
      "name": "name",
      "summary": "summary",
      "type": "Document",
      "url": "http://pasture-one-actor/images/100.png"
    },
    "attributedTo": "http://actor.example",
    "content": "text",
    "id": "http://actor.example/object/cdQScTTkw-M",
    "published": "2025-08-15T08:43:29Z",
    "to": [
      "as:Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-08-15T08:43:29Z",
  "to": [
    "as:Public",
    "http://remote.example/"
  ],
  "type": "Create"
}

Activity 3

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "actor": "http://actor.example",
  "id": "http://actor.example/activity/EEqyRr-Jqpo",
  "object": {
    "attachment": [
      {
        "imageType": "image/jpeg",
        "name": "name",
        "type": "Document",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png"
      }
    ],
    "attributedTo": "http://actor.example",
    "content": "text",
    "id": "http://actor.example/object/mvGc7EGWF68",
    "published": "2025-08-15T08:43:29Z",
    "to": [
      "as:Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-08-15T08:43:29Z",
  "to": [
    "as:Public",
    "http://remote.example/"
  ],
  "type": "Create"
}

Activity 4

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "actor": "http://actor.example",
  "id": "http://actor.example/activity/-uKoAFo1JUE",
  "object": {
    "attachment": [
      {
        "type": "Document",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png"
      }
    ],
    "attributedTo": "http://actor.example",
    "content": "no comment in attachment",
    "id": "http://actor.example/object/l_6T7z-P7Tw",
    "published": "2025-08-15T08:43:29Z",
    "to": [
      "as:Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-08-15T08:43:29Z",
  "to": [
    "as:Public",
    "http://remote.example/"
  ],
  "type": "Create"
}