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/oBIhGbhVX3Y",
  "published": "2025-07-15T09:07:23Z",
  "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/EW5rjSbprbM",
  "published": "2025-07-15T09:07:23Z",
  "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/T_VveAwZYY8",
  "published": "2025-07-15T09:07:23Z",
  "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/W18Dh1HaQJU",
  "published": "2025-07-15T09:07:23Z",
  "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/uVMv65lJphw",
  "object": {
    "attachment": {
      "type": "Document",
      "url": "http://pasture-one-actor/images/100.png"
    },
    "attributedTo": "http://actor.example",
    "content": "no description",
    "id": "http://actor.example/ViyldKroFxA",
    "published": "2025-07-15T09:07:23Z",
    "to": [
      "as:Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-07-15T09:07:23Z",
  "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/bmB2wkcMC-0",
  "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/QsQwVxONkQE",
    "published": "2025-07-15T09:07:23Z",
    "to": [
      "as:Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-07-15T09:07:23Z",
  "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/n8rnPPXLc3M",
  "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/3ixl4pi8dI8",
    "published": "2025-07-15T09:07:23Z",
    "to": [
      "as:Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-07-15T09:07:23Z",
  "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/zw6PsFmvuCg",
  "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/8WQDcZLduVY",
    "published": "2025-07-15T09:07:23Z",
    "to": [
      "as:Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-07-15T09:07:23Z",
  "to": [
    "as:Public",
    "http://remote.example/"
  ],
  "type": "Create"
}