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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.

Support Table Preview

attachment Object Activity
no description
{
  “type”: “Image”,
  “url”: “http://pasture-one-actor/images/e8b0766b-7cb5-4156-bb17-16c6d98639b3.png”
}
 Object Activity
name, summary and content
{
  “type”: “Image”,
  “url”: “http://pasture-one-actor/images/eeb67658-e42b-4760-917a-c45523fcb915.png”,
  “name”: “name”,
  “summary”: “summary”,
  “content”: “content”
}
 Object Activity
only name
[
  {
    “type”: “Image”,
    “url”: “http://pasture-one-actor/assets/FediverseLogo.png”,
    “name”: “name”,
    “imageType”: “image/jpeg”
  }
]
 Object Activity
only summary
{
  “type”: “Image”,
  “url”: “http://pasture-one-actor/images/e39e0e9b-8a13-42ca-898a-52993b9566f8.png”,
  “name”: “name”,
  “summary”: “summary”,
  “content”: “content”
}
 Object Activity
only content
{
  “type”: “Image”,
  “url”: “http://pasture-one-actor/images/2bff5333-7be4-4a48-a128-93959fd41ffe.png”,
  “content”: “content”
}
 Object Activity
no comment in attachment; reused image
[
  {
    “type”: “Image”,
    “url”: “http://pasture-one-actor/assets/FediverseLogo.png”
  }
]
 Object Activity

Objects

Object 1

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "attachment": {
    "type": "Image",
    "url": "http://pasture-one-actor/images/e8b0766b-7cb5-4156-bb17-16c6d98639b3.png"
  },
  "attributedTo": "http://actor.example",
  "content": "no description",
  "id": "http://actor.example/object/4nNF90WDkeg",
  "published": "2025-10-12T08:53:00Z",
  "to": [
    "https://www.w3.org/ns/activitystreams#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": "Image",
    "url": "http://pasture-one-actor/images/eeb67658-e42b-4760-917a-c45523fcb915.png"
  },
  "attributedTo": "http://actor.example",
  "content": "name, summary and content",
  "id": "http://actor.example/object/YJrLUZBLpQY",
  "published": "2025-10-12T08:53:00Z",
  "to": [
    "https://www.w3.org/ns/activitystreams#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": "Image",
      "url": "http://pasture-one-actor/assets/FediverseLogo.png"
    }
  ],
  "attributedTo": "http://actor.example",
  "content": "only name",
  "id": "http://actor.example/object/Klqb2VZKMeI",
  "published": "2025-10-12T08:53:00Z",
  "to": [
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type": "Note"
}

Object 4

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "attachment": {
    "content": "content",
    "name": "name",
    "summary": "summary",
    "type": "Image",
    "url": "http://pasture-one-actor/images/e39e0e9b-8a13-42ca-898a-52993b9566f8.png"
  },
  "attributedTo": "http://actor.example",
  "content": "only summary",
  "id": "http://actor.example/object/gI4HbyIuxoY",
  "published": "2025-10-12T08:53:00Z",
  "to": [
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type": "Note"
}

Object 5

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "attachment": {
    "content": "content",
    "type": "Image",
    "url": "http://pasture-one-actor/images/2bff5333-7be4-4a48-a128-93959fd41ffe.png"
  },
  "attributedTo": "http://actor.example",
  "content": "only content",
  "id": "http://actor.example/object/7JxsT18pddM",
  "published": "2025-10-12T08:53:00Z",
  "to": [
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type": "Note"
}

Object 6

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "attachment": [
    {
      "type": "Image",
      "url": "http://pasture-one-actor/assets/FediverseLogo.png"
    }
  ],
  "attributedTo": "http://actor.example",
  "content": "no comment in attachment; reused image",
  "id": "http://actor.example/object/AL7zE3pNl9Y",
  "published": "2025-10-12T08:53:00Z",
  "to": [
    "https://www.w3.org/ns/activitystreams#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/xebh1HfuIOY",
  "object": {
    "attachment": {
      "type": "Image",
      "url": "http://pasture-one-actor/images/e8b0766b-7cb5-4156-bb17-16c6d98639b3.png"
    },
    "attributedTo": "http://actor.example",
    "content": "no description",
    "id": "http://actor.example/object/R7SdpinKh_0",
    "published": "2025-10-12T08:53:00Z",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-10-12T08:53:00Z",
  "to": [
    "http://remote.example/",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "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/vkqWCHHqaa4",
  "object": {
    "attachment": {
      "content": "content",
      "name": "name",
      "summary": "summary",
      "type": "Image",
      "url": "http://pasture-one-actor/images/eeb67658-e42b-4760-917a-c45523fcb915.png"
    },
    "attributedTo": "http://actor.example",
    "content": "name, summary and content",
    "id": "http://actor.example/object/lm71rS_2_OU",
    "published": "2025-10-12T08:53:00Z",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-10-12T08:53:00Z",
  "to": [
    "http://remote.example/",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "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/yq0Jfw8gi28",
  "object": {
    "attachment": [
      {
        "imageType": "image/jpeg",
        "name": "name",
        "type": "Image",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png"
      }
    ],
    "attributedTo": "http://actor.example",
    "content": "only name",
    "id": "http://actor.example/object/iken8fz5HvA",
    "published": "2025-10-12T08:53:00Z",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-10-12T08:53:00Z",
  "to": [
    "http://remote.example/",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "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/iLmiY7_7sVU",
  "object": {
    "attachment": {
      "content": "content",
      "name": "name",
      "summary": "summary",
      "type": "Image",
      "url": "http://pasture-one-actor/images/e39e0e9b-8a13-42ca-898a-52993b9566f8.png"
    },
    "attributedTo": "http://actor.example",
    "content": "only summary",
    "id": "http://actor.example/object/t6Ayl2TsVnE",
    "published": "2025-10-12T08:53:00Z",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-10-12T08:53:00Z",
  "to": [
    "http://remote.example/",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type": "Create"
}

Activity 5

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "actor": "http://actor.example",
  "id": "http://actor.example/activity/lch-DTjm5dY",
  "object": {
    "attachment": {
      "content": "content",
      "type": "Image",
      "url": "http://pasture-one-actor/images/2bff5333-7be4-4a48-a128-93959fd41ffe.png"
    },
    "attributedTo": "http://actor.example",
    "content": "only content",
    "id": "http://actor.example/object/bNDMhhCxrVA",
    "published": "2025-10-12T08:53:00Z",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-10-12T08:53:00Z",
  "to": [
    "http://remote.example/",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type": "Create"
}

Activity 6

{
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    {
      "Hashtag": "as:Hashtag",
      "sensitive": "as:sensitive"
    }
  ],
  "actor": "http://actor.example",
  "id": "http://actor.example/activity/eBJ0QrrgaTQ",
  "object": {
    "attachment": [
      {
        "type": "Image",
        "url": "http://pasture-one-actor/assets/FediverseLogo.png"
      }
    ],
    "attributedTo": "http://actor.example",
    "content": "no comment in attachment; reused image",
    "id": "http://actor.example/object/ZEPgmF1CajU",
    "published": "2025-10-12T08:53:00Z",
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "http://remote.example/"
    ],
    "type": "Note"
  },
  "published": "2025-10-12T08:53:00Z",
  "to": [
    "http://remote.example/",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "type": "Create"
}