> ## Documentation Index
> Fetch the complete documentation index at: https://heygen-1fa696a7.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Avatar IV

> Avatar IV is the default HeyGen v3 rendering engine - the broadest avatar support, arbitrary image animation, motion_prompt, and expressiveness control.

## Supported avatar types

`studio_avatar`, `digital_twin`, `photo_avatar`, `image` (arbitrary), `prompt`

## Exclusive features

* `motion_prompt` — a natural-language string controlling body motion and hand gestures (e.g. `"walk towards the camera slowly"`). Photo avatars and arbitrary images only.
* `expressiveness` — controls energy and range of movement: `high`, `medium`, or `low`. Photo avatars and arbitrary images only. Defaults to `low`.
* **Arbitrary image support** — animate any image by setting `type: "image"`, with no registered avatar required.

## Example

Because Avatar IV is the default, you can omit `engine` entirely:

```json theme={null}
{
  "type": "avatar",
  "avatar_id": "YOUR_LOOK_ID",
  "script": "Hello from Avatar IV.",
  "voice_id": "YOUR_VOICE_ID",
  "resolution": "1080p"
}
```

> `engine` is omitted — Avatar IV is selected automatically. You can also request it explicitly with `"engine": { "type": "avatar_iv" }`.

For a full walkthrough of creating a Digital Twin video and polling for completion, see the [Digital Twin guide](/generate-avatar-video). To compare engines, see [Models](/models); for the highest-fidelity motion, see [Avatar V](/avatar-v).