> ## 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.

<Note>
  **Free usage:** Avatar IV videos created through CLI OAuth (`heygen auth login --oauth`) draw from your free monthly avatar-video quota — no pay-as-you-go charge. See [Free usage for agents](/docs/for-ai-agents#free-usage).
</Note>

## 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"`). Available for photo avatars and arbitrary images.
* `expressiveness` — controls energy and range of movement: `high`, `medium`, or `low`. Available for photo avatars and arbitrary images. Defaults to `low`.
* **Arbitrary image support** — animate any image by setting `type: "image"`, with no registered avatar required.

## Example request

Avatar IV is the default engine, so you can omit `engine` entirely in your [`POST /v3/videos`](/reference/create-video) request:

```bash theme={null}
curl -X POST "https://api.heygen.com/v3/videos" \
  -H "x-api-key: YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "type": "avatar",
    "avatar_id": "YOUR_LOOK_ID",
    "script": "Hello from Avatar IV.",
    "voice_id": "YOUR_VOICE_ID",
    "resolution": "1080p"
  }'
```

To request it explicitly — or to be unambiguous in code that switches engines — pass `"engine": { "type": "avatar_iv" }`:

```bash theme={null}
curl -X POST "https://api.heygen.com/v3/videos" \
  -H "x-api-key: YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "type": "avatar",
    "avatar_id": "YOUR_LOOK_ID",
    "script": "Hello from Avatar IV.",
    "voice_id": "YOUR_VOICE_ID",
    "resolution": "1080p",
    "engine": { "type": "avatar_iv" }
  }'
```

Video generation is asynchronous — the response returns a `video_id` you poll with `GET /v3/videos/{video_id}`. 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).