> ## 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.

# Models

> Browse every HeyGen avatar rendering engine behind the API - Avatar V, Avatar IV, and Avatar III - and learn how to choose between them.

<video controls muted playsInline className="w-full aspect-video rounded-xl" src="https://mintcdn.com/heygen-1fa696a7/xiWXoDBsEfd3CTgQ/images/videos/avatar-models.mp4?fit=max&auto=format&n=xiWXoDBsEfd3CTgQ&q=85&s=6d3d157629e0e8f62b00cab49b1440ed" title="Avatar rendering engines in 30 seconds — Avatar III vs IV vs V and how to choose" data-path="images/videos/avatar-models.mp4" />

# HeyGen Avatar Rendering Engines

HeyGen has three avatar rendering engines, all available on the v3 API and selected per request through the `engine` field on `POST /v3/videos`. New integrations should default to [Avatar IV](/avatar-iv) for broad coverage, opt into [Avatar V](/avatar-v) for the highest-fidelity motion and lip-sync, and use [Avatar III](/avatar-iii) for its dedicated photo-to-video pipeline.

```json theme={null}
{
  "type": "avatar",
  "avatar_id": "YOUR_LOOK_ID",
  "script": "Hello from HeyGen.",
  "voice_id": "YOUR_VOICE_ID",
  "engine": { "type": "avatar_iv" }
}
```

Before requesting **Avatar V**, confirm the avatar look supports it by checking `supported_api_engines` on the look (`GET /v3/avatars/looks/{look_id}`) — requesting it for a look that doesn't list it returns a `400` error. **Avatar III** does not appear in `supported_api_engines`; it accepts any avatar look shown in the table below.

## Choosing an engine

|                            | [Avatar V](/avatar-v) | [Avatar IV](/avatar-iv) | [Avatar III](/avatar-iii)   |
| -------------------------- | --------------------- | ----------------------- | --------------------------- |
| `engine.type`              | `avatar_v`            | `avatar_iv`             | `avatar_iii`                |
| API version                | v3                    | v3                      | v3                          |
| Default engine             | ✗ (explicit opt-in)   | ✓                       | ✗ (explicit opt-in)         |
| Available to new users     | ✓                     | ✓                       | ✓                           |
| Digital Twin               | ✓                     | ✓                       | ✓                           |
| Photo Avatar               | ✗                     | ✓                       | ✓ (dedicated pipeline)      |
| Studio Avatar              | ✗                     | ✓                       | ✓ (renders as Digital Twin) |
| Arbitrary image input      | ✗                     | ✓                       | ✗                           |
| `motion_prompt`            | ✓                     | ✓                       | ✗                           |
| `expressiveness`           | ✗                     | ✓                       | ✗                           |
| Cross-reference animation  | ✓                     | ✗                       | ✗                           |
| 4K output                  | ✗                     | ✗                       | ✓ (except Photo Avatar)     |
| Eligibility check required | ✓                     | ✗                       | ✗                           |

<Note>
  A separate, older Avatar III engine remains available to existing customers through the **legacy v1/v2 endpoints**. It uses a different pipeline and is **not** the same as the Avatar III (`avatar_iii`) engine documented here. It is not offered on the new developer platform. See [Avatar III](/avatar-iii#legacy-avatar-iii-v1v2) for the distinction.
</Note>