> ## 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.
# 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" }
}
```
> Omit `engine` to use **Avatar IV** automatically. Pass `{ "type": "avatar_v" }` or `{ "type": "avatar_iii" }` to select a different engine.
Before requesting a non-default engine, confirm the avatar look supports it by checking `supported_api_engines` on the look (`GET /v3/avatars/looks/{look_id}`). Requesting an engine that isn't listed returns a `400` error.
## 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 | ✗ | ✓ | ✗ |
| Arbitrary image input | ✗ | ✓ | ✗ |
| `motion_prompt` | ✓ | ✓ | ✗ |
| `expressiveness` | ✗ | ✓ | ✗ |
| Cross-reference animation | ✓ | ✗ | ✗ |
| 4K output | ✗ | ✗ | Digital Twin only |
| Eligibility check required | ✓ | ✗ | ✓ |
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.