---
license: apache-2.0
tags:
- instance-segmentation
- panoptic-segmentation
- semantic-segmentation
- zero-shot
- open-vocabulary
- sam2
- beit3
- fiftyone
library_name: transformers
pipeline_tag: image-segmentation
---

# OpenWorldSAM — Zero-Shot Universal Image Segmentation

OpenWorldSAM extends SAM2 with language understanding to enable
open-vocabulary instance, panoptic, semantic, and referring segmentation
from arbitrary text prompts — **without any task-specific fine-tuning**.

This repository is an **unofficial HuggingFace Hub mirror** of the
[GinnyXiao/OpenWorldSAM](https://github.com/GinnyXiao/OpenWorldSAM) checkpoint,
uploaded to enable loading via `transformers` and the
[FiftyOne Model Zoo](https://docs.voxel51.com/user_guide/model_zoo/index.html).
All credit belongs to the original authors.

## Attribution

> **Paper:** "Extending SAM2 for Universal Image Segmentation with Language Prompts"  
> Fangxun Shu\*, Yiwen Ye\*, Jianhua Han, Jiwen Yu, Qize Yang, Xiao-Ping Zhang, Hang Xu, Bei Yu, Xiaodan Liang.  
> NeurIPS 2025 Spotlight · [arXiv 2507.05427](https://arxiv.org/abs/2507.05427)

> **Original code:** [GinnyXiao/OpenWorldSAM](https://github.com/GinnyXiao/OpenWorldSAM) — Apache-2.0 License

> **Checkpoint:** ADE20K instance segmentation (`openworld_sam_ade20k.pt`)
> from the official [Google Drive release](https://drive.google.com/drive/folders/1kPCkl3D4GJ2jJT3E39FyQoZiuuZOV9N8)

## Usage

### Via FiftyOne Model Zoo

```python
import fiftyone as fo
import fiftyone.zoo as foz

dataset = foz.load_zoo_dataset("quickstart", max_samples=5)
model = foz.load_zoo_model(
    "openworld-sam-ade20k-torch",
    class_names=["person", "car", "chair", "table", "sky", "tree"],
)
dataset.apply_model(model, label_field="owsam_pred")
session = fo.launch_app(dataset)
```

### Standalone (`trust_remote_code`)

```python
import torch
from transformers import AutoModel

model = AutoModel.from_pretrained(
    "neerajaabhyankar/openworld-sam", trust_remote_code=True
)
model.eval()

import numpy as np
arr = np.array(your_pil_image)  # HWC uint8 RGB

batched_inputs = [{
    "image":             model.preprocess_image(arr),
    "evf_image":         model.preprocess_image_beit3(arr),
    "height":            arr.shape[0],
    "width":             arr.shape[1],
    "prompt":            ["person", "car", "tree"],
    "unique_categories": [0, 1, 2],
}]

with torch.no_grad():
    outputs = model(batched_inputs)

# outputs[0]["instances"]:
#   "masks"     — bool tensor [N, H, W]
#   "scores"    — float tensor [N]
#   "class_ids" — long tensor [N]
```

## Architecture

| Component | Detail |
|-----------|--------|
| Visual backbone | SAM2 Hiera-Large (frozen, 224M params) |
| Multimodal encoder | BEiT-3 Large (frozen, 675M params) |
| Trainable params | ~4.5M — projection MLP + positional tokens + 3-layer cross-attention |
| Total params | ~902M |
| Vocabulary | ADE20K-150 classes (default); any text at inference |
| SAM2 input | 1024×1024, normalised with ImageNet pixel stats |
| BEiT-3 input | 224×224, normalised to mean=0.5 std=0.5 |

## Notes

Loading this checkpoint via `from_pretrained` prints a LOAD REPORT with a few
MISSING/UNEXPECTED keys. Both are understood and safe to ignore for the
single-image zero-shot segmentation path documented above:

- **`evf_sam2.text_hidden_fcs.*` — MISSING (dead duplicate module, harmless).**
  Both `EvfSam2Model.__init__` (`model/evf_sam2.py`) and
  `OpenWorldSAMModel.__init__` (`modeling_openworld_sam.py`) independently
  construct their own `text_hidden_fcs` projection `ModuleList`. The
  checkpoint only ever stored one copy, at the top-level `text_hidden_fcs.*`
  key — that's the one `OpenWorldSAMModel` actually uses in `forward()`
  (`self.text_hidden_fcs[0](feat)`) and it loads correctly. The nested
  `evf_sam2.text_hidden_fcs` copy is never referenced anywhere in
  `OpenWorldSAMModel.forward()`, so it gets randomly initialized and simply
  sits unused. No effect on inference output. (Could be cleaned up by
  removing the unused construction in `EvfSam2Model.__init__`, but there's
  no correctness reason to.)

- **`memory_encoder.fuser.layers.{0,1}` — gamma/weight name mismatch (real
  bug, but on an unused code path).** In
  `model/segment_anything_2/sam2/modeling/memory_encoder.py`, `CXBlock`
  has a comment claiming the layer-scale parameter was renamed:
  `# modified by ZhangYx from self.gamma to self.weight. Due to
  (facebookresearch/segment-anything-2#85)` — but the code still declares
  `self.gamma`. The checkpoint was produced by the actual ZhangYx fork,
  which *did* apply the rename, so its keys are `...weight`. Since the
  bundled code never got the rename, `gamma` and `weight` don't match:
  the checkpoint's `weight` values show as UNEXPECTED, and the model's
  `gamma` parameters show as MISSING (randomly initialized instead of
  loaded).
  In practice this doesn't affect the model as used here: `memory_encoder`
  (and its `fuser`) is only invoked from SAM2's video mask-propagation path
  (`self.memory_encoder(...)` in `sam2_base.py`, called during track-step),
  which `OpenWorldSAMModel.forward()` never exercises — it only calls
  `visual_model.forward_image()` + `_prepare_backbone_features()` for
  single-image inference. This would only matter if a future integration
  adds video/memory-based mask propagation on top of this checkpoint; at
  that point, fix it by renaming `self.gamma` → `self.weight` in `CXBlock`
  to match the checkpoint.

## Requirements

```
torch torchvision transformers safetensors timm einops
```

No `detectron2` required — this mirror is self-contained.

## License

Apache-2.0 (same as original GinnyXiao/OpenWorldSAM)