LabelImg is a widely used open-source tool for image annotation, mainly designed for object detection tasks. Many users want to know whether it supports polygon (free-shape) annotation in addition to bounding boxes.
The short answer is no — LabelImg does not support polygon annotation.
What LabelImg Supports
LabelImg is built specifically for bounding box annotation, where objects are labeled using rectangular boxes.
Its core features include:
- Rectangle (bounding box) annotation
- Class labeling
- YOLO and Pascal VOC format export
- Image navigation and dataset labeling
These features are focused on object detection datasets.
Why Polygon Annotation Is Not Available
Polygon annotation requires drawing multiple points around an object to create an irregular shape. This is typically used for segmentation tasks.
LabelImg was designed to stay lightweight and simple, so it only supports rectangular bounding boxes instead of complex shapes like polygons.
Difference Between Bounding Box and Polygon
Bounding boxes in LabelImg are simple rectangles that surround an object. They are easier and faster to create but less precise.
Polygon annotations, on the other hand, trace the exact shape of an object, providing higher precision for tasks like image segmentation.
Use Cases of LabelImg
Since it only supports bounding boxes, LabelImg is best suited for:
- Object detection models
- YOLO training datasets
- Simple classification with localization
- Quick dataset labeling
It is not suitable for segmentation-based AI tasks.
Alternatives for Polygon Annotation
If polygon annotation is required, users need more advanced tools such as:
- CVAT
- Label Studio
- Supervisely
These tools support segmentation, polygons, and other advanced annotation types that LabelImg does not offer.
Why LabelImg Is Still Popular
Even without polygon support, LabelImg remains popular because it is:
- Lightweight
- Easy to use
- Fast for bounding boxes
- Free and open-source
- Ideal for YOLO datasets
For many object detection projects, bounding boxes are sufficient.
Conclusion
LabelImg does not support polygon annotation because it is designed specifically for simple bounding box labeling. While this limits its use for segmentation tasks, it remains highly effective for object detection dataset preparation.