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Oriented Bounding Boxes (OBB)

Oriented bounding boxes (OBB) represent rotated objects with a 5th coordinate—the angle—in addition to the 4 coordinates used for axis-aligned boxes (HBB). OBB is useful for aerial imagery, boats, vehicles, and any scene where objects can be rotated.

HBB vs OBB

  • HBB (axis-aligned): 4 coordinates—[x_min, y_min, x_max, y_max] or equivalent in other formats. The box edges are parallel to the image axes.
  • OBB (oriented): 5 coordinates—the same 4 as HBB plus angle as the 5th value. The box can be rotated.

Original boats with OBB annotations

All 5 Formats for OBB

All coordinate formats supported for HBB also work for OBB. Append angle as the 5th coordinate:

FormatHBBOBB
coco[x_min, y_min, w, h][x_min, y_min, w, h, angle]
pascal_voc[x_min, y_min, x_max, y_max][x_min, y_min, x_max, y_max, angle]
albumentations[x_min, y_min, x_max, y_max] normalized[x_min, y_min, x_max, y_max, angle]
yolo[cx, cy, w, h] normalized[cx, cy, w, h, angle]
cxcywh[cx, cy, w, h] pixels[cx, cy, w, h, angle]

Angle Semantics

The angle is the rotation of the width edge from horizontal, in [-90, 90) degrees.

  • Width = length of the edge more parallel to horizontal
  • Height = length of the other edge
  • Angle = rotation of the width edge from horizontal

The base rectangle has corners [-w/2,-h/2], [w/2,-h/2], [w/2,h/2], [-w/2,h/2]; at 0° the width edge is horizontal. Albumentations uses the same convention as OpenCV's cv2.minAreaRect and cv2.boxPoints for compatibility.

BboxParams for OBB

Set bbox_type='obb' and choose your coord_format:

import albumentations as A

# OBB in cxcywh format (center, width, height, angle in degrees)
bbox_params = A.BboxParams(
    coord_format='cxcywh',
    bbox_type='obb',
    label_fields=['labels'],
)

transform = A.Compose([
    A.Affine(scale=(0.9, 1.1), rotate=(-15, 15), shear=(-5, 5), p=0.9),
    A.HorizontalFlip(p=0.5),
], bbox_params=bbox_params)

Transform Support

Most Dual transforms support OBB, including Affine, Rotate, HorizontalFlip, and VerticalFlip. Check the Supported Targets by Transform reference for details. If a transform does not support OBB, the pipeline will raise an error at initialization.

Affine + OBB Examples

Affine supports scale, translate, rotate, and shear—all of which correctly transform OBB angles.

Affine: single-parameter examples

Affine: rotation grid

Affine: scale × rotation grid

Affine: shear × rotation grid

Random Affine (training-style augmentation)

Pipeline: Affine + HFlip + BrightnessContrast

Clipping for OBB

  • clip_bboxes_on_input: Clips boxes to image bounds once before the pipeline. For OBB this is lossy—boxes with corners outside [0, 1] become axis-aligned (angle=0). Recommend False for OBB when using Affine or rotation.
  • clip_after_transform: Clips after each transform. For OBB, clips all 4 rotated corners and returns a wrapping axis-aligned box (angle=0). Default: True. Set to False if you want to allow temporary excursions outside bounds during the pipeline.

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