How To: Normalized Coordinates of the Bounding Box [ymin, xmin, ymax, xmax] for TFDS, TensorFlow

Paul Xiong
2 min readSep 29, 2022

#tfds.features.BBoxFeature

TFDS BBoxFeature uses normalized coordinates.

What is the normalized coordinates then?

Normalized device coordinate or NDC space is a screen independent display coordinate system; it encompasses a cube where the x, y, and z components range from −1 to 1.

An example for xmin, ymin, xmax, ymax

The bounding box has the following (x, y) coordinates of its corners: top-left is (x_min, y_min) or (98px, 345px), top-right is (x_max, y_min) or (420px, 345px), bottom-left is (x_min, y_max) or (98px, 462px), bottom-right is (x_max, y_max) or (420px, 462px). As you see, coordinates of the bounding box's corners are calculated with respect to the top-left corner of the image which has (x, y) coordinates (0, 0).

pascal_voc

pascal_voc is a format used by the Pascal VOC dataset. Coordinates of a bounding box are encoded with four values in pixels: [x_min, y_min, x_max, y_max]. x_min and y_min are coordinates of the top-left corner of the bounding box. x_max and y_max are coordinates of bottom-right corner of the bounding box.

Coordinates of the example bounding box in this format are [98, 345, 420, 462].

albumentations

albumentations is similar to pascal_voc, because it also uses four values [x_min, y_min, x_max, y_max] to represent a bounding box. But unlike pascal_voc, albumentations uses normalized values. To normalize values, we divide coordinates in pixels for the x- and y-axis by the width and the height of the image.

Coordinates of the example bounding box in this format are [98 / 640, 345 / 480, 420 / 640, 462 / 480] which are [0.153125, 0.71875, 0.65625, 0.9625].

Albumentations uses this format internally to work with bounding boxes and augment them.

--

--

Paul Xiong
Paul Xiong

Written by Paul Xiong

Predicting the next word (token) is what powers ChatGPT, while predicting the next photo (embedding) forms the foundation of ImageGPT.

No responses yet