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Object localization

Object localization is the machine learning problem that encompasses object detection–finding whether an object exists exists in an image–and finding the location of the object an image.

The location of an object in an image is typically represented as a “bounding box”.

An example of 2-dimensional object localization

Below is a rough example of what a machine learning model would output if trained and evaluated on 2D images for the purpose of drawing 2D bounding boxes around a single object. In the real world, many of the exact details may vary.

To train a machine learning model to identify the presence or absence of a single object, the model’s output would be a single 1 (for the object’s presence) or 0 (for the object’s absence). We’ll refer to this as \(p_{\mathrm{exists}}\).

But to draw a two-dimensional bounding box showing where the object is in the image, the model would need to process the image ane return a vector that looked like the following:

\[ \begin{bmatrix} p_{\mathrm{exists}} \\ b_{\mathrm x} \\ b_{\mathrm y} \\ b_{\mathrm{width}} \\ b_{\mathrm{height}} \end{bmatrix} \]

If the object exists, then the remaining values in the vector have the following meanings:

  • \(b_{\mathrm x}\) is the X position of the object in the image. This may either be the center of the object, or the X position of one of the corners.
  • \(b_{\mathrm y}\) is the Y position of the object in the image. This may either be the center of the object, or the Y position of one of the corners.
  • \(b_{\mathrm{width}}\) is the width of the bounding box.
  • \(b_{\mathrm{height}}\) is the height of the bounding box.

When the object does not exist in the given image, many machine learning models may return vectors with random numbers.

When training an object localization model, one typically uses a dataset where \(p_{\mathrm{exists}}\) is known to be 1 or 0. When evaluating an object localization model, the model typically returns \(p_{\mathrm{exists}}\) as a probability between 0 and 1. When using this probability, we may pick a threshold–such as 0.5–where we assume the model has found an object.

Different variations in object localization

In addition to 2D images from digital cameras, object localization is also relevant in other sensor data that might capture objects–such as a RADAR or LIDAR scan.

Object localization models may be trained to identify multiple objects, or to identify objects with 3D bounding boxes or non-rectangular bounding “boxes”.