Girish Mahajan (Editor)

Top hat transform

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In mathematical morphology and digital image processing, top-hat transform is an operation that extracts small elements and details from given images. There exist two types of top-hat transform: The white top-hat transform is defined as the difference between the input image and its opening by some structuring element; The black top-hat transform is defined dually as the difference between the closing and the input image. Top-hat transforms are used for various image processing tasks, such as feature extraction, background equalization, image enhancement, and others.

Contents

Mathematical definitions

Let f : E R be a grayscale image, mapping points from an Euclidean space or discrete grid E (such as R2 or Z2) into the real line. Let b ( x ) be a grayscale structuring element.

Then, the white top-hat transform of f is given by:

T w ( f ) = f f b ,

where denotes the opening operation.

The black top-hat transform of f (sometimes called the bottom-hat transform ) is given by:

T b ( f ) = f b f ,

where is the closing operation.

Properties

The white top-hat transform returns an image, containing those "objects" or "elements" of an input image that:

  • Are "smaller" than the structuring element (i.e., places where the structuring element does not fit in), and
  • are brighter than their surroundings.
  • The black top-hat returns an image, containing the "objects" or "elements" that:

  • Are "smaller" than the structuring element, and
  • are darker than their surroundings.
  • The size, or width, of the elements that are extracted by the top-hat transforms can be controlled by the choice of the structuring element b . The bigger the latter, the larger the elements extracted.

    Both top-hat transforms are images that contain only non-negative values at all pixels.

    References

    Top-hat transform Wikipedia