Back to FabImage Library website

You are here: Start » Function Reference » Image » Image Features » ImageLocalMaxima


Header: FIL.h
Namespace: fil
Module: FoundationLite

Finds image locations characterized by locally maximal pixel values.

Applications: Detection of characteristic points, usually after some image transformations.


void fil::ImageLocalMaxima
	const fil::Image& inImage,
	ftl::Optional<const fil::Region&> inRoi,
	bool inConsiderPlateaus,
	ftl::Optional<float> inMinValue,
	ftl::Optional<float> inMaxValue,
	float inMinDistance,
	ftl::Optional<const fil::ImageLocalExtremaVerification&> inMaximaVerification,
	ftl::Optional<ftl::Array<fil::Extremum2D>&> outLocalMaxima,
	ftl::Optional<ftl::Array<fil::Region>&> outMaximaRegions = ftl::NIL


Name Type Range Default Description
Input value inImage const Image& Input image
Input value inRoi Optional<const Region&> NIL Range of pixels to be processed
Input value inConsiderPlateaus bool Consider multi-pixel maxima (plateaus) or not
Input value inMinValue Optional<float> NIL Minimal value of maximum to be considered
Input value inMaxValue Optional<float> NIL Maximal value of maximum to be considered
Input value inMinDistance float 0.0 - Minimal distance between two found maxima
Input value inMaximaVerification Optional<const ImageLocalExtremaVerification&> NIL Maxima verification structure
Output value outLocalMaxima Optional<Array<Extremum2D>&> Found local maxima
Output value outMaximaRegions Optional<Array<Region>&> NIL Regions of local maxima (plateaus and singletons)

Optional Outputs

The computation of following outputs can be switched off by passing value ftl::NIL to these parameters: outLocalMaxima, outMaximaRegions.

Read more about Optional Outputs.


The operation finds local maxima on the image inImage within specific region inRoi.

It returns three arrays of the same length:

  • Subpixel precise positions of the maxima
  • Values of the maxima
  • Regions of equally bright pixels around the maxima

If the given image consists of more than one channel, they are averaged to obtain monochromatic image, so that the values of the extrema are of type float.

There are two types of local extrema:

  • Peak/hole: single pixel (strictly) brighter/darker than its neighbourhood (eight pixels). The extremum's position is determined with subpixel precision using gradient method with Sobel gradient. Returned region contains only one pixel.
  • Plateau/lowland: a connected set of equally bright pixels (strictly) brighter/darker from their neighbourhood. Returned position for this extremum is the center of mass of the plateau.

If parameter inConsiderPlateaus is set to true, the filter finds both types. Otherwise, it finds only strict maxima.

Parameter inMinValue determines the minimal value of the maximum to be considered.


  • If some points are not detected, try decreasing inMinValue or setting inConsiderPlateaus to True.
  • Consider adding some Gaussian smoothing before this function.


Example image

Output for example image


If a plateau/lowland is concave, it's center of mass may lie outside it. In such case, the value of the extremum (which is equal to the common value of its pixels) may be different from the value of pixel indicated by position of the extremum.


List of possible exceptions:

Error type Description
DomainError Region exceeds an input image in ImageLocalMaxima.

See Also

  • ImageLocalMinima – Finds image locations characterized by locally minimal pixel values.