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ImageLocalMaxima

Header:	AVL.h
Namespace:	avl
Module:	FoundationLite

Finds image locations characterized by locally maximal pixel values.

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

Syntax

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

Parameters

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

Description

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.

Hints

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

Examples

Example image

Output for example image

Remarks

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.

Errors

List of possible exceptions:

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