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Header: AVL.h
Namespace: avl
Module: MetrologyBasic

Locates a specified number of the strongest dark or bright pixel peak along a given path (without a scan map).

Applications: Very fast detection (or presence verification) of thin structures like wires or scale marks.


void avl::ScanExactlyNRidges_Direct
	const avl::Image& inImage,
	const avl::Path& inScanPath,
	atl::Optional<const avl::CoordinateSystem2D&> inScanPathAlignment,
	int inScanWidth,
	const avl::SamplingParams& inSamplingParams,
	const avl::RidgeScanParams& inRidgeScanParams,
	int inRidgeCount,
	avl::Selection::Type inRidgeSelection,
	float inMinDistance,
	atl::Optional<float> inMaxDistance,
	atl::Optional<const avl::LocalBlindness&> inLocalBlindness,
	atl::Conditional<atl::Array<avl::Ridge1D> >& outRidges,
	atl::Conditional<atl::Array<avl::Gap1D> >& outGaps,
	atl::Optional<avl::Path&> outAlignedScanPath = atl::NIL,
	atl::Optional<avl::Profile&> outBrightnessProfile = atl::NIL,
	atl::Optional<avl::Profile&> outResponseProfile = atl::NIL,
	atl::Array<avl::Path>& diagSamplingPoints,
	float& diagSamplingStep


Name Type Range Default Description
Input value
inImage const Image& Input image
Input value
inScanPath const Path& Path along which the scan is performed
Input value
inScanPathAlignment Optional<const CoordinateSystem2D&> NIL Adjusts the scan path to the position of the inspected object
Input value
inScanWidth int 1 - 5 Width of the scan field in pixels
Input value
inSamplingParams const SamplingParams& SamplingParams ( Interpolation: Bilinear SamplingStep: 1.0f SampleCount: Nil ) Parameters controlling the sampling process
Input value
inRidgeScanParams const RidgeScanParams& RidgeScanParams ( ProfileInterpolation: Quadratic4 SmoothingStdDev: 0.6f RidgeWidth: 5 RidgeMargin: 2 MinMagnitude: 5.0f RidgePolarity: Dark ) Parameters controlling the ridge extraction process
Input value
inRidgeCount int 0 - 1 Number of ridges to be found
Input value
inRidgeSelection Selection::Type Selection mode of the resulting ridges
Input value
inMinDistance float 0.0 - 0.0f Minimal distance between consecutive ridges
Input value
inMaxDistance Optional<float> 0.0 - NIL Maximal distance between consecutive ridges
Input value
inLocalBlindness Optional<const LocalBlindness&> NIL Defines conditions in which weaker ridges can be detected in the vicinity of stronger ridges
Output value
outRidges Conditional<Array<Ridge1D> >& Found ridges
Output value
outGaps Conditional<Array<Gap1D> >& Gaps between consecutive edges
Output value
outAlignedScanPath Optional<Path&> NIL Transformed input path
Output value
outBrightnessProfile Optional<Profile&> NIL Extracted image profile
Output value
outResponseProfile Optional<Profile&> NIL Profile of the ridge operator response
Diagnostic input
diagSamplingPoints Array<Path>& Array of paths each one containing the sampling points that contributes to a single value of the extracted profile
Diagnostic input
diagSamplingStep float& Used distance between consecutive sampling points on the scan path

Optional Outputs

The computation of following outputs can be switched off by passing value atl::NIL to these parameters: outAlignedScanPath, outBrightnessProfile, outResponseProfile.

Read more about Optional Outputs.


The operation scans the image along inScanPath and finds a set of inRidgeCount image ridges perpendicular to the path. If no subset (of inRidgeCount elements) of detected ridges meets the requirements of inRidgeScanParams.minMagnitude, inMinDistance, inRidgeScanParams.ridgePolarity then the outputs are set to NIL.

The optional parameter inScanPathAlignment defines a transform to be performed on the inScanPath so that the actual scan path (outAlignedScanPath) is adjusted to the position of the object, typically detected by one of Template Matching filters.

Note that in case of a scan path which is closed, the parameters controlling the distances between consecutive found objects do not control the distance between the first and the last of the found objects (counting from the beginning of the scan path).


  • Set inRidgeCount to the number of ridges that are to be found (the N number).
  • Define inEdgeScanParams.RidgePolarity to detect a particular ridge type, and only that type.
  • If the expected number of ridges cannot be found, try decreasing inRidgeScanParams.MinMagnitude. Verify this with the values on the outResponseProfile output.
  • If consecutive ridges are very close to each other, change inEdgeScanParams.ProfileInterpolation to Quadratic3.
  • Adjust inMinDistance (in pixels) to filter out false ridges that appear in proximity to other ridges.


ScanMultipleRidges_Direct locates the ridges across inScanPath.


Read more about Local Coordinate Systems in Machine Vision Guide: Local Coordinate Systems.

This filter is a part of the 1D Edge Detection toolset. For a comprehensive introduction to this technique please refer to 1D Edge Detection and 1D Edge Detection - Subpixel Precision chapters of our Machine Vision Guide.

See Also

  • CreateScanMap – Precomputes a data object that is required for fast 1D edge detection.
  • ScanSingleRidge_Direct – Locates the strongest dark or bright pixel peak along a given path (without a scan map).
  • ScanExactlyNRidges_Direct – Locates a specified number of the strongest dark or bright pixel peak along a given path (without a scan map).