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Header: AVL.h
Namespace: avl
Module: MetrologyBasic

Locates multiple transitions between dark and bright pixels along a given path.

Applications: Very fast detection of multiple edge points - usually for object counting or displacement detection.


void avl::ScanMultipleEdges
	const avl::Image& inImage,
	const ScanMap& inScanMap,
	const EdgeScanParams& inEdgeScanParams,
	float inMinDistance,
	atl::Optional<float> inMaxDistance,
	atl::Optional<const avl::LocalBlindness&> inLocalBlindness,
	atl::Array<avl::Edge1D>& outEdges,
	atl::Array<avl::Gap1D>& outGaps,
	avl::Profile& diagBrightnessProfile,
	avl::Profile& diagResponseProfile


Name Type Range Default Description
inImage const Image& Input image
inScanMap const ScanMap& Data precomputed with CreateScanMap
inEdgeScanParams const EdgeScanParams& EdgeScanParams ( ProfileInterpolation: Quadratic4 SmoothingStdDev: 0.6f MinMagnitude: 5.0f EdgeTransition: BrightToDark ) Parameters controlling the edge extraction process
inMinDistance float 0.0 - 0.0f Minimal distance between consecutive edges
inMaxDistance Optional<float> 0.0 - NIL Maximal distance between consecutive edges
inLocalBlindness Optional<const LocalBlindness&> NIL Defines conditions in which weaker edges can be detected in the vicinity of stronger edges
outEdges Array<Edge1D>& Found edges
outGaps Array<Gap1D>& Gaps between consecutive edges
diagBrightnessProfile Profile& Extracted image profile
diagResponseProfile Profile& Profile of the edge (derivative) operator response


The operation scans the image using inScanMap previously generated from a scan path and detects image edges perpendicular to the path. Depending on the inEdgeScanParams.edgeTransition parameter, edges representing increase or decrease (or both) of image brightness along the path will be taken into account.

When the number of edges to be found is known, one can use the ScanExactlyNEdges.

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).


  • Define inEdgeScanParams.EdgeTransition to detect a particular edge type, and only that type.
  • If too few edges are found, try decreasing inEdgeScanParams.MinMagnitude. Verify this with the values on the diagResponseProfile output.
  • If consecutive edges are closer than 6 pixels apart, change inEdgeScanParams.ProfileInterpolation to Quadratic3.
  • Adjust inMinDistance (in pixels) to filter out false edges that appear in proximity to other edges.


ScanExactlyNEdges locates the edges using a scan map representing the scan path above.


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.
  • ScanSingleEdge – Locates the strongest transition between dark and bright pixels along a given path.
  • ScanExactlyNEdges – Locates a specified number of the strongest transitions between dark and bright pixels along a given path.
  • ScanExactlyNEdges_Direct – Locates a specified number of the strongest transitions between dark and bright pixels along a given path (without a scan map).