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Header: AVL.h
Namespace: avl
Module: MetrologyPro

Performs a series of 1D ridge detections and finds an arc that best matches the detected points.

Applications: Precise detection of a thin arciform line, whose rough location is known beforehand.


void avl::FitArcToRidges
	const avl::Image& inImage,
	const avl::ArcFittingMap& inFittingMap,
	const RidgeScanParams& inRidgeScanParams,
	avl::Selection::Type inRidgeSelection,
	atl::Optional<const avl::LocalBlindness&> inLocalBlindness,
	float inMaxIncompleteness,
	avl::CircleFittingMethod::Type inFittingMethod,
	atl::Optional<avl::MEstimator::Type> inOutlierSuppression,
	atl::Conditional<avl::Arc2D>& outArc,
	atl::Optional<atl::Array<atl::Conditional<avl::Ridge1D> >&> outRidges = atl::NIL,
	atl::Optional<atl::Conditional<avl::Profile>&> outDeviationProfile = atl::NIL,
	atl::Optional<atl::Array<avl::Point2D>&> outInliers = atl::NIL,
	atl::Array<avl::Profile>& diagBrightnessProfiles,
	atl::Array<avl::Profile>& diagResponseProfiles


Name Type Range Default Description
inImage const Image& Input image to fit arc to
inFittingMap const ArcFittingMap& Input fitting map
inRidgeScanParams const RidgeScanParams& Parameters controlling the ridge extraction process
inRidgeSelection Selection::Type Selection::​Best Selection mode of ridges
inLocalBlindness Optional<const LocalBlindness&> NIL Defines conditions in which weaker ridges can be detected in the vicinity of stronger ridges
inMaxIncompleteness float 0.0 - 0.999 0.1f Maximal fraction of ridge points not found
inFittingMethod CircleFittingMethod::Type AlgebraicTaubin Method used to fit an arc
inOutlierSuppression Optional<MEstimator::Type> NIL Selects a method for ignoring incorrectly detected points
outArc Conditional<Arc2D>& Fitted arc or nothing if the fitting fails
outRidges Optional<Array<Conditional<Ridge1D> >&> NIL Found ridges
outDeviationProfile Optional<Conditional<Profile>&> NIL Profile of distances between the actual arc points and the corresponding reference arc points
outInliers Optional<Array<Point2D>&> NIL Points matching the fitting Arc
diagBrightnessProfiles Array<Profile>& Extracted image profiles
diagResponseProfiles Array<Profile>& Profiles of the ridge operator response

Optional Outputs

The computation of following outputs can be switched off by passing value atl::NIL to these parameters: outRidges, outDeviationProfile, outInliers.

Read more about Optional Outputs.


The operation tries to fit a given arc to ridges present in the inImage image. Internally, it performs a series of scans with the ScanSingleRidge filter using inFittingMap previously generated from the object being fitted. The found points are then used to determine the actual position of the arc in the image. Only inMaxIncompleteness fraction of these scans may fail. If the fitting according to the given parameters is not possible, outArc is set to Nil.


  • Connect an input image to the inImage input.
  • Define inRidgeScanParams.RidgePolarity to detect a particular ridge type, and only that type.
  • Adjust inRidgeScanParams.RidgeWidth to the expected thickness of the ridge (in pixels).
  • If no or too few ridge points are found, try decreasing inRidgeScanParams.MinMagnitude.
  • If some of the scans may fail, set the inMaxIncompleteness input accordingly.
  • If some of the scans may produce false results, try different values of the inOutlierSuppression input.
  • Use the outRidges outputs to visualize the scanning results.


Fitting an arc to a curved wall of a plastic capsule
(inRidgeScanParams.RidgeWidth = 3, inRidgeScanParams.Polarity = Dark).


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

This filter is a part of the Shape Fitting toolset. To read more about this technique, one can refer to the Shape Fitting chapter of our Machine Vision Guide

Hardware Acceleration

This operation supports automatic parallelization for multicore and multiprocessor systems.

See Also

  • CreateArcFittingMap – Precomputes a data object that is required for fast arc fitting on images.
  • FitArcToEdges – Performs a series of 1D edge detections and finds an arc that best matches the detected points.
  • FitArcToStripe – Performs a series of 1D stripe detections and finds an arc that best matches the detected points.
  • FitArcToRidges_Direct – Performs a series of 1D ridge detections and finds an arc that best matches the detected points.