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Module: MetrologyPro

Performs a series of 1D edge detections and finds a circle that best matches the detected points.


Precise detection of a circular object or hole, whose rough location is known beforehand.
Name Type Range Description
Input value
inImage Image Image to fit the circle to
Input value
inFittingField CircleFittingField Circle fitting field
Input value
inFittingFieldAlignment CoordinateSystem2D* Adjusts the fitting field to the position of the inspected object
Input value
inScanCount Integer 3 - The number of points that will be searched to estimate the position of the circle
Input value
inScanWidth Integer 1 - The width of each scan field (in pixels)
Input value
inSamplingParams SamplingParams Parameters controlling the sampling process
Input value
inEdgeScanParams EdgeScanParams Parameters controlling the edge extraction process
Input value
inEdgeSelection Selection Selection mode of edges
Input value
inLocalBlindness LocalBlindness* Defines conditions in which weaker edges can be detected in the vicinity of stronger edges
Input value
inMaxIncompleteness Real 0.0 - 0.999 Maximal fraction of edge points not found
Input value
inFittingMethod CircleFittingMethod Method used to fit a circle
Input value
inOutlierSuppression MEstimator* Selects a method for ignoring incorrectly detected points
Output value
outCircle Circle2D? Fitted circle or nothing if the fitting fails
Output value
outEdges Edge1D?Array Found edges
Output value
outDeviationProfile Profile? Profile of distances between the actual circle points and the corresponding reference circle points
Output value
outAlignedFittingField CircleFittingField Fitting field used; in the image coordinate system
Output value
outInliers Point2DArray Points matching the fitting Circle
Diagnostic input
diagScanSegments Segment2DArray Segments along which the scans were run
Diagnostic input
diagSamplingAreas Rectangle2DArray Areas from which the input image is sampled
Diagnostic input
diagBrightnessProfiles ProfileArray Extracted image profiles
Diagnostic input
diagResponseProfiles ProfileArray Profiles of the edge (derivative) operator response


The operation tries to fit a given circle to edges present in the inImage image. Internally, it performs a series of scans with the ScanSingleEdge filter along inScanCount specific scan segments which length is always equal to the inFittingField width and cannot be less than 4. The found points are then used to determine the actual position of the circle in the image. Only inMaxIncompleteness fraction of these scans may fail. If the fitting according to the given parameters is not possible, outCircle is set to Nil.


  • Connect an input image to the inImage input.
  • If the object location is variable, connect inFittingFieldAlignment to an appropriate local coordinate system (e.g. from a template matching filter).
  • Define the inFittingField input to specify the area in which edge scanning will be performed.
  • Define inEdgeScanParams.EdgeTransition to detect a particular edge type, and only that type.
  • If the noise level is high, try increasing inScanWidth and/or inEdgeScanParams.SmoothingStdDev.
  • If no or too few edge points are found, try decreasing inEdgeScanParams.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 diagScanSegments and outEdges outputs to visualize the scanning results.


Description of usage of this filter can be found in examples and tutorial: Badge Measurement, Calibration World Coordinates With Remapped Images, Gasket Inspection, Radiator Holes, Parallel Image Saving, Shape Fitting: Measuring holes, Shape Fitting: Gasket.

Fitting a circles to the edges of a hole
(inEdgeScanParams.Transition = BrightToDark).


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.

Complexity Level

This filter is available on Basic Complexity Level.

Filter Group

This filter is member of FitCircle filter group.

See Also

  • FitCircleToRidges – Performs a series of 1D ridge detections and finds a circle that best matches the detected points.
  • FitCircleToStripe – Performs a series 1D stripe detections and finds a circle that best matches the detected points.