Fit Circle to Surface Edge

Description

Use this Step to fit a circle to the circular edge detected from the surface data.

Basic Concepts

Calipers, circular path, edge points

The Step will generate a circular path on the basis of the set feature region, which takes the width (or length, the same) of the feature region as the diameter and the center point as the circle center. Calipers are then placed along the circular path. They are evenly spaced and arranged radially, with their centers on the circular path. In each caliper, profiles are extracted along the length of the caliper based on the specified profile spacing. From each profile, points that meet requirements are detected and marked as edge points. Finally, these edge points will be used to fit a circle.

basic concepts

As shown in the figure, the brownish-yellow dots at the edge in the calipers represent the edge points, and the bright yellow line represents the radius of the fitted circle.

In summary, the basic concepts you should grasp while using this Step are as follows:

  • Caliper

    The calipers will be evenly spaced along the circular path. Within each caliper, several profiles (not displayed) are extracted along the caliper’s length at the specified spacing.

  • Circular path

    The circle that takes the width (or length, the same) of the feature region as the diameter and the center point as the circle center, which is used for determining the positions of calipers.

    If no feature region is set, the diameter of the circular path will be equal to the shorter side of the image’s bounding box, with the center of the circle located at the center of the bounding box.

  • Edge point

    Points that meet the parameter requirements are detected from the profiles in each caliper and are used for circle fitting.

Workflow

The process of configuring this Step is shown below.

fit circle to surface edge process

  1. Configure the input. Connect the Step ports in the graphical programming workspace or select the input under Input in the parameter configuration panel.

  2. Set a feature region, and then set the general parameters.

  3. According to the selected edge detection mode, set the relevant parameters for either step edges or corner edges.

  4. Select the desired output items under Output. For an expandable output item, click ▶ and configure the Min and Max values to determine the acceptable range for the item.

  5. Run the Step and view output.

Parameter Description

General Parameters

Parameter Description

Use Intensity Image

Select this option to use the intensity image for edge detection when the target detection region is flat, and the color differences in the region are prominent.

Once selected, the Step uses the intensity image for circular edge detection and circle fitting; otherwise, the Step uses the depth map by default.

Use Feature Region

When this parameter is not selected, the Step uses the entire surface data to detect the circular edge and fit the circle. When selected, the Step only uses the data within the feature region for detection and fitting.

See Feature Region to learn about how to adjust a feature region.

Use Masks

After selecting this option, you can set 1 to 4 mask regions to block out part of the data. Data within the mask regions is excluded from circular edge detection and fitting.

Start Angle

Set the angle to determine the position of the first caliper, and the remaining calipers will be placed counterclockwise at equal intervals according to relevant settings.

Interval between calipers = Angle Span ÷ Caliper Count. After the start angle is set, the first caliper will be placed at half an interval angle away from the start angle. Then, the remaining calipers are placed sequentially in a counterclockwise direction at the specified intervals.

Angle Span

The angle range for evenly arranging the specified number of calipers.

As shown in the figure below, it is recommended that the set Angle Span, Caliper Count, Caliper Length, and Caliper Width ensure the detected edge points evenly cover the entire circular edge.

optimal settings

Caliper Count

The number of calipers that need to be evenly placed along the circular path.

Caliper Length

The caliper’s length direction is perpendicular to the tangent to the circular path at the caliper’s center.

Caliper Width

The caliper’s width direction parallels the tangent to the circular path at the caliper’s center.

Search Direction

The direction for detecting edge points. The Step will extract profiles along the search direction in the calipers to detect edge points.

Value list: Outward, Inward.

Circle Fitting Method

The method for fitting a circle, which can be used to calculate and assess the roundness of a fitted circle.

  • Least square circle (LSC): This method uses the least squares approach to fitting a circle.

  • Minimum circumscribed circle (MCC): This method fits the smallest circle that contains all the edge points.

  • Maximum inscribed circle (MIC): This method fits the largest circle that is completely surrounded by all the edge points.

  • Minimum zone circle (MZC): This method fits two concentric circles that completely enclose all edge points, with the distance between the two circles kept to a minimum.

    When selecting this method, you need to set the Circle Type to Inner or Outer.

Outlier Fraction

The percentage of outlier points to be removed during circle fitting.

Only visible when the “Circle Fitting Method” is Least square circle (LSC).

Profile Spacing

The distance between adjacent profiles in the calipers. If the set spacing is less than the pixel width of a pixel, the Step will take the pixel width as the profile spacing.

The smaller the spacing, the more profiles there are, the more edge points are detected, and the more accurate the fitted straight line, but the computational cost is also higher.

Profile Averaging Width

The size of the windows for calculating the average of data points along the direction perpendicular to the search direction to obtain a smoother profile and thus reduce the impact of noise. When it is set to 0, no averaging operation will be performed on the profiles.

Set the parameter according to your actual needs. A window that is too large may lead to excessive smoothing and loss of important details; if a window is too small, however, the noise may not be effectively reduced.

Edge Detection Mode

The depth or intensity changing mode of data points at the edge.

  • Step: The depth or intensity values of points at the edge change abruptly. Once you select the step mode, set step edge parameters.

  • Corner: The depth or intensity values of points at the edge change continuously. Once you select the corner mode, set corner edge parameters.

Show Detail

Select this parameter to display the calipers and circular path in the visualization window.

Step Edge Parameters

Set the parameters below when you set the edge detection mode to Step.

Parameter Description

Window Size for Profile Smoothing

The window size for averaging profiles along the search direction. After averaging, the profile will be smoother. When it is set to 0, no averaging operation will be performed on the profiles along the search direction.

Gap Filling Threshold

The data points on a profile may have gaps due to occlusion. When the size of a gap on a profile is less than or equal to the set threshold, use the first valid point from the lower side of the gap to fill the gap. When the gap size exceeds the set threshold, leave the gap as null point(s).

If the gap size exceeds the set threshold, you can decide whether to Include Null points or not in the detection process.

Include Null Points

Use this parameter to determine whether to include null points or not in the edge detection process. (Null points are points without depth or intensity information, which are often the result of data loss or inappropriate settings of the feature region(s).) Once this parameter is selected, set Null Filling Value of Depth.

Null Filling Value of Depth

A depth value for filling null points.

Only needs to be set after Include Null Points is selected.

Null Filling Value of Intensity

An intensity value for filling null points.

Only needs to be set after both Use Intensity Image and Include Null Points are selected.

Step Width

The width of the step on a profile. When the step on a profile is a slope, set this parameter can help you accurately measure the step height and locate the position of the edge.

The step width should be slightly greater than the edge with so that the Step can accurately measure the step height on the basis of the flat region on both sides of the edge.

Step Direction

The step direction along the search direction.

Value list: Rising, Falling, Rising or falling.

Step Type

Set the type of step that will be selected from each profile.

Value list: Best, First, Last.

Absolute Threshold

The minimum height variation for a potential step on a profile. A step that satisfies the absolute threshold can produce an edge point.

When the parameter Use Intensity Image is selected, absolute threshold means the minimum intensity variation for a potential step on a profile.

Use Relative Threshold

Use this parameter to determine that a step is valid if the height (or intensity) change of the step is not less than the set percentage of the maximum change found on a profile. Once this parameter is selected, set Relative Threshold.

A step is valid only when both Absolute Threshold and Relative Threshold are satisfied.

Relative Threshold

Only visible when Use Relative Threshold is selected.

Corner Edge Parameters

Set the parameters below when you set the edge detection mode to Corner.

Parameter Description

Corner Type

Select a proper corner type from the drop-down list to determine the edge point.

Value list: First, Last, Top, Bottom, Best.

Epsilon

This parameter reflects the sensibility to the change of the profile shape. A larger value means that fewer points can meet the requirements, and thus it is more challenging to detect corners.

In most cases, set the value between 0 and 1 can meet the needs.

Output Description

Select the output item(s) to add the output port(s) to the Step, and the corresponding data will be output after the Step is run. You can select the output according to the actual measurement requirements.

If you select an expandable output item, you should expand it by clicking ▶, and then set the Min and Max values to determine the acceptable range. If the output value falls within the acceptable range, the measurement item is judged as passing (OK), or else it is judged as failing (NG).
Output item Description

Circle Center X

X value of the center of the fitted circle.

Circle Center Y

Y value of the center of the fitted circle.

Radius

The radius of the fitted circle.

Roundness

The degree to which the fitted circle approximates the ideal circle. A smaller value indicates a closer approximation, meaning the fitted circle is more accurate.

Min Error

The error value of the edge point farthest within the fitted circle, which is a negative number.

Max Error

The error value of the edge point farthest outside the fitted circle, which is a positive number.

Center of Fitted Circle

The center of the fitted circle.

Fitted Circle

The circle fitted to the circular edge.

Min Error Point

The point corresponds to Min Error.

Max Error Point

The point corresponds to Max Error.

Troubleshooting

  • For common errors, see Error Code List.

  • If the error code is inconsistent with the error message, please contact Technical Support for help.

CV-W3401

Error: The “Profile Spacing” value is not within the valid range.

Possible cause: The parameter value is less than 0 or greater than the length of the longest side of the image.

Solution: Ensure the parameter value is greater than or equal to 0 and less than the length of the longest side of the image.

CV-W3402

Error: The set “Window Size for Profile Smoothing” is invalid.

Possible causes:

  • The parameter value is less than 0 or greater than the length of the longest side of the image.

  • The parameter value is greater than the caliper length.

Solutions:

  • Ensure the parameter value is greater than or equal to 0 and less than the length of the longest side of the image.

  • Ensure the parameter value is less than the caliper length.

CV-W3403

Error: The set “Step Width” is invalid.

Possible causes:

  • The parameter value is less than 0 or greater than the length of the longest side of the image.

  • The parameter value is greater than the caliper length.

Solutions:

  • Ensure the parameter value is greater than or equal to 0 and less than the length of the longest side of the image.

  • Ensure the parameter value is less than the caliper length.

CV-W3404

Error: The set “Gap Filling Threshold” is invalid.

Possible cause: The parameter value is less than 0 or greater than the length of the longest side of the image.

Solution: Ensure the parameter value is greater than or equal to 0 and less than the length of the longest side of the image.

CV-W3405

Error: The set “Caliper Width” is invalid.

Possible cause: The parameter value is less than or equal to 0, or it is greater than the length of the longest side of the image.

Solution: Ensure the parameter value is greater than 0 and less than the length of the longest side of the image.

CV-W3406

Error: The set “Caliper Width” is invalid.

Possible cause: The parameter value is less than or equal to 0, or it is greater than the length of the longest side of the image.

Solution: Ensure the parameter value is greater than 0 and less than the length of the longest side of the image.

CV-W3407

Error: No edge points detected.

Possible causes:

  • The calipers are not set properly.

  • “Absolute Threshold” and “Relative Threshold” are too large.

Solutions:

  • Adjust the caliper settings to ensure edge points can be detected from the calipers.

  • Lower “Absolute Threshold” and “Relative Threshold” values.

CV-W3408

Error: The “Outlier Fraction” value is outside of the valid range.

Possible cause: The parameter value is less than 0 or greater than 1.

Solution: Make sure the parameter value is within 0–1.

CV-W3409

Error: The “Profile Averaging Width” value is outside of the valid range.

Possible causes:

  • The parameter value is less than 0 or greater than the length of the longest side of the image.

  • The parameter value is greater than the caliper width.

Solutions:

  • Ensure the parameter value is greater than or equal to 0 and less than the length of the longest side of the image.

  • Lower the parameter value or raise the caliper width to ensure that the parameter value is less than the caliper width.

CV-W340A

Error: The “Caliper Count” value is outside of the valid range.

Possible cause: The parameter value is less than 1 or greater than or equal to 1000.

Solution: Make sure the parameter value is within 1–1000.

CV-W340B

Error: The “Start Angle” value is outside of the valid range.

Possible cause: The parameter value is less than −180° or greater than 180°.

Solution: Make sure the parameter value is within −180°–180°.

CV-W340C

Error: The “Angle Span” value is outside of the valid range.

Possible cause: The parameter value is less than 0° or greater than 360°.

Solution: Make sure the parameter value is within 0°–360°.

CV-W340D

Error: The set “Circle Fitting Method” is invalid.

Solution: Select a valid circle fitting method from the drop-down list.

CV-W340E

Error: The “Null Filling Value of Intensity” value is the valid range.

Possible cause: The parameter value is less than 0 or greater than 255.

Solution: Make sure the parameter value is within 0–255.

CV-W340F

Error: The “Null Filling Value of Depth” is outside of the valid range.

Possible cause: The parameter value is less than −10000 or greater than 10000.

Solution: Make sure the parameter value is within −10000–10000.

CV-W3410

The “Epsilon” value is outside of the valid range.

Possible cause: The parameter value is less than 0.

Solution: Make sure the parameter value is greater than or equal to 0.

CV-W3411

Error: The set “Corner Type” is invalid.

Solution: Select a valid corner type from the drop-down list.

CV-W3412

Error: The “Absolute Threshold” value is outside of the valid range.

Possible cause: The parameter value is less than 0 or greater than 255.

Solution: When the “Use Intensity Image” parameter is selected, ensure the absolute threshold is within 0–255.

CV-W3413

Error: The “Absolute Threshold” value is outside of the valid range.

Possible cause: The parameter value is less than 0 or greater than 10000.

Solution: Make sure the parameter value is within 0–10000.

CV-W3414

Error: The “Relative Threshold” value is outside of the valid range.

Possible cause: The parameter value is less than 0 or greater than 1.

Solution: Make sure the parameter value is within 0–1.

CV-W3415

Error: The set “Edge Detection Mode” is invalid.

Solution: Select a valid edge detection mode from the drop-down list.

CV-W3416

Error: The set “Search Direction” is invalid.

Solution: Select a valid search direction from the drop-down list.

CV-W3417

Error: The set “Step Direction” is invalid.

Solution: Select a valid step direction from the drop-down list.

CV-W3418

Error: The set “Step Type” is invalid.

Solution: Select a valid step type from the drop-down list.

CV-W3419

Error: The set “Circle Type” is invalid.

Solution: Select a valid circle type from the drop-down list.

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