Extract Arc

## Extract Arc

Find points that meet the gradient response value, then use the RANSAC algorithm to fit a circular arc.

## Parameter Configuration Description

### Main Interface Parameters

![](/media/202601/提取圆圆弧_1768369210.png)

**[Gradient Threshold]** The grayscale gradient of the edge position along the scanning direction; points greater than this threshold are retained.
*   For clear images with obvious edges: Set a higher value (>40).
*   For blurry images or indistinct edges: Set a lower value (10-30).
![](/media/202601/tidu_1769763009.png)

**[Scanning Direction]** Scan from inside out or from outside in, indicated by the arrow on the ROI.

**[Polarity]**:
*   **Black to White:** Along the ROI arrow, pointing from a low grayscale area to a high grayscale area.
*   **White to Black:** Along the ROI arrow, pointing from a high grayscale area to a low grayscale area.
*   **All:** Can detect both black-to-white and white-to-black transitions.

**[Fitting Point Selection]**:
*   **All Points:** Use all detected edge points.
*   **Strongest Point:** Use the edge point with the maximum gradient value encountered along the scanning direction.
*   **First Row Points:** Use the first edge point encountered along the scanning direction.
*   **Last Row Points:** Use the last edge point encountered along the scanning direction.

**[Arc Filtering]**:
*   **Optimal Fit:** Comprehensively considers the best-fitted arc.
*   **Most Fitting Points:** The arc with the highest number of qualifying fitting points among all fitted circles/arcs.
*   **Strongest Fit:** The arc with the strongest gradient values for its fitting points among all fitted circles/arcs.
*   **First Arc:** The first arc fitted along the scanning direction.
*   **Last Arc:** The last arc fitted along the scanning direction.
*   **Specified ID Arc:** Filters out the arc with the specified ID along the scanning direction.

### Advanced Parameters
![](/media/202601//1767687645.9347115.png)

#### Filtering Parameters:

**[Gaussian Filter Threshold]** The σ in the Gaussian function, which can be understood as half the width of the transition region pixels. (A larger value reduces noise impact but blurs edges; a smaller value retains more detail but increases noise impact.)
![](/media/202602/滤波阈值_1770111631.png)

**[Sampling Points Count]** The number of uniform divisions of the ROI along the sector arc direction.
![](/media/202602/2026-02-13_155648_7257680.44161060302276745.png)

**[Sampling Point Width]** Based on the center points divided evenly by the sampling points count, take the sampling point width to form small rectangular regions. Calculate and judge along the scanning direction within these small rectangular regions to check for qualifying edge points.
![](/media/202602/2026-02-13_155920_9387490.205714192935314.png)

**[Minimum Fitting Points Count]** The minimum number of fitting points required for arc fitting. If the count is less than this value, the fitting is considered failed.
![](/media/202602/2026-02-13_160203_3967900.7973577897029431.png)

**[Distance Threshold]** Edge response points whose perpendicular distance to the fitted circle is less than this value are considered fitting points (also known as inliers). As shown in the figure below, edge points greater than the set distance threshold are displayed in red.
![](/media/202602/2026-02-13_151438_4592480.33066702526903646.png)

**[Maximum Iterations]** The maximum number of iterations required. If an arc meeting the conditions is fitted early, the fitting process can stop prematurely.

**[Iterations Without Improvement]** Stop iteration if the arc fitting effect does not continue to improve during the fitting process.

**[Score Threshold]** The final score used to judge the arc. If the score is less than this threshold, the fitting is considered failed. The range is 0-1. When all fitting points lie exactly on the fitted circle, the score is 1.

#### Judgment Parameters:

**[Size Conversion]** Two options: Scale and Calibration, used to calculate the arc's diameter.
*   If **Calibration** is selected, an additional port will be added to import the calibration file.
*   If **Scale** is selected, enter the scale value in the subsequent box.

**[Diameter Compensation]** Used to compensate for the diameter of the fitted arc.

**[Diameter Filtering]** When this judgment condition is enabled, if the diameter of the fitted arc is not within the specified range, the fitting is considered failed.

### ROI Settings

![](/media/202601/ROI设置_1768369374.png)

**[ROI Source]**
*   **Manual ROI:** Allows manual dragging of the ROI position and size in the window.
*   **Referenced ROI:** Allows connecting to an external ROI via a port.

**[Position Correction]** Selecting pose correction allows connection to a coordinate system, enabling the ROI area to move with the coordinate system.

**[Position Adjustment]** Adjust the position and size of the ROI.

## Output Description
*   **Run Result:** Outputs `True` if successful; outputs `False` if failed.
*   **Fitting Error:** The average error of the edge points used for circle fitting relative to the circle.
*   **Fitting Count:** The number of edge points used for circle fitting.
*   **Diameter Output:** The diameter of the detected arc.
*   **Center Output:** The center point of the detected arc.
*   **Arc Output:** The detected arc.
*   **Annular ROI:** The ROI used for detection.
*   **Fitting Points Array:** The set of edge points used for arc fitting.
![](/media/202602/2026-02-13_160231_8106350.2641124485866745.png)

---

Click here to jump to the corresponding Chinese manual page: [Extract Arc](/doc/87)