Extract Parallel Lines

## Extract Parallel Lines

Find edge points that meet the gradient response value, then use the RANSAC algorithm to fit two parallel lines.

## Parameter Configuration Description

### Main Interface Parameters
![](/media/202602/2026-02-10_150315_1194380.660786409187248.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_1769762616.png)

**[Line Scanning Direction]** Scan from top to bottom or from bottom to top, indicated by the arrow on the ROI.

**[Line Polarity]**:
*   **Black to White:** Along the rectangular arrow, pointing from a low grayscale area to a high grayscale area.
*   **White to Black:** Along the rectangular arrow, pointing from a high grayscale area to a low grayscale area.
*   **All:** Can detect both black-to-white and white-to-black transitions.
![](/media/202601/jixing_1769766940.png)

**[Line Type]**:
*   **First Line:** The first line fitted along the scanning direction.
*   **Last Line:** The last line fitted along the scanning direction.

### Advanced Parameters
![](/media/202602/2026-02-10_150603_2474120.9152114183120532.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)

**[Angle Range Relative to ROI]** The allowable deviation angle for the fitted line relative to the ROI. As shown in the figure below, if the "Angle Range Relative to ROI" is set to 16, and the difference between the Line Angle and ROI Angle is 9 (which is within 16), the line is considered valid.
![](/media/202602/与ROI角度_1770110521.png)

**[Sampling Points Count]** The number of uniform divisions perpendicular to the ROI scanning line direction.
![](/media/202602/采样点数_1770016300.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/采样点宽度_1770017004.png)

**[Fitting Points Count]** The minimum number of fitting points required for line fitting. If the count is less than this value, the fitting is considered failed. As shown in the figure below, with the fitting points count set to 5, there are a total of 6 points marked by white circles.
![](/media/202602/最少拟合点数_1770111223.png)

**[Distance Threshold]** Edge points whose perpendicular distance to the fitted line is less than this value will be used for line fitting. As shown in the figure below, detected edge points with a distance to the line less than the distance threshold are displayed in blue, while those greater than the threshold are displayed in red.
![](/media/202602/距离阈值_1770630645.png)

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

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

**[Score Threshold]** The final score used to judge the line. If the score is less than this threshold, the fitting is considered failed. The range is 0-1. The score is calculated by summing the perpendicular distances from all edge points to the fitted line; the smaller the sum, the higher the score. When all fitting points lie exactly on the fitted line, the score is 1.

### ROI Settings
![](/media/202601//1767686688.3281448.png)

**[ROI Type]** The default ROI type for the detection area is a Skewed Rectangle.

**[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 lines are successfully found; outputs `False` if failed.
*   **Skewed Rectangle ROI:** The ROI used for detection.
*   **Line 1:** The first detected line.
*   **Line 1 Angle:** The angle of the first detected line.
*   **Line 2:** The second detected line.
*   **Line 2 Angle:** The angle of the second detected line.
*   **Bisector:** The angle bisector of Line 1 and Line 2.
*   **Bisector Angle:** The angle of the angle bisector of Line 1 and Line 2.
*   **Distance Segment:** The perpendicular line segment between Line 1 and Line 2.
*   **Distance Segment Angle:** The angle of the perpendicular line segment between Line 1 and Line 2.
*   **Distance Segment Midpoint:** The midpoint of the perpendicular line segment between Line 1 and Line 2.
*   **Distance Segment Length:** The length (distance) of the perpendicular line segment between Line 1 and Line 2.
![](/media/202602/2026-02-10_150812_3596680.9658928286385875.png)

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Click here to jump to the corresponding Chinese manual page: [Extract Parallel Lines](/doc/91)