Detect and Fit Line

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Description

This Step is used to fit a line to the straight edge detected from the image.

Usage Scenario

This Step is a general geometric feature detection and fitting Step.

Basic Concepts

Caliper

In image processing, a virtual measurement tool detects edges within a specified region. Adjusting its count, width, and length affects the accuracy and stability of edge detection.
Edge polarity

It refers to the direction of grayscale change across an edge, e.g., dark-to-light or light-to-dark.

Input and Output

Input

Input ports	Data type	Description
Image	Image	The image used for detecting a straight edge.
Alignment Parameter Group	Pose2D	Used to adjust the target region’s pose in sync with the target object’s pose changes.

Input ports

Data type

Description

Image

The image used for detecting a straight edge.

Alignment Parameter Group

Pose2D

Used to adjust the target region’s pose in sync with the target object’s pose changes.

Output

Output Port	Data Type	Description
Fitted Line	Shape2D/Line	The line obtained by fitting.
Midpoint	Shape2D/Point	The midpoint of the fitted line.
Midpoint X	Number	The X coordinate of the midpoint of the fitted line.
Midpoint Y	Number	The Y coordinate of the midpoint of the fitted line.
Rotation Angle	Number	The rotation angle of the fitted line relative to the positive X-axis.

Output Port

Data Type

Description

Fitted Line

Shape2D/Line

The line obtained by fitting.

Midpoint

Shape2D/Point

The midpoint of the fitted line.

Midpoint X

Number

The X coordinate of the midpoint of the fitted line.

Midpoint Y

Number

The Y coordinate of the midpoint of the fitted line.

Rotation Angle

Number

The rotation angle of the fitted line relative to the positive X-axis.

Parameter Description

Parameter	Description
ROI Settings	You can draw a rectangular ROI. The system extracts multiple columns of pixels according to the caliper settings, and the detected edge points in each column will be used for line fitting. In the data visualization window, the arrow on each caliper indicates the direction in which the caliper detects edges. The Step detects edge points along the caliper’s direction. For stable edge detection, this direction should be as perpendicular as possible to the line edge. If the line to be detected is tilted, you can adjust the rotation angle of the rectangular ROI so that the caliper and its detection direction rotate along with the ROI, allowing them to align with the line’s orientation.
Fitted Line Direction	Select an appropriate option from the drop-down list to determine the direction vector of the fitted line, thus determining the direction of the fitted line. Positive X: The X component of the direction vector is positive. Negative X: The X component of the direction vector is negative. Positive Y: The Y component of the direction vector is positive. Negative Y: The Y component of the direction vector is negative.
Edge Polarity	This parameter specifies the direction in which the grayscale values change at the edges. Value list: Dark to light: Detects edges where grayscale values increase from low to high, corresponding to a visual transition from dark to bright. Light to dark: Detects edges where grayscale values decrease from high to low, corresponding to a visual transition from bright to dark. Any: No limit on the direction of grayscale value change of the edge. Both types can be detected.
Filter Window Size	Specify the window size to be used when filtering each column of pixels in its direction. Filtering can reduce noise and improve the stability of edge detection results.
Edge Type	Define the edge types to be retained in edge detection. Value list: Best, First, Last.
Gray Value Change Threshold	This parameter specifies the minimum gray value change between adjacent pixels at the edge of an extracted pixel column required for edge point detection. Setting this parameter value properly can effectively filter out weak edges and noise.
Use Relative Threshold	Once selected, an edge point is detected only if the gray value change between adjacent pixels at the edge of a pixel column meets or exceeds the specified percentage of the maximum change in that column. Once this parameter is selected, set Relative Threshold.
Outlier Fraction	The percentage of outlier points to be removed during line fitting.

Parameter

Description

ROI Settings

You can draw a rectangular ROI. The system extracts multiple columns of pixels according to the caliper settings, and the detected edge points in each column will be used for line fitting.

In the data visualization window, the arrow on each caliper indicates the direction in which the caliper detects edges. The Step detects edge points along the caliper’s direction. For stable edge detection, this direction should be as perpendicular as possible to the line edge.

If the line to be detected is tilted, you can adjust the rotation angle of the rectangular ROI so that the caliper and its detection direction rotate along with the ROI, allowing them to align with the line’s orientation.

Fitted Line Direction

Select an appropriate option from the drop-down list to determine the direction vector of the fitted line, thus determining the direction of the fitted line.

Positive X: The X component of the direction vector is positive.
Negative X: The X component of the direction vector is negative.
Positive Y: The Y component of the direction vector is positive.
Negative Y: The Y component of the direction vector is negative.

Edge Polarity

This parameter specifies the direction in which the grayscale values change at the edges.

Value list:

Dark to light: Detects edges where grayscale values increase from low to high, corresponding to a visual transition from dark to bright.
Light to dark: Detects edges where grayscale values decrease from high to low, corresponding to a visual transition from bright to dark.
Any: No limit on the direction of grayscale value change of the edge. Both types can be detected.

Filter Window Size

Specify the window size to be used when filtering each column of pixels in its direction. Filtering can reduce noise and improve the stability of edge detection results.

Edge Type

Define the edge types to be retained in edge detection.

Value list: Best, First, Last.

Gray Value Change Threshold

This parameter specifies the minimum gray value change between adjacent pixels at the edge of an extracted pixel column required for edge point detection. Setting this parameter value properly can effectively filter out weak edges and noise.

Use Relative Threshold

Once selected, an edge point is detected only if the gray value change between adjacent pixels at the edge of a pixel column meets or exceeds the specified percentage of the maximum change in that column. Once this parameter is selected, set Relative Threshold.

Outlier Fraction

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

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