Operators

hough_line_trans_dir (Operator)

Name

hough_line_trans_dir — Compute the Hough transform for lines using local gradient direction.

Signature

hough_line_trans_dir(ImageDir : HoughImage : DirectionUncertainty, AngleResolution : )

Description

The operator hough_line_trans_dir calculates the Hough transform for lines in those regions passed in the domain of ImageDir. To do so, the angles and the lengths of the lines' normal vectors are registered in the parameter space (the so-called Hough or accumulator space).

In contrast to hough_line_trans, additionally the edge direction in ImageDir (e.g., returned by sobel_dir or edges_image) is taken into account. This results in a more efficient computation and in a reduction of the noise in the Hough space.

The parameter DirectionUncertainty describes how much the edge direction of the individual points within a line is allowed to vary. For example, with DirectionUncertainty = 10 a horizontal line (i.e., edge direction = 0 degrees) may contain points with an edge direction between -10 and +10 degrees. The higher DirectionUncertainty is chosen, the higher the computation time will be. For DirectionUncertainty = 180 hough_line_trans_dir shows the same behavior as hough_line_trans, i.e., the edge direction is ignored. DirectionUncertainty should be chosen at least as high as the step width of the edge direction stored in ImageDir. The minimum step width is 2 degrees (defined by the image type 'direction').

The result is stored in a newly generated UINT2-Image (HoughImage), where the x-axis (i.e., columns) represents the angle between the normal vector and the x-axis of the original image, and the y-axis (i.e., rows) represents the distance of the line from the origin.

The angle ranges from -90 to 180 degrees and will be stored with a resolution of 1 / AngleResolution, which means that one pixel in x-direction is equivalent to 1 / AngleResolution degrees and that the HoughImage has a width of 270 * AngleResolution + 1 pixels. The height of the HoughImage corresponds to the distance between the lower right corner of the surrounding rectangle of the input region and the origin.

The local maxima in the result image are equivalent to the parameter values of the lines in the original image.

Parallelization

• Multithreading type: reentrant (runs in parallel with non-exclusive operators).
• Processed without parallelization.

Parameters

ImageDir (input_object)  singlechannelimage object (direction)

Image containing the edge direction. The edges must be described by the image domain.

HoughImage (output_object)  image object (uint2)

Hough transform.

DirectionUncertainty (input_control)  angle.deg (integer)

Uncertainty of the edge direction (in degrees).

Default value: 2

Typical range of values: 2 ≤ DirectionUncertainty ≤ 180

Minimum increment: 2

AngleResolution (input_control)  integer (integer)

Resolution in the angle area (in 1/degrees).

Default value: 4

List of values: 1, 2, 4, 8

Result

The operator hough_line_trans_dir returns the value 2 (H_MSG_TRUE) if the input is not empty. The behavior in case of empty input is set via the operator set_system('no_object_result',<Result>). If necessary an exception is raised.