hough_lines_dir🔗
Short description🔗
hough_lines_dir — Detect lines in edge images with the help of the Hough transform
using local gradient direction and return them in normal form.
Signature🔗
hough_lines_dir( image ImageDir, out image HoughImage, out region Lines, angle.deg DirectionUncertainty, integer AngleResolution, string Smoothing, integer FilterSize, integer Threshold, integer AngleGap, integer DistGap, string GenLines, out hesseline.angle.rad Angle, out hesseline.distance Dist )
Description🔗
The operator hough_lines_dir selects line-like
structures in a region based on the Hough transform. The individual
points of a line can be unconnected. The region is given by the domain
of ImageDir.
In contrast to hough_lines, 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_lines_dir shows the same behavior as
hough_lines, 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 parameter AngleResolution defines how accurately the
angles are determined. The accuracy amounts to \(1 /
\textrm{AngleResolution}\) degrees. A subsequent smoothing of the
Hough space results in an increased stability. The smoothing filter can be
selected by Smoothing, the degree of smoothing by the
parameter FilterSize (see mean_image or
gauss_image for details).
The parameter Threshold
determines by how many points of the original region a line’s hypothesis
must at least be supported in order to be selected into the output.
The parameters AngleGap and DistGap define a
neighborhood of the points in the Hough image in order to determine the
local maxima: AngleGap describes the minimum distance of two
maxima in the Hough image in angle direction and DistGap in
distance direction, respectively. Thus, maxima exceeding
Threshold but lying close to an even higher maximum are eliminated.
If multiple maxima in this neighborhood are equally high, all of them are
returned. This elimination can particularly be helpful when searching for
short and long lines simultaneously. Besides the not smoothed Hough image
HoughImage, the lines are returned in Hessian Normal Form (HNF),
that is by the direction Angle and length Dist of their
normal vectors.
If the parameter GenLines is set to 'true', additionally
those regions in ImageDir are returned that contributed to the
local maxima in Hough space. They are stored in the parameter
Lines.
Execution information🔗
Execution information
-
Multithreading type: reentrant (runs in parallel with non-exclusive operators).
-
Multithreading scope: global (may be called from any thread).
-
Processed without parallelization.
Parameters🔗
ImageDir (input_object) singlechannelimage → object (direction)
Image containing the edge direction. The edges are described by the image domain.
HoughImage (output_object) image → object (uint2)
Hough transform.
Lines (output_object) region-array → object
Regions of the input image that contributed to the local maxima.
DirectionUncertainty (input_control) angle.deg → (integer)
Uncertainty of edge direction (in degrees).
Default: 2
Value range: 2 ≤ DirectionUncertainty ≤ 180
Minimum increment: 2
AngleResolution (input_control) integer → (integer)
Resolution in the angle area (in 1/degrees).
Default: 4
List of values: 1, 2, 4, 8
Smoothing (input_control) string → (string)
Smoothing filter for hough image.
Default: 'mean'
List of values: 'gauss', 'mean', 'none'
FilterSize (input_control) integer → (integer)
Required smoothing filter size.
Default: 5
List of values: 3, 5, 7, 9, 11
Threshold (input_control) integer → (integer)
Threshold value in the Hough image.
Default: 100
Value range: 1 ≤ Threshold
AngleGap (input_control) integer → (integer)
Minimum distance of two maxima in the Hough image (direction: angle).
Default: 5
Value range: 0 ≤ AngleGap
DistGap (input_control) integer → (integer)
Minimum distance of two maxima in the Hough image (direction: distance).
Default: 5
Value range: 0 ≤ DistGap
GenLines (input_control) string → (string)
Create line regions if 'true'.
Default: 'true'
List of values: 'false', 'true'
Angle (output_control) hesseline.angle.rad-array → (real)
Angles (in radians) of the detected lines’ normal vectors.
Value range: -1.5707963 ≤ Angle ≤ 3.1415927
Dist (output_control) hesseline.distance-array → (real)
Distance of the detected lines from the origin.
Number of elements: Dist == Angle
Value range: 0 ≤ Dist
Result🔗
The operator hough_lines returns the value 2 (H_MSG_TRUE)
if the input is not empty.
The behavior in case of empty input (no input regions available) is
set via the operator set_system('no_object_result',<Result>).
If necessary an exception is raised.
Combinations with other operators🔗
Combinations
Possible predecessors
edges_image, sobel_dir, threshold, nonmax_suppression_dir, reduce_domain, skeleton
Possible successors
gen_region_hline, select_matching_lines
See also
hough_line_trans_dir, hough_line_trans, gen_region_hline, hough_circles
Module🔗
Foundation