edges_sub_pixedges_sub_pixEdgesSubPixEdgesSubPix (Operator)


edges_sub_pixedges_sub_pixEdgesSubPixEdgesSubPix — Extract sub-pixel precise edges using Deriche, Lanser, Shen, or Canny filters.


edges_sub_pix(Image : Edges : Filter, Alpha, Low, High : )

Herror edges_sub_pix(const Hobject Image, Hobject* Edges, const char* Filter, double Alpha, const Hlong Low, const Hlong High)

Herror T_edges_sub_pix(const Hobject Image, Hobject* Edges, const Htuple Filter, const Htuple Alpha, const Htuple Low, const Htuple High)

void EdgesSubPix(const HObject& Image, HObject* Edges, const HTuple& Filter, const HTuple& Alpha, const HTuple& Low, const HTuple& High)

HXLDCont HImage::EdgesSubPix(const HString& Filter, double Alpha, const HTuple& Low, const HTuple& High) const

HXLDCont HImage::EdgesSubPix(const HString& Filter, double Alpha, Hlong Low, Hlong High) const

HXLDCont HImage::EdgesSubPix(const char* Filter, double Alpha, Hlong Low, Hlong High) const

HXLDCont HImage::EdgesSubPix(const wchar_t* Filter, double Alpha, Hlong Low, Hlong High) const   (Windows only)

static void HOperatorSet.EdgesSubPix(HObject image, out HObject edges, HTuple filter, HTuple alpha, HTuple low, HTuple high)

HXLDCont HImage.EdgesSubPix(string filter, double alpha, HTuple low, HTuple high)

HXLDCont HImage.EdgesSubPix(string filter, double alpha, int low, int high)


edges_sub_pixedges_sub_pixEdgesSubPixEdgesSubPixEdgesSubPix detects step edges using recursively implemented filters (according to Deriche, Lanser and Shen) or the conventionally implemented “derivative of Gaussian” filter (using filter masks) proposed by Canny. Thus, the following edge operators are available:

'deriche1', 'lanser1', 'deriche2', 'lanser2', 'shen', 'mshen', 'canny', 'sobel', and 'sobel_fast'

(parameter FilterFilterFilterFilterfilter).

The extracted edges are returned as sub-pixel precise XLD contours in EdgesEdgesEdgesEdgesedges. For all edge operators except 'sobel_fast'"sobel_fast""sobel_fast""sobel_fast""sobel_fast", the following attributes are defined for each edge point (see get_contour_attrib_xldget_contour_attrib_xldGetContourAttribXldGetContourAttribXldGetContourAttribXld for further details):


Gives the direction of the edge (not of the XLD contour), calculated from the image gradients in horizontal and vertical direction. The angles [rad] are given with respect to the column axis of the image.


Direction of the normal vectors to the contour in radians (oriented such that the normal vectors point to the right side of the contour as the contour is traversed from start to end point; the angles are given with respect to the row axis of the image).


Edge amplitude (gradient magnitude).

The “filter width” (i.e., the amount of smoothing) can be chosen arbitrarily for all edge operators except 'sobel'"sobel""sobel""sobel""sobel" and 'sobel_fast'"sobel_fast""sobel_fast""sobel_fast""sobel_fast", and can be estimated by calling info_edgesinfo_edgesInfoEdgesInfoEdgesInfoEdges for concrete values of the parameter AlphaAlphaAlphaAlphaalpha. For all filters (Deriche, Lanser and Shen filters), the “filter width” decreases for increasing AlphaAlphaAlphaAlphaalpha. The only exception is the Canny filter, where an increasing AlphaAlphaAlphaAlphaalpha also causes an increase of the “filter width”. “Wide” filters exhibit a larger invariance to noise, but also a decreased ability to detect small details. Non-recursive filters, such as the Canny filter, are realized using filter masks, and thus the execution time increases for increasing filter width. In contrast, the execution time for recursive filters does not depend on the filter width. Thus, arbitrary filter widths are possible using the Deriche, Lanser and Shen filters without increasing the run time of the operator. The resulting advantage in speed compared to the Canny operator naturally increases for larger filter widths. As border treatment, the recursive operators assume that the images to be zero outside of the image, while the Canny operator repeats the gray value at the image's border. The signal-noise-ratio of the filters is comparable for the following choices of AlphaAlphaAlphaAlphaalpha: Alpha('lanser1') = Alpha('deriche1'), Alpha('deriche2') = Alpha('deriche1') / 2, Alpha('lanser2') = Alpha('deriche2'), Alpha('shen') = Alpha('deriche1') / 2, Alpha('mshen') = Alpha('shen'), Alpha('canny') = 1.77 / Alpha('deriche1'). The originally proposed recursive filters ('deriche1', 'deriche2', 'shen') return a biased estimate of the amplitude of diagonal edges. This bias is removed in the corresponding modified version of the operators ('lanser1', 'lanser2' and 'mshen'), while maintaining the same execution speed.

For relatively small filter widths (11 x 11), i.e., for Alpha ('lanser2' = 0.5), all filters yield similar results. Only for “wider” filters differences begin to appear: the Shen filters begin to yield qualitatively inferior results. However, they are the fastest of the implemented operators that supprt arbitrary mask sizes, closely followed by the Deriche operators. The two Sobel filters, which use a fixed mask size of (3 x 3), are faster than the other filters. Of these two, the filter 'sobel_fast'"sobel_fast""sobel_fast""sobel_fast""sobel_fast" is significantly faster than 'sobel'"sobel""sobel""sobel""sobel".

edges_sub_pixedges_sub_pixEdgesSubPixEdgesSubPixEdgesSubPix links the edge points into edges by using an algorithm similar to a hysteresis threshold operation, which is also used in lines_gausslines_gaussLinesGaussLinesGaussLinesGauss. Points with an amplitude larger than HighHighHighHighhigh are immediately accepted as belonging to an edge, while points with an amplitude smaller than LowLowLowLowlow are rejected. All other points are accepted as edges if they are connected to accepted edge points (see also lines_gausslines_gaussLinesGaussLinesGaussLinesGauss and hysteresis_thresholdhysteresis_thresholdHysteresisThresholdHysteresisThresholdHysteresisThreshold).

Because edge extractors are often unable to extract certain junctions, a mode that tries to extract these missing junctions by different means can be selected by appending '_junctions'"_junctions""_junctions""_junctions""_junctions" to the values of FilterFilterFilterFilterfilter that are described above. This mode is analogous to the mode for completing junctions that is available in lines_gausslines_gaussLinesGaussLinesGaussLinesGauss.

The edge operator 'sobel_fast'"sobel_fast""sobel_fast""sobel_fast""sobel_fast" has the same semantics as all the other edge operators. Internally, however, it is based on significantly simplified variants of the individual processing steps (hysteresis thresholding, edge point linking, and extraction of the subpixel edge positions). Therefore, 'sobel_fast'"sobel_fast""sobel_fast""sobel_fast""sobel_fast" in some cases may return slightly less accurate edge positions and may select different edge parts.

edges_sub_pixedges_sub_pixEdgesSubPixEdgesSubPixEdgesSubPix can be executed on OpenCL devices for the filter types 'canny'"canny""canny""canny""canny" and 'sobel_fast'"sobel_fast""sobel_fast""sobel_fast""sobel_fast". This will require up to width*height*29 bytes of pinned memory. Since allocating memory is an expensive operation, it would make sense to set the pinned memory cache to at least this size (using set_compute_device_paramset_compute_device_paramSetComputeDeviceParamSetComputeDeviceParamSetComputeDeviceParam for parameter 'pinned_mem_cache_capacity'"pinned_mem_cache_capacity""pinned_mem_cache_capacity""pinned_mem_cache_capacity""pinned_mem_cache_capacity", or to disable pinned memory completely (using set_compute_device_paramset_compute_device_paramSetComputeDeviceParamSetComputeDeviceParamSetComputeDeviceParam for parameter 'alloc_pinned'"alloc_pinned""alloc_pinned""alloc_pinned""alloc_pinned"), in which case the normal memory cache is used. Note that the results can vary from the CPU implementation.


Since edges_sub_pixedges_sub_pixEdgesSubPixEdgesSubPixEdgesSubPix uses Gauss convolution internally for the 'canny'"canny""canny""canny""canny" filter, the same limitations for OpenCL apply as for derivate_gaussderivate_gaussDerivateGaussDerivateGaussDerivateGauss: AlphaAlphaAlphaAlphaalpha must be chosen small enough that the required filter mask is less than 129 pixels in size. Also, edges_sub_pixedges_sub_pixEdgesSubPixEdgesSubPixEdgesSubPix is not available on OpenCL devices for HALCON XL, as double precision floating point arithmetic would be required, but OpenCL devices are optimized for single precision arithmetic.

Note that filter operators may return unexpected results if an image with a reduced domain is used as input. Please refer to the chapter Filters.

Execution Information

This operator supports cancelling timeouts and interrupts.


ImageImageImageImageimage (input_object)  singlechannelimage objectHImageHImageHobject (byte / uint2 / real)

Input image.

EdgesEdgesEdgesEdgesedges (output_object)  xld_cont-array objectHXLDContHXLDContHobject *

Extracted edges.

FilterFilterFilterFilterfilter (input_control)  string HTupleHTupleHtuple (string) (string) (HString) (char*)

Edge operator to be applied.

Default value: 'canny' "canny" "canny" "canny" "canny"

List of values: 'canny'"canny""canny""canny""canny", 'canny_junctions'"canny_junctions""canny_junctions""canny_junctions""canny_junctions", 'deriche1'"deriche1""deriche1""deriche1""deriche1", 'deriche1_junctions'"deriche1_junctions""deriche1_junctions""deriche1_junctions""deriche1_junctions", 'deriche2'"deriche2""deriche2""deriche2""deriche2", 'deriche2_junctions'"deriche2_junctions""deriche2_junctions""deriche2_junctions""deriche2_junctions", 'lanser1'"lanser1""lanser1""lanser1""lanser1", 'lanser1_junctions'"lanser1_junctions""lanser1_junctions""lanser1_junctions""lanser1_junctions", 'lanser2'"lanser2""lanser2""lanser2""lanser2", 'lanser2_junctions'"lanser2_junctions""lanser2_junctions""lanser2_junctions""lanser2_junctions", 'mshen'"mshen""mshen""mshen""mshen", 'mshen_junctions'"mshen_junctions""mshen_junctions""mshen_junctions""mshen_junctions", 'shen'"shen""shen""shen""shen", 'shen_junctions'"shen_junctions""shen_junctions""shen_junctions""shen_junctions", 'sobel'"sobel""sobel""sobel""sobel", 'sobel_fast'"sobel_fast""sobel_fast""sobel_fast""sobel_fast", 'sobel_junctions'"sobel_junctions""sobel_junctions""sobel_junctions""sobel_junctions"

List of values (for compute devices): 'canny'"canny""canny""canny""canny", 'sobel_fast'"sobel_fast""sobel_fast""sobel_fast""sobel_fast"

AlphaAlphaAlphaAlphaalpha (input_control)  real HTupleHTupleHtuple (real) (double) (double) (double)

Filter parameter: small values result in strong smoothing, and thus less detail (opposite for 'canny').

Default value: 1.0

Suggested values: 0.1, 0.2, 0.3, 0.4, 0.5, 0.7, 0.9, 1.1

Typical range of values: 0.2 ≤ Alpha Alpha Alpha Alpha alpha ≤ 50.0

Minimum increment: 0.01

Recommended increment: 0.1

Restriction: Alpha > 0.0

LowLowLowLowlow (input_control)  integer HTupleHTupleHtuple (integer / real) (int / long / double) (Hlong / double) (Hlong / double)

Lower threshold for the hysteresis threshold operation.

Default value: 20

Suggested values: 5, 10, 15, 20, 25, 30, 40

Typical range of values: 1 ≤ Low Low Low Low low ≤ 255

Minimum increment: 1

Recommended increment: 5

Restriction: Low > 0

HighHighHighHighhigh (input_control)  integer HTupleHTupleHtuple (integer / real) (int / long / double) (Hlong / double) (Hlong / double)

Upper threshold for the hysteresis threshold operation.

Default value: 40

Suggested values: 10, 15, 20, 25, 30, 40, 50, 60, 70

Typical range of values: 1 ≤ High High High High high ≤ 255

Minimum increment: 1

Recommended increment: 5

Restriction: High > 0 && High >= Low

Example (HDevelop)


Example (C)


Example (HDevelop)


Example (HDevelop)


Example (HDevelop)



Let A be the number of pixels in the domain of ImageImageImageImageimage. Then the runtime complexity is O(A*Alpha) for the Canny filter and O(A) for the recursive Lanser, Deriche, and Shen filters.

The amount of temporary memory required is dependent on the height H of the domain of ImageImageImageImageimage and the width W of ImageImageImageImageimage. Let S = W*H, then edges_sub_pixedges_sub_pixEdgesSubPixEdgesSubPixEdgesSubPix requires at least 60*S bytes of temporary memory during execution for all edge operators except 'sobel_fast'"sobel_fast""sobel_fast""sobel_fast""sobel_fast". For 'sobel_fast'"sobel_fast""sobel_fast""sobel_fast""sobel_fast", at least 9*S bytes of temporary memory are required.


edges_sub_pixedges_sub_pixEdgesSubPixEdgesSubPixEdgesSubPix returns 2 (H_MSG_TRUE) if all parameters are correct and no error occurs during execution. If the input is empty the behavior can be set via set_system('no_object_result',<Result>)set_system("no_object_result",<Result>)SetSystem("no_object_result",<Result>)SetSystem("no_object_result",<Result>)SetSystem("no_object_result",<Result>). If necessary, an exception is raised.

Possible Successors

segment_contours_xldsegment_contours_xldSegmentContoursXldSegmentContoursXldSegmentContoursXld, gen_polygons_xldgen_polygons_xldGenPolygonsXldGenPolygonsXldGenPolygonsXld, select_shape_xldselect_shape_xldSelectShapeXldSelectShapeXldSelectShapeXld


sobel_dirsobel_dirSobelDirSobelDirSobelDir, frei_dirfrei_dirFreiDirFreiDirFreiDir, kirsch_dirkirsch_dirKirschDirKirschDirKirschDir, prewitt_dirprewitt_dirPrewittDirPrewittDirPrewittDir, robinson_dirrobinson_dirRobinsonDirRobinsonDirRobinsonDir, edges_imageedges_imageEdgesImageEdgesImageEdgesImage

See also

info_edgesinfo_edgesInfoEdgesInfoEdgesInfoEdges, hysteresis_thresholdhysteresis_thresholdHysteresisThresholdHysteresisThresholdHysteresisThreshold, bandpass_imagebandpass_imageBandpassImageBandpassImageBandpassImage, lines_gausslines_gaussLinesGaussLinesGaussLinesGauss, lines_facetlines_facetLinesFacetLinesFacetLinesFacet


S.Lanser, W.Eckstein: “Eine Modifikation des Deriche-Verfahrens zur Kantendetektion”; 13. DAGM-Symposium, München; Informatik Fachberichte 290; Seite 151 - 158; Springer-Verlag; 1991.
S.Lanser: “Detektion von Stufenkanten mittels rekursiver Filter nach Deriche”; Diplomarbeit; Technische Universität München, Institut für Informatik, Lehrstuhl Prof. Radig; 1991.
J.Canny: “Finding Edges and Lines in Images”; Report, AI-TR-720; M.I.T. Artificial Intelligence Lab., Cambridge; 1983.
J.Canny: “A Computational Approach to Edge Detection”; IEEE Transactions on Pattern Analysis and Machine Intelligence; PAMI-8, vol. 6; S. 679-698; 1986.
R.Deriche: “Using Canny's Criteria to Derive a Recursively Implemented Optimal Edge Detector”; International Journal of Computer Vision; vol. 1, no. 2; S. 167-187; 1987.
R.Deriche: “Optimal Edge Detection Using Recursive Filtering”; Proc. of the First International Conference on Computer Vision, London; S. 501-505; 1987.
R.Deriche: “Fast Algorithms for Low-Level Vision”; IEEE Transactions on Pattern Analysis and Machine Intelligence; PAMI-12, no. 1; S. 78-87; 1990.
S.Castan, J.Zhao und J.Shen: “Optimal Filter for Edge Detection Methods and Results”; Proc. of the First European Conference on Computer Vision, Antibes; Lecture Notes on computer Science; no. 427; S. 12-17; Springer-Verlag; 1990.


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