median_imagemedian_imageMedianImageMedianImage (Operator)


median_imagemedian_imageMedianImageMedianImage — Compute a median filter with various masks.


median_image(Image : ImageMedian : MaskType, Radius, Margin : )

Herror median_image(const Hobject Image, Hobject* ImageMedian, const char* MaskType, const Hlong Radius, const char* Margin)

Herror T_median_image(const Hobject Image, Hobject* ImageMedian, const Htuple MaskType, const Htuple Radius, const Htuple Margin)

void MedianImage(const HObject& Image, HObject* ImageMedian, const HTuple& MaskType, const HTuple& Radius, const HTuple& Margin)

HImage HImage::MedianImage(const HString& MaskType, Hlong Radius, const HTuple& Margin) const

HImage HImage::MedianImage(const HString& MaskType, Hlong Radius, const HString& Margin) const

HImage HImage::MedianImage(const char* MaskType, Hlong Radius, const char* Margin) const

HImage HImage::MedianImage(const wchar_t* MaskType, Hlong Radius, const wchar_t* Margin) const   (Windows only)

static void HOperatorSet.MedianImage(HObject image, out HObject imageMedian, HTuple maskType, HTuple radius, HTuple margin)

HImage HImage.MedianImage(string maskType, int radius, HTuple margin)

HImage HImage.MedianImage(string maskType, int radius, string margin)


median_imagemedian_imageMedianImageMedianImageMedianImage performs a median filter on the input image ImageImageImageImageimage with a square or circular mask and returns the filtered image in ImageMedianImageMedianImageMedianImageMedianimageMedian. The shape of the mask can be selected with MaskTypeMaskTypeMaskTypeMaskTypemaskType. The radius of the mask can be selected with RadiusRadiusRadiusRadiusradius.

Conceptually, the median filter sorts all gray values within the mask in ascending order and then selects the median of the gray values. The median is the “middle” one of the sorted gray values, i.e., the gray value with rank (position) (N - 1) / 2 + 1 of the sorted gray values, where N denotes the number of pixels covered by the filter mask. Here, the rank 1 corresponds to the smallest gray value and the rank N corresponds to the largest gray value within the mask (see also rank_imagerank_imageRankImageRankImageRankImage).

The filter mask is determined by RadiusRadiusRadiusRadiusradius, defining the size, and MaskTypeMaskTypeMaskTypeMaskTypemaskType, defining the shape of the mask. For latter one, the following options are available:

median_imagemedian_imageMedianImageMedianImageMedianImage can be used, for example, to smooth images, to suppress unwanted objects (e.g., point-like or line-like structures) that are smaller than the mask, and can therefore be used to estimate the background illumination for a shading correction or as a preprocessing step for the dynamic threshold operation (see dyn_thresholddyn_thresholdDynThresholdDynThresholdDynThreshold).

Several border treatments can be chosen for filtering via the parameter MarginMarginMarginMarginmargin:

When using the MaskTypeMaskTypeMaskTypeMaskTypemaskType 'square'"square""square""square""square" with RadiusRadiusRadiusRadiusradius 1 or 2 (resulting in a 3x3 or 5x5 pixel filter mask) and the border treatment 'mirrored', median_image can be executed on OpenCL devices.

For an explanation of the concept of smoothing filters see the introduction of chapter Filters / Smoothing.


median_imagemedian_imageMedianImageMedianImageMedianImage uses an algorithm with a runtime per pixel that depends on the mask height 2 * RadiusRadiusRadiusRadiusradius + 1. Therefore, median_imagemedian_imageMedianImageMedianImageMedianImage is slower than median_rectmedian_rectMedianRectMedianRectMedianRect for square masks with a large mask height. The precise mask height for which median_rectmedian_rectMedianRectMedianRectMedianRect will become faster than median_imagemedian_imageMedianImageMedianImageMedianImage depends on the computer architecture (processor type, availability of SIMD instructions like SSE2 or MMX, cache size and throughput, memory throughput). Typically, this is the case for mask heights > 15, but can also be the case only for larger mask sizes, e.g., if SIMD instructions are unavailable and memory throughput is low.

Furthermore, it should be noted that median_rectmedian_rectMedianRectMedianRectMedianRect uses a recursive implementation, which internally computes the filter response on the smallest enclosing rectangle of the domain of the input image. Therefore, if the domain of the input image only covers a small fraction of the smallest enclosing rectangle, it can happen that median_imagemedian_imageMedianImageMedianImageMedianImage is faster than median_rectmedian_rectMedianRectMedianRectMedianRect even for larger mask heights.

Due to performance reasons, the input ImageImageImageImageimage is not checked whether it contains NaNs. Using an input image with NaNs crashes HALCON.

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


ImageImageImageImageimage (input_object)  (multichannel-)image(-array) objectHImageHImageHobject (byte* / int2* / uint2* / int4* / real*) *allowed for compute devices

Image to be filtered.

ImageMedianImageMedianImageMedianImageMedianimageMedian (output_object)  (multichannel-)image(-array) objectHImageHImageHobject * (byte / int2 / uint2 / int4 / real)

Filtered image.

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

Filter mask type.

Default value: 'circle' "circle" "circle" "circle" "circle"

List of values: 'circle'"circle""circle""circle""circle", 'square'"square""square""square""square"

List of values (for compute devices): 'square'"square""square""square""square"

RadiusRadiusRadiusRadiusradius (input_control)  integer HTupleHTupleHtuple (integer) (int / long) (Hlong) (Hlong)

Radius of the filter mask.

Default value: 1

List of values (for compute devices): 1, 2

Suggested values: 1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 15, 19, 25, 31, 39, 47, 59

Typical range of values: 1 ≤ Radius Radius Radius Radius radius ≤ 4095

Minimum increment: 1

Recommended increment: 2

MarginMarginMarginMarginmargin (input_control)  string HTupleHTupleHtuple (string / integer / real) (string / int / long / double) (HString / Hlong / double) (char* / Hlong / double)

Border treatment.

Default value: 'mirrored' "mirrored" "mirrored" "mirrored" "mirrored"

List of values (for compute devices): 'mirrored'"mirrored""mirrored""mirrored""mirrored"

Suggested values: 'mirrored'"mirrored""mirrored""mirrored""mirrored", 'cyclic'"cyclic""cyclic""cyclic""cyclic", 'continued'"continued""continued""continued""continued", 0, 30, 60, 90, 120, 150, 180, 210, 240, 255

Example (HDevelop)

read_image (Image, 'fabrik')
median_image (Image, Median, 'circle', 3, 'continued')


For each pixel: O(2 * RadiusRadiusRadiusRadiusradius + 1).


If the parameter values are correct the operator median_imagemedian_imageMedianImageMedianImageMedianImage returns the value 2 (H_MSG_TRUE). The behavior in case of empty input (no input images available) is set via the operator 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 Predecessors


Possible Successors

thresholdthresholdThresholdThresholdThreshold, dyn_thresholddyn_thresholdDynThresholdDynThresholdDynThreshold, regiongrowingregiongrowingRegiongrowingRegiongrowingRegiongrowing


median_rectmedian_rectMedianRectMedianRectMedianRect, rank_imagerank_imageRankImageRankImageRankImage, rank_rectrank_rectRankRectRankRectRankRect

See also

gray_erosion_rectgray_erosion_rectGrayErosionRectGrayErosionRectGrayErosionRect, gray_dilation_rectgray_dilation_rectGrayDilationRectGrayDilationRectGrayDilationRect, gray_erosion_shapegray_erosion_shapeGrayErosionShapeGrayErosionShapeGrayErosionShape, gray_dilation_shapegray_dilation_shapeGrayDilationShapeGrayDilationShapeGrayDilationShape, gray_erosiongray_erosionGrayErosionGrayErosionGrayErosion, gray_dilationgray_dilationGrayDilationGrayDilationGrayDilation


T.S. Huang, G.J. Yang, G.Y. Tang; “A Fast Two-Dimensional Median Filtering Algorithm”; IEEE Transactions on Acoustics, Speech, and Signal Processing, vol. 27, no. 1, pp. 13-18, 1979.
R. Haralick, L. Shapiro; “Computer and Robot Vision”; Addison-Wesley, 1992, pp. 318-320.