median_imagemedian_imageMedianImageMedianImagemedian_image (算子)

名称

median_imagemedian_imageMedianImageMedianImagemedian_image — 使用各种掩模计算中值滤波器。

签名

描述

median_imagemedian_imageMedianImageMedianImageMedianImagemedian_image performs a median filter on the input image ImageImageImageImageimageimage with a square or circular mask and returns the filtered image in ImageMedianImageMedianImageMedianImageMedianimageMedianimage_median. The shape of the mask can be selected with MaskTypeMaskTypeMaskTypeMaskTypemaskTypemask_type. The radius of the mask can be selected with RadiusRadiusRadiusRadiusradiusradius。

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_imageRankImageRankImageRankImagerank_image).

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

• 'circle'"circle""circle""circle""circle""circle"：The mask consists of the pixel within a circle with RadiusRadiusRadiusRadiusradiusradius around the pixel of the mask center.

• 'square'"square""square""square""square""square"：The mask consists of the pixel within a square with an edge length of RadiusRadiusRadiusRadiusradiusradius pixel.

median_imagemedian_imageMedianImageMedianImageMedianImagemedian_image 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_thresholdDynThresholdDynThresholdDynThresholddyn_threshold).

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

• gray value: Pixels outside of the image borders are assumed to be constant (with the specified gray value).

• 'continued'"continued""continued""continued""continued""continued"：Continuation of border pixels.

• 'cyclic'"cyclic""cyclic""cyclic""cyclic""cyclic"：Cyclic continuation of image borders.

• 'mirrored'"mirrored""mirrored""mirrored""mirrored""mirrored"：Reflection of pixels at the image borders.

When using the MaskTypeMaskTypeMaskTypeMaskTypemaskTypemask_type 'square'"square""square""square""square""square" with RadiusRadiusRadiusRadiusradiusradius 1 or 2 (resulting in a 3x3 or 5x5 pixel filter mask) and the border treatment 'mirrored', median_imagemedian_imageMedianImageMedianImageMedianImagemedian_image 可在 OpenCL 设备上执行。

有关平滑滤波器概念的说明，请参阅 滤波器 / 平滑 一章的引言。

注意

median_imagemedian_imageMedianImageMedianImageMedianImagemedian_image uses an algorithm with a runtime per pixel that depends on the mask height 2 * RadiusRadiusRadiusRadiusradiusradius + 1. Therefore, median_imagemedian_imageMedianImageMedianImageMedianImagemedian_image is slower than median_rectmedian_rectMedianRectMedianRectMedianRectmedian_rect for square masks with a large mask height. The precise mask height for which median_rectmedian_rectMedianRectMedianRectMedianRectmedian_rect will become faster than median_imagemedian_imageMedianImageMedianImageMedianImagemedian_image 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_rectMedianRectMedianRectMedianRectmedian_rect 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_imageMedianImageMedianImageMedianImagemedian_image is faster than median_rectmedian_rectMedianRectMedianRectMedianRectmedian_rect even for larger mask heights.

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

请注意，若使用域缩减后的图像作为输入，滤波器算子可能会返回意外结果。请参阅 滤波器 一章

执行信息

• 支持 OpenCL 计算设备。
• 多线程类型：可重入（与非独占算子并行运行）。
• 多线程作用域：全局（可从任何线程调用）。
• 在元组级别上自动并行化。
• 在通道级别上自动并行化。
• 在域级别上自动并行化。

参数

示例（HDevelop）

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

复杂度

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

结果

如果参数值正确，算子 median_imagemedian_imageMedianImageMedianImageMedianImagemedian_image 返回值 2 ( H_MSG_TRUE )。当输入为空（无可用输入图像）时可设置行为通过算子 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>)set_system("no_object_result",<Result>)。如有必要，则抛出异常。

可能的前趋

read_imageread_imageReadImageReadImageReadImageread_image

可能的后继

thresholdthresholdThresholdThresholdThresholdthreshold, dyn_thresholddyn_thresholdDynThresholdDynThresholdDynThresholddyn_threshold, regiongrowingregiongrowingRegiongrowingRegiongrowingRegiongrowingregiongrowing

替代

median_rectmedian_rectMedianRectMedianRectMedianRectmedian_rect, rank_imagerank_imageRankImageRankImageRankImagerank_image, rank_rectrank_rectRankRectRankRectRankRectrank_rect

另见

gray_erosion_rectgray_erosion_rectGrayErosionRectGrayErosionRectGrayErosionRectgray_erosion_rect, gray_dilation_rectgray_dilation_rectGrayDilationRectGrayDilationRectGrayDilationRectgray_dilation_rect, gray_erosion_shapegray_erosion_shapeGrayErosionShapeGrayErosionShapeGrayErosionShapegray_erosion_shape, gray_dilation_shapegray_dilation_shapeGrayDilationShapeGrayDilationShapeGrayDilationShapegray_dilation_shape, gray_erosiongray_erosionGrayErosionGrayErosionGrayErosiongray_erosion, gray_dilationgray_dilationGrayDilationGrayDilationGrayDilationgray_dilation

参考文献

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.

模块

基础

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