iwiFilterCanny

Intel IPP Integration Wrappers Developer Guide and Reference

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ID 751823

Date 9/30/2019

Version 2020

Public

Visible to Intel only — GUID: GUID-CE6FBB08-CE67-4AF2-92C1-823B5A249497

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<span class='option'>iwiFilterCanny</span>

Syntax

typedef struct _IwiFilterCannyParams
{
    IppiDifferentialKernel  kernel; 
    IppiMaskSize            kernelSize; 
    IppNormType             norm;
} IwiFilterCannyParams;

static IW_INLINE void iwiFilterCanny_SetDefaultParams(
    IwiFilterCannyParams *pParams 
)

IW_DECL(IppStatus) iwiFilterCanny(
    const IwiImage             *pSrcImage,   
    IwiImage                   *pDstImage, 
    Ipp32f                      treshLow,   
    Ipp32f                      treshHigh,  
    const IwiFilterCannyParams *pAuxParams,
    IppiBorderType              border,     
    const Ipp64f               *pBorderVal  
);

Parameters

pSrcImage

Pointer to the source image.

pDstImage

Pointer to the destination image.

treshLow

Lower threshold for edges detection.

treshHigh

Upper threshold for edges detection.

pAuxParams

Pointer to the auxiliary parameters structure. If NULL, default parameters are used.

border

Extrapolation algorithm for out of image pixels processing. Supported values:

ippBorderConst	Values of all border pixels are set to a constant.
ippBorderRepl	Border is replicated from the edge pixels.
ippBorderInMem	Border is obtained from the source image pixels in memory.
ippBorderMirror	Border pixels are mirrored from the source image boundary pixels.

pBorderVal

Pointer to an array of values to assign to pixels of the constant border, one element of the array for each channel of the image. This parameter is applicable only to the ippBorderConst border type and can be NULL for other border types.

Auxiliary Arguments

Argument	Default Value	Description
kernel	ippFilterSobel	Type of the differential kernel. Supported values: ippFilterSobel, ippFilterScharr.
kernelSize	ippMskSize3x3	Size of the filter kernel. Supported values for ippFilterSobel: ippMskSize3x3, ippMskSize5x5; for ippFilterScharr: ippMskSize3x3.
norm	ippNormL2	Normalization mode. Supported values: ippNormL1 and ippNormL2.

Description

This function applies the Canny edge detector [Canny86] to the source image. The detector uses a grayscale image as an input and outputs a black-and-white image, where non-zero pixels mark detected edges. The algorithm consists of three stages described below.

Stage 1: Differentiation

The image data is differentiated in x and y directions. From the computed x and y gradient components, Canny edge detector functions calculate the magnitude and angle of the gradient vector.

Stage 2: Non-maximum Suppression

With the rate of intensity change found at each point in the image, edges must be placed at the points of maximum values of gradient magnitude. It is preferable to suppress non-maximum points that are perpendicular to the edge direction, rather than parallel to the edge direction, as the edge is strong along an extended contour.

The algorithm starts off with sorting the direction of gradient to one of four sectors shown in the figure below.

Gradient Sectors

The algorithm passes a 3x3 neighborhood across the magnitude array. At each point, the center element of the neighborhood is compared with its two neighbors along the line of the gradient given by the sector value. If the central value is not greater than the neighbors, it is suppressed.

Stage 3: Edge Thresholding

The Canny operator uses the so-called “hysteresis” thresholding. Most thresholders use a single threshold limit, which means that if the edge values fluctuate above and below this value, the line appears broken. This phenomenon is commonly referred to as “streaking”. Hysteresis counters streaking by setting an upper and lower edge value limit. Considering a line segment, if a value lies above the upper threshold limit, it is immediately accepted. If the value lies below the low threshold, it is rejected. Points which lie between the two limits are accepted if they are connected to pixels which are also accepted. The likelihood of streaking is small, since the line segment points must fluctuate above the upper limit and below the lower limit for streaking to occur.

J.Canny recommends the ratio of high to low limit be in the range two or three to one, based on predicted signal-to-noise ratios.

This function supports the following features:

Feature	Support
Internal threading	No
Manual tiling	No
IwiTile simple tiling	No
IwiTile pipeline tiling	No

Return Values

ippStsNoErr	No errors.
ippStsMaskSizeErr	The kernel value is illegal.
ippStsDataTypeErr	The dataType value is illegal.
ippStsNumChannelsErr	The channels value is illegal.
ippStsNotEvenStepErr	The srcStep and/or dstStep value is not divisible by size of elements.
ippStsBorderErr	The border value is illegal.
ippStsNoMemErr	Failed to allocate memory.
ippStsSizeErr	Values of the size fields are illegal.
ippStsInplaceModeNotSupportedErr	In-place operation is not supported: pSrc is equal to pDst.
ippStsNullPtrErr	At least one of the pointers (except pTile) is NULL.

Parent topic: Filtering Functions

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