Image processing – Edge Detection filters with c++ and GDI+

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Introduction

Edge detection is a fundamental tool used in most image processing applications to obtain information from the frames as a precursor step to feature extraction and object segmentation. This process detects outlines of an object and boundaries between objects and the background in the image. An edge-detection filter can also be used to improve the appearance of blurred or anti-aliased video streams.

In this article I will show 2 different ways to do edge detection. First method is based on convolution filters operators like: Sobel , Prewitt and Kirsh. The second one is based on the difference between pairs of pixel around the current pixel.

Convolution Filters - Sobel, Prewitt and Kirsh

The convolution filters are described in one of my preview articles which can be found here.

In this approach, three different masks are used to detect edges named after their inventors, Sobel, Prewitt, and Kirsh. In each case, a horizontal version of the filter is applied to one bitmap, vertical version to another, and one of these two images is used to merge both horizontal and vertical version together using the following formula:

pixel = sqrt(pixel1 *pixel1 + pixel2 * pixel2)

Source code for Sobel, Prewitt and Kirsh edge detection filters

void CEdgeDetectionFilters::ApplyInternalConvolutionEdge(BYTE *pBuffer , int nWidth , int nHeight ) { memcpy(tempBuff1,pBuffer,nWidth*nHeight*3); UINT x,y; int nPixel_0; int nPixel_1; int dwStride = (nWidth * (24 / 8) + 3) & ~3; long dwOffset = dwStride - nWidth * 3; nHeight = nHeight - 2; nWidth = nWidth - 2; for( y = 0; y< nHeight;++y) { for( x = 0 ; x<nWidth; ++x) { long dep = (x*3) +(y*dwStride)/*+(y*dwOffset)*/; long depNextLine = dep + dwStride; long depNextNextLine = depNextLine + dwStride; nPixel_0 = ((tempBuff1[2 + dep] * horizontalFilter[0][0]) + (tempBuff1[5 + dep] * horizontalFilter[0][1]) + (tempBuff1[8 + dep] * horizontalFilter[0][2]) + (tempBuff1[2 + depNextLine] * horizontalFilter[1][0])+ (tempBuff1[5 + depNextLine ] * horizontalFilter[1][1]) + (tempBuff1[8 +depNextLine] * horizontalFilter[1][2]) + (tempBuff1[2 + depNextNextLine]* horizontalFilter[2][0]) + (tempBuff1[5 +depNextNextLine]* horizontalFilter[2][1]) + (tempBuff1[8 + depNextNextLine]* horizontalFilter[2][2])) ; nPixel_1 = ((tempBuff1[2 + dep] * verticalFilter[0][0]) + (tempBuff1[5 + dep] * verticalFilter[0][1]) + (tempBuff1[8 + dep] * verticalFilter[0][2]) + (tempBuff1[2 + depNextLine] * verticalFilter[1][0])+ (tempBuff1[5 + depNextLine ] * verticalFilter[1][1]) + (tempBuff1[8 +depNextLine] * verticalFilter[1][2]) + (tempBuff1[2 + depNextNextLine]* verticalFilter[2][0]) + (tempBuff1[5 +depNextNextLine]* verticalFilter[2][1]) + (tempBuff1[8 + depNextNextLine]* verticalFilter[2][2])) ; pBuffer[5 + depNextLine] = max(dwThreshold,min(sqrt(pow(nPixel_0,2)+pow(nPixel_1,2)), 255 )); nPixel_0 = ((tempBuff1[1 + dep] * horizontalFilter[0][0]) + (tempBuff1[4 + dep] * horizontalFilter[0][1]) + (tempBuff1[7 + dep] * horizontalFilter[0][2]) + (tempBuff1[1 + depNextLine] * horizontalFilter[1][0])+ (tempBuff1[4 + depNextLine ] * horizontalFilter[1][1]) + (tempBuff1[7 +depNextLine] * horizontalFilter[1][2]) + (tempBuff1[1 + depNextNextLine]* horizontalFilter[2][0]) + (tempBuff1[4 +depNextNextLine]* horizontalFilter[2][1]) + (tempBuff1[7 + depNextNextLine]* horizontalFilter[2][2])) ; nPixel_1 = ((tempBuff1[1 + dep] * verticalFilter[0][0]) + (tempBuff1[4 + dep] * verticalFilter[0][1]) + (tempBuff1[7 + dep] * verticalFilter[0][2]) + (tempBuff1[1 + depNextLine] * verticalFilter[1][0])+ (tempBuff1[4 + depNextLine ] * verticalFilter[1][1]) + (tempBuff1[7 +depNextLine] * verticalFilter[1][2]) + (tempBuff1[1 + depNextNextLine]* verticalFilter[2][0]) + (tempBuff1[4 +depNextNextLine]* verticalFilter[2][1]) + (tempBuff1[7 + depNextNextLine]* verticalFilter[2][2])) ; pBuffer[4 + depNextLine] = max(dwThreshold,min(sqrt(pow(nPixel_0,2)+pow(nPixel_1,2)), 255 )); nPixel_0 = ((tempBuff1[0 + dep] * horizontalFilter[0][0]) + (tempBuff1[3 + dep] * horizontalFilter[0][1]) + (tempBuff1[6 + dep] * horizontalFilter[0][2]) + (tempBuff1[0 + depNextLine] * horizontalFilter[1][0])+ (tempBuff1[3 + depNextLine ] * horizontalFilter[1][1]) + (tempBuff1[6 +depNextLine] * horizontalFilter[1][2]) + (tempBuff1[0 + depNextNextLine]* horizontalFilter[2][0]) + (tempBuff1[3 +depNextNextLine]* horizontalFilter[2][1]) + (tempBuff1[6 + depNextNextLine]* horizontalFilter[2][2])) ; nPixel_1 = ((tempBuff1[0 + dep] * verticalFilter[0][0]) + (tempBuff1[3 + dep] * verticalFilter[0][1]) + (tempBuff1[6 + dep] * verticalFilter[0][2]) + (tempBuff1[0 + depNextLine] * verticalFilter[1][0])+ (tempBuff1[3 + depNextLine ] * verticalFilter[1][1]) + (tempBuff1[6 +depNextLine] * verticalFilter[1][2]) + (tempBuff1[0 + depNextNextLine]* verticalFilter[2][0]) + (tempBuff1[3 +depNextNextLine]* verticalFilter[2][1]) + (tempBuff1[6 + depNextNextLine]* verticalFilter[2][2])) ; pBuffer[3 + depNextLine] = max(dwThreshold,min(sqrt(pow(nPixel_0,2)+pow(nPixel_1,2)), 255 )); } } }

The above function can be found in CEdgeDetectionFilters.cpp .

Difference Edge Detection

The difference edge detection detects the difference between pairs of pixel around the current processed pixel. It used the highest value from the difference of the four pairs of pixels that can be used to form a line through the middle pixel.

void CEdgeDetectionFilters::ApplyInternalDifferenceEdge(BYTE *pBuffer , int nWidth , int nHeight ) { memcpy(tempBuff1,pBuffer,nWidth*nHeight*3); UINT x,y; int nPixelMax , nPixel; int dwStride = (nWidth * (24 / 8) + 3) & ~3; nHeight = nHeight - 1; nWidth = (nWidth *3) -3; for( y = 1; y< nHeight;++y) { for( x = 3 ; x<nWidth ; ++x) { long dep = y*dwStride+x; nPixelMax = abs(tempBuff1[dep - dwStride +3] - tempBuff1[dep + dwStride -3]); nPixel = abs(tempBuff1[dep + dwStride +3] - tempBuff1[dep - dwStride -3]); nPixelMax = max(nPixel , nPixelMax); nPixel = abs(tempBuff1[dep+3] - tempBuff1[dep-3]); nPixelMax = max(nPixel , nPixelMax); if(nPixelMax < dwThreshold) nPixelMax = 0; pBuffer[dep] = nPixelMax; } } }

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License

This article, along with any associated source code and files, is licensed under The Intelliproject Open License (IPOL)

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