OpenCV: Detecting Horizontal Edges

since: 2013/12/27
update: 2013/12/27
reference:
1. Amazon.com: Practical OpenCV eBook
2. Welcome to opencv documentation!
3. I touchs: Using OpenCV on Mac OS X


A. 前置作業:
     1. 先依照 I touchs: Using OpenCV on Mac OS X 的說明, 建置好開發環境.

     2. 需要加入到專案的 OpenCV 函式庫爲: (Add Files to Project...)
          libopencv_core.dylib
          libopencv_highgui.dylib

     3. 將 main.cpp 的 main function 更名.(不作為程式執行的進入點)
          //int main(int argc, const char * argv[])  
          int main_main(int argc, const char * argv[])

     4. 爲專案新增 C++ 檔案:      
         點選專案 > New File... > OS X > C and C++ > C++ Class > Next >
         Save as: detectHorizontalEdges.cpp > Create

     5. 說明:    
          a. 使用 OpenCV 的 filter2D() 函式, 來作 kernel-based filtering.

          b. 在 filter matrix(or kernel) 與被 kernel 涵蓋住的像素間, 會作
              Element-wise multiplication(矩陣元素對應相乘)之和的運算.

          c. filter2D() 函式使用的演算法:
              DFT(Discrete Fourier transform)-based algorithm for large kernels;
              direct algorithm for small kernels.

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B. 撰寫程式:
    1. 開啓 detectHorizontalEdges.h 檔案, 修改如下:
#ifndef __HelloOpenCV__detectHorizontalEdges__
#define __HelloOpenCV__detectHorizontalEdges__

#include <iostream>
//@add
#include <opencv2/opencv.hpp>
#include <opencv2/highgui/highgui.hpp>

#endif /* defined(__HelloOpenCV__detectHorizontalEdges__) */

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    2. 開啓 detectHorizontalEdges.cpp 檔案, 修改如下:
#include "detectHorizontalEdges.h"

using namespace std;
using namespace cv;

int main()
{
    Mat img = imread("/Lanli/RD/Projects/OpenCV_Mac/HelloOpenCV/officeRoom.png", CV_LOAD_IMAGE_GRAYSCALE);
    Mat img_filtered;
   
    // Filter kernel for detecting vertical edges
    //float vertical_fk[5][5] = {{0,0,0,0,0}, {0,0,0,0,0}, {-1,-2,6,-2,-1}, {0,0,0,0,0}, {0,0,0,0,0}};
    /*
     0  0   -1  0   0
     0  0   -2  0   0
     0  0    6  0   0
     0  0   -2  0   0
     0  0   -1  0   0
     */
    //Mat filter_kernel = Mat(5, 5, CV_32FC1, vertical_fk);
   
   
    // Filter kernel for detecting horizontal edges
    float horizontal_fk[5][5] = {{0,0,-1,0,0}, {0,0,-2,0,0}, {0,0,6,0,0}, {0,0,-2,0,0}, {0,0,-1,0,0}};
    /*
      0   0   0   0    0
      0   0   0   0    0
     -1  -2   6  -2   -1
      0   0   0   0    0
      0   0   0   0    0
     */
    Mat filter_kernel = Mat(5, 5, CV_32FC1, horizontal_fk); // for float 32 bits,  1 channel
   
    // Apply filter
    // -1: the output image will have the same depth as the source.
    filter2D(img, img_filtered, -1, filter_kernel);
   
    namedWindow("Image");
    imshow("Image", img);
   
    namedWindow("Filtered image");
    imshow("Filtered image", img_filtered);
   
    imwrite("/Lanli/RD/Projects/OpenCV_Mac/HelloOpenCV/officeRoom_filtered.png", img_filtered);
    while (char(waitKey(1)) != 'q') {
    }
   
    return 0;
}

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C. 執行結果:
     原始圖片:

     水平濾波: