OpenCV 三種圖像遍歷方法

實驗目的：

通過顏色壓縮（color reduction）示例理解學習OpenCV中遍歷圖像的三種方法；

實驗代碼：

#include<opencv2/opencv.hpp>
#include<iostream>
#include<cmath>

using namespace cv;
using namespace std;

// 1：使用指針遍歷圖像
void colorReduceByPointer(Mat& image, int div = 64)
{
    int nRows = image.rows;
    int nCols = image.cols * image.channels();

    if (image.isContinuous())
    {
        nCols *= nRows;
        nRows = 1; // 如果連續，外層循環執行只需一次，以此提高執行效率
    }

    for (int i = 0; i < nRows; ++i)
    {
        uchar* data = image.ptr<uchar>(i);
        for (int j = 0; j < nCols; ++j)
        {
            data[j] = data[j] / div*div + div / 2;
        }
    }
}

// 2：使用at遍歷圖像
void colorReduceByAtMethod(Mat& image, int div = 64)
{
    int nRows = image.rows;
    int nCols = image.cols;

    if (image.channels() == 1)
    {
        for (int i = 0; i < nRows; ++i)
        {
            for (int j = 0; j < nCols; ++j)
            {
                image.at<uchar>(i, j) = image.at<uchar>(i, j) / div * div + div / 2;
            }
        }
    }
    else if (image.channels() == 3)
    {
        for (int i = 0; i < nRows; ++i)
        {
            for (int j = 0; j < nCols; ++j)
            {
                image.at<Vec3b>(i, j)[0] = image.at<Vec3b>(i, j)[0] / div*div + div / 2;
                image.at<Vec3b>(i, j)[1] = image.at<Vec3b>(i, j)[1] / div*div + div / 2;
                image.at<Vec3b>(i, j)[2] = image.at<Vec3b>(i, j)[2] / div*div + div / 2;
            }
        }
    }
}

// 3：使用迭代器進行圖像的遍歷
void colorReduceByIterator(Mat& image, int div = 64)
{
    const int channels = image.channels();
    switch (channels)
    {
        case 1:
        {
            MatIterator_<uchar>begin, end;
            for (begin = image.begin<uchar>(), end = image.end<uchar>(); begin != end; ++begin)
            {
                *begin = *begin / div*div + div / 2;
            }
            break;
        }
        case 3:
        {
            MatIterator_<Vec3b> begin, end;
            for (begin = image.begin<Vec3b>(), end = image.end<Vec3b>(); begin != end; ++begin)
            {
                (*begin)[0] = (*begin)[0] / div * div + div / 2;
                (*begin)[1] = (*begin)[1] / div * div + div / 2;
                (*begin)[2] = (*begin)[2] / div * div + div / 2;
            }
            break;
        }
    }
}


int main()
{
    Mat image;
    image = imread("E:\\dataset\\image.jpg");
    if (!image.data)
        return -1;
    //cvtColor(image, image, CV_BGR2GRAY);
    //colorReduceByAtMethod(image);
    //colorReduceByPointer(image);
    colorReduceByIterator(image);
    namedWindow("after");
    imshow("after", image);
    cvWaitKey(-1);
    destroyAllWindows();
    return 0;
}