opencv 3.1.0 访问像素值的三种方法（C++）

#include <opencv.hpp>
#include <iostream>

using namespace std;

void colorReduce_ptr(cv::Mat &inputImage, cv::Mat &outputImage, int div);
void colorReduce_iterator(cv::Mat &inputImage, cv::Mat &outputImage, int div);
void colorReduce_at(cv::Mat &inputImage, cv::Mat &outputImage, int div);

void colorReduce_ptr(cv::Mat &inputImage, cv::Mat &outputImage, int div)
{
    outputImage = inputImage.clone();
    int rowNumber = outputImage.rows;
    int colNumber = outputImage.cols*outputImage.channels();
    for (size_t i = 0; i < rowNumber; i++)
    {
        uchar* data = outputImage.ptr<uchar>(i);
        for (size_t j = 0; j < colNumber; j++)
        {
            data[j] = data[j] / div*div + div / 2;
        }
    }
}

void colorReduce_iterator(cv::Mat &inputImage, cv::Mat &outputImage, int div)
{
    outputImage = inputImage.clone();
    cv::Mat_<cv::Vec3b>::iterator it = outputImage.begin<cv::Vec3b>();
    cv::Mat_<cv::Vec3b>::iterator it_end = outputImage.end<cv::Vec3b>();

    for (;it != it_end; ++it)
    {
        for (size_t px = 0; px < 3; px++)
        {
            (*it)[px] = (*it)[px] / div*div + div / 2;
        }
    }
}

void colorReduce_at(cv::Mat &inputImage, cv::Mat &outputImage, int div)
{
    outputImage = inputImage.clone();
    int rowNumber = outputImage.rows;
    int colNumber = outputImage.cols;

    for (size_t i = 0; i < rowNumber; i++)
    {
        for (size_t j = 0; j < colNumber; j++)
        {
            for (size_t px = 0; px < 3; px++)
            {
                outputImage.at<cv::Vec3b>(i, j)[px] = outputImage.at<cv::Vec3b>(i, j)[px] / div * div + div / 2;
            }
        }
    }
}

int main(int argc, char** argv[])
{
    cv::Mat srcImg = cv::imread("E:/data/lena.jpg");
    cv::imshow("srcImg", srcImg);

    cv::Mat dstImg;
    dstImg.create(srcImg.rows, srcImg.cols, srcImg.type());
    
    double timeStart;
    timeStart = static_cast<double>(cv::getTickCount());
    colorReduce_ptr(srcImg, dstImg, 32);
    timeStart = ((double)cv::getTickCount() - timeStart) / cv::getTickFrequency();
    cout << "ptr方法运行时间为：" << timeStart << "秒" << endl;

    timeStart = static_cast<double>(cv::getTickCount());
    colorReduce_iterator(srcImg, dstImg, 32);
    timeStart = ((double)cv::getTickCount() - timeStart) / cv::getTickFrequency();
    cout << "iterator方法运行时间为：" << timeStart << "秒" << endl;

    timeStart = static_cast<double>(cv::getTickCount());
    colorReduce_at(srcImg, dstImg, 32);
    timeStart = ((double)cv::getTickCount() - timeStart) / cv::getTickFrequency();
    cout << "at方法运行时间为：" << timeStart << "秒" << endl;
    
    cv::imshow("dstImg", dstImg);
    cv::waitKey(0);
}