访问彩色像素的理解及疑惑

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

using namespace std;
using namespace cv;

int main()
{
//Mat img = imread("Lena.jpg");直接采用imread()函数就会报内存读取错误，没找出来原因。

IplImage *image = cvLoadImage("E:\\···Lena.jpg");
Mat img(image,true); //转换成Mat类型

if (!img.data)
{
cout<<"Error";
}

for (int i=0;i<img.rows;i++)
{
for (int j=0;j<img.cols;j++)
{//彩色图像分为三个通道，每个像素值由三个分量组成

Vec3b pixe;

pixe[0] = i%255;
pixe[1] = j%55;
pixe[2] = 200;//分别设置三个分量的值，并把值存到容器中

img.at<Vec3b>(i,j) = pixe;//此处对每个像素赋值
}
}

imshow("img",img);
waitKey(0);

return 0;
}

#include <iostream>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>

using namespace std;
using namespace cv;

const int num = 300;

void salt_pepper(Mat &img)
{

for (int i=0;i<num;i++)
{
int x = rand() % 255;
int y = rand() % 255;

if (img.channels() == 3)
{
img.at<Vec3b>(x,y)[0] = 255;
img.at<Vec3b>(x,y)[1] = 255;
img.at<Vec3b>(x,y)[2] = 255;
}
}
}

int main()
{
Mat img = imread("Lena.jpg");

if (!img.data)
{
cout<<"error";
}

salt_pepper(img);

imshow("salt_pepper",img);
waitKey(0);

return 0;
}