1. 读写像素

(1)读一个灰度图像的某点像素值

Scalar intensity=img.at<uchar>(y,x);
或者Scalar intensity =img.at<uchar>(Point(x,y))

(2)读一个RGB像素点的像素值

int b = dst.at<Vec3b>(row, col)[0];  //读取像素
int g = dst.at<Vec3b>(row, col)[1];
int r = dst.at<Vec3b>(row, col)[2];

2. 修改像素值

（1）灰度图像

img.at<uchar>(y,x)=128;

（2）RGB三通道图像

dst.at<Vec3b>(row, col)[0] = newb;
dst.at<Vec3b>(row, col)[1] = newg;
dst.at<Vec3b>(row, col)[2] = newr;

（3）空白图像赋值

img=Scalar(0);

3. Vec3b与Vec3F

Vec3b对应的三通道的顺序是blue,green,red的uchar类型数据
Vec3f对应三通道的float类型
把CV_8UC1转换到CV32F1实现如下：
src.convertTo(dst,CV_32F);
 
代码演示：

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

using namespace std;
using namespace cv;

int main(int argc, char** argv) {
	Mat src,gray_src;
	src = imread("D:/image/test.jpg");
	if (src.empty()) {
		cout << "could not load image...." << endl;
		return -1;
	}
	namedWindow("input", WINDOW_AUTOSIZE);
	imshow("input", src);

	cvtColor(src, gray_src, COLOR_BGR2GRAY);		//转成灰度图像
	namedWindow("output", WINDOW_AUTOSIZE);
	imshow("output", gray_src);
	
	int height_gray = gray_src.rows;
	int width_gray = gray_src.cols;

	//单通道给图像做反向操作
	for (int row = 0; row < height_gray; row++) {
		for (int col = 0; col < width_gray; col++) {
			int gray = gray_src.at<uchar>(row, col);
			gray_src.at<uchar>(row, col) = 255 - gray;
		}
	}
	namedWindow("反向", WINDOW_AUTOSIZE);
	imshow("反向", gray_src);

	//三通道给图像做反向操作
	Mat dst;
	dst.create(src.size(), src.type());
	int height_color = src.rows;
	int width_color = src.cols;
	int channels = src.channels();

	for (int row = 0; row < height_color; row++) {
		for (int col = 0; col < width_color; col++) {
			int b = dst.at<Vec3b>(row, col)[0];  //读取像素
			int g = dst.at<Vec3b>(row, col)[1];
			int r = dst.at<Vec3b>(row, col)[2];
			int newb = 255 - b;
			int newg = 255 - g;
			int newr = 255 - r;
			dst.at<Vec3b>(row, col)[0] = newb;
			dst.at<Vec3b>(row, col)[1] = newg;
			dst.at<Vec3b>(row, col)[2] = newr;
		}
	}
	
	//三通道反向操作的API
	bitwise_not(src, dst);


	namedWindow("output", WINDOW_AUTOSIZE);
	imshow("output", dst);

	waitKey(0);
	return 0;
}