opencv4opencv视频教程 C++ 5读写图像imreadimwrite读写像素at＜＞()修改像素值ROI区域选择（图像裁剪）RectVec3b与Vec3FCV_32F）

Posted 2022-02-16 Dontla

上一讲：【opencv4】opencv教程 C++ 4、Mat对象（深拷贝：clone()、copyTo()，create()创建图片，zeros()、eye()初始化空白图像，Scalar()创建向量）

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文章目录

• • 读写图像
  • 读写像素`at<>()`
  • 修改像素值、区域选择（图像裁剪）Rect
  • Vec3b与Vec3F
  • 代码示例

读写图像

读写像素at<>()

修改像素值、区域选择（图像裁剪）Rect

貌似opencv4没有img这个函数。。。（不是img函数，而是图像名称！）

示例：区域裁剪

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

using namespace cv;
using namespace std;

int main(int argc, const char* argv[]) 

	Mat src, dst, dst1, dst2, dst3, difference;

	src = imread("./test.jpg");
	//if (src.empty()) 
	if (!src.data)
		printf("could not load image...\\n");
		return -1;
	
	namedWindow("input img");	//默认自动窗口大小
	imshow("input img", src);

	Rect r(100,50,300,200);	//（起始x,起始y,截取x,截取y）
	dst = src(r);
	imshow("output img", dst);

	waitKey(0);
	return 0;

VS编译运行：

Vec3b与Vec3F

示例：我这转换后咋变这样了？

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

using namespace cv;
using namespace std;

int main(int argc, const char* argv[]) 

	Mat src, dst, dst1, dst2, dst3, difference;

	src = imread("./test.jpg");
	//if (src.empty()) 
	if (!src.data)
		printf("could not load image...\\n");
		return -1;
	
	namedWindow("input img");	//默认自动窗口大小
	imshow("input img", src);

	src.convertTo(dst,CV_32F);
	imshow("output img", dst);

	waitKey(0);
	return 0;

转换成CV_32F浮点数，加上一个参数就没事了，但是打印像素值却有点让人看不懂呢/??显示跟像素值是怎么映射的？不明觉厉，已上知乎问了

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

using namespace cv;
using namespace std;

int main(int argc, const char* argv[]) 

	Mat src, dst, dst1, dst2, dst3, difference;

	src = imread("./test.jpg");
	//if (src.empty()) 
	if (!src.data) 
		printf("could not load image...\\n");
		return -1;
	
	namedWindow("input img");	//默认自动窗口大小
	imshow("input img", src);

	//src.convertTo(dst, CV_32F);
	src.convertTo(dst, CV_32F, 1 / 255.0);

	printf("dst type:%d\\n", dst.type());	//dst type:21

	printf("%f\\n", dst.at<float>(0, 0));	//0.133333
	printf("%f\\n", dst.at<float>(0, 1));	//0.054902
	
	imshow("output img", dst);

	waitKey(0);
	return 0;

参考文章：Opencv中Mat数据类型CV_8UC3转换为 CV_32FC3注意事项

代码示例

手动求一张彩色图片的灰度图，并且与opencv内置求灰度函数cvtColor函数作比较

原图：

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

using namespace cv;
using namespace std;

int main(int argc, const char* argv[]) 

	Mat src, dst, dst1, dst2, dst3, difference;

	src = imread("./test.jpg");
	//if (src.empty()) 
	if (!src.data)
		printf("could not load image...\\n");
		return -1;
	
	namedWindow("input img");	//默认自动窗口大小
	imshow("input img", src);

	cvtColor(src,dst,COLOR_BGR2GRAY);
	imshow("output img", dst);

	//dst1 = Mat(src.size(),src.type());
	dst1.create(src.size(), CV_8UC1);
	dst2.create(src.size(), CV_8UC1);
	dst3.create(src.size(), CV_8UC1);
	difference.create(src.size(), CV_8UC1);

	int height = src.rows;
	int width = src.cols;
	int sc = src.channels();

	for (int row = 0; row < height; row++) 
		for (int col = 0; col < width; col++) 
			int b = src.at<Vec3b>(row, col)[0];
			int g = src.at<Vec3b>(row, col)[1];
			int r = src.at<Vec3b>(row, col)[2];
			dst1.at<char>(row, col) = min(b, min(g, r));
			dst2.at<char>(row, col) = b + g + r - max(b, max(g, r)) - min(b, min(g, r));	//灰度取中间值
			dst3.at<char>(row, col) = max(b, max(g, r));
		
	

	//namedWindow("output img1");	//默认自动窗口大小
	imshow("output img1", dst1);
	imshow("output img2", dst2);
	imshow("output img3", dst3);

	//求两灰度图差
	for (int row = 0; row < height; row++) 
		for (int col = 0; col < width; col++) 
			//difference.at<char>(row, col) = abs(dst.at<char>(row, col) - dst2.at<char>(row, col));
			//difference.at<char>(row, col) = dst.at<char>(row, col) - dst2.at<char>(row, col);
			difference.at<char>(row, col) = (dst.at<char>(row, col) - dst2.at<char>(row, col)) > 0 ? 0 : 255;	//从结果看来，opencv转换灰度方法确实跟我们自己用的不一样
		
	


	//imshow("output img3", dst - dst2);
	imshow("output img4", difference);
	//imshow("output img4", abs(dst-dst2));



	waitKey(0);
	return 0;

VS编译运行结果：