OpenCV：添加透明通道/分离图像和视频的RGB通道

为三通道图像添加alpha透明通道

int main()
{
    //@ 读取图像
    string image_path = "**Desktop\\image\\person\\girl101.jpeg";
    Mat src = imread(image_path, IMREAD_COLOR);

    //@ 分离通道，有两种方式
    // (1)分离到向量
    vector rgb3Channels;
    split(src, rgb3Channels);
    // (2)分离到数组
    //Mat aChannels[3];
    //split(src, aChannels);

    //@ 创建一个alpha通道,全0。然后选定一个矩形区域，将其赋值为255
    Mat zero_mat = Mat::zeros(Size(src.cols, src.rows), CV_8UC1);
    Mat roi(zero_mat, Rect(0, 0, src.cols/2, src.rows/2));  // 证明alpha赋值为0的做法是正确有效的。
    roi = Scalar(255);//单通道情况时，Scalar中一个参数
    namedWindow("zero_mat", CV_WINDOW_NORMAL);
    imshow("zero_mat", zero_mat);

    //@ 合并通道，同样有两种方式，这里不写了，一种数组，一种vector
    vector channels_4;
    channels_4.push_back(rgb3Channels[0]);	//b
    channels_4.push_back(rgb3Channels[1]);	//g
    channels_4.push_back(rgb3Channels[2]);	//r
    channels_4.push_back(zero_mat);         //alpha=0

    Mat img_alpha_0;
    merge(channels_4, img_alpha_0);
    imwrite("img_alpha_01.png", img_alpha_0);
    namedWindow("img_alpha_0", CV_WINDOW_NORMAL);
    //@ 实际上已经添加了透明通道了，可以从保存的图像文件查看
    //@ 但是这里假如直接imshow的话，不会显示透明通道
    imshow("img_alpha_0", img_alpha_0);
    waitKey(0);

    return 0;
}

这里imshow img_alpha_0的时候，没有显示alpha通道，但是查看一下保存的图像可以看到alpha

---

opencv还有一种分离通道的方法，摘自：

OpenCV学习笔记（二十一）---三通道和四通道之间的区别 - JavaShuo

int main()
{
    Mat rgba(4, 4, CV_8UC4, Scalar(1, 2, 3, 4));
    cout << "rgba =" << endl << rgba << endl;
    cout << "rgba.rows=" << rgba.rows << ",rgba.cols=" << rgba.cols << endl;
    Mat bgr(rgba.rows, rgba.cols, CV_8UC3);
    Mat alpha(rgba.rows, rgba.cols, CV_8UC1);
    // forming an array of matrices is a quite efficient operation,
    // because the matrix data is not copied, only the headers
    Mat out[] = { bgr, alpha };		//注意out[]的含义，分裂到一个Mat数组中
    // rgba[0] -> bgr[2], rgba[1] -> bgr[1],
    // rgba[2] -> bgr[0], rgba[3] -> alpha[0]
    int from_to[] = { 0,2, 1,1, 2,0, 3,3 };	//指定分裂规则
    mixChannels(&rgba, 1, out, 2, from_to, 4);

    cout << "bgr =" << endl << bgr << endl;
    cout << "alpha =" << endl << alpha << endl;
    system("pause");
    waitKey(0);
    return 0;
}

---

 

对图像

#include 
#include 
using namespace std;
using namespace cv;
string imgName = "lena256.tif";

int main(int argc, char* argv[])
{
	Mat src = imread(imgName);
	Mat img;
	resize(src, img, Size(src.cols*0.5, src.rows * 0.5)); // 缩小操作
	Mat channel[3];
	split(img, channel);
	imshow("original", img);
	imshow("B", channel[0]);
	imshow("G", channel[1]);
	imshow("R", channel[2]);

	//set blue channel to 0
	channel[0] = Mat::zeros(img.rows, img.cols, CV_8UC1);
	//merge red and green channels
	merge(channel, 3, img);
	imshow("R_G_merge", img);
	waitKey(0);
	return 1;
}

对摄像头视频流

#include 
#include 
using namespace std;
using namespace cv;
int main(int argc, char* argv[])
{
	VideoCapture cap;
	cap.open(0);
	if (!cap.isOpened())
	{
		exit(0);
	}

	cap.set(CV_CAP_PROP_FRAME_WIDTH, 250);
	cap.set(CV_CAP_PROP_FRAME_HEIGHT, 250);

	cout << "Frame Width: " << cap.get(CV_CAP_PROP_FRAME_WIDTH) << endl;
	cout << "Frame Height: " << cap.get(CV_CAP_PROP_FRAME_HEIGHT) << endl;

	Mat frame;
	vector rgb;
	cap >> frame;

	namedWindow("original", 1);
	namedWindow("red", 1);
	namedWindow("green", 1);
	namedWindow("blue", 1);
	for (;;)
	{
		cap >> frame;
		imshow("original", frame);
		split(frame, rgb);

		imshow("red", rgb.at(2));
		imshow("green", rgb.at(1));
		imshow("blue", rgb.at(0));

		if (waitKey(30) >= 0)
			break;
	}

	waitKey(0);
	return 1;
}