使用cv::imread()讀取圖片

經過上一節的學習,我們了解到opencv處理的圖片其實就是對矩陣進行操作,使用cv::imread()對圖片進行讀取并回傳一個矩陣,來看一個例子:

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

using namespace std;

int main() {
	//imread(const String &filedir,int flags)
    cv::Mat img = cv::imread("D:/null.jpg", 1);
    //另一種判空的寫法:img.data == NULL;
    if (img.empty()) {
        cout << "open the file failed!" << endl;
    }
    cv::Mat gray_img = cv::imread("D:/cat.jpg", 0);
    if (!gray_img.empty()) {
        cv::imshow("gray_img", gray_img);
        cv::waitKey(0);
    }
    cv::Mat nochange_img = cv::imread("D:/cat.jpg", -1);
    if (!nochange_img.empty()) {
        cv::imshow("no_chang", nochange_img);
        cv::waitKey(0);
    }
    return 0;
}

效果顯示
open the file failed!

上述代碼展示了讀取圖片的三種不同的狀態,這個狀態由flags引數決定,flags=1(>0)表示函式需要回傳三通道影像,若圖片不是三通道則強制轉成3通道.flags=0表示函式回傳單通道影像,若圖片是多通道圖片則強制轉成單通道.flags=-1(<0)表示不對影像進行通道轉換處理.imread()支持影像格式:jpg,jpeg,dib,bmp,png,pbm,ppm,sr,ras,tiff,exr,jp2

使用cv::imwrite()存盤圖片

上面我們學習了如何讀取指定路徑下的圖片,接下來來看一下如何將處理過的圖片進行指定存盤.

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

using namespace std;
using namespace cv;

int main() {

    Mat mat = imread("D:/cat.jpg", 1);
    if (!mat.empty()) {
        //Rect(x,y,width,height)
        Mat roi(mat, Rect(200, 200, 400, 500));
        //bool imwrite(const String &filedir,InputArray img)
        //imwrite("D:/cat_roi_write.jpg", roi);
        //imwrite("D:/cat_roi_write.bmp", roi);
        bool is_write = imwrite("D:/cat_roi_write.png", roi);
        cout << "write is succeed?:" << is_write << endl;
    } else {
        cerr << "openfile is failed" << endl;
    }
    return 0;
}

效果顯示
write is succeed?:1

png是無損圖片
jpg是壓縮成的有損圖片
bmp是不壓縮的無損圖片,大小較大
注意并不是所有Mat物件都能存盤為圖片,一般來說只有8U型別的單通道和3通道矩陣可以進行影像存盤,若需要保存成16U型別的圖片,則只能使用png,jpeg 2000,tiff格式進行存盤.
為了將其他格式的矩陣保存為圖片,opencv提供了Mat::convertTo(),從v::cvtColor()能夠將矩陣轉換為能夠保存的型別格式,若指定目錄下已經存在要保存的檔案,則會對其進行覆寫.(其中cvtColor()是cv類的成員函式,需要宣告頭檔案#include<opencv2/opencv.hpp>)

使用cv::VideoCapture::open()讀取視頻

VideoCapture既可以從視頻檔案讀取影像,也可以從攝像頭中讀取影像.使用VideoCapture::open()打開.

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

using namespace std;
using namespace cv;

int main() {
    VideoCapture capture;
    Mat frame;
    frame = capture.open("D:/test_video.mp4");
    if (!capture.isOpened()) {
        cout << "this video can not open" << endl;
        return -1;
    } else {
        cout << "video is successful open=" << capture.isOpened() << endl;
    }
    //視窗大小自適應,靈活調整,寫在cv::imshow()之前,防止圖片尺寸太大imshow顯示不全，注意名字和imshow相同，不然可能會顯示兩個視窗
    namedWindow("show_frame", WINDOW_AUTOSIZE);
    while (capture.read(frame)) {
        imshow("show_frame", frame);
        //每一幀的等待時間,數字越小讀取越快
        waitKey(1);
    }
    //釋放視頻流,手動呼叫虛解構式,open()和VideoCapture的解構式會自動呼叫,故可以不手動釋放
    capture.release();
    return 0;
}

列印是否打開成功的結果:video is successful open=1
呼叫本地攝像頭很容易,將capture.open("D:/test_video.mp4");換成capture.open(0);即可.

使用cv::VideoWriter::write()存盤視頻

使用opencv存盤視頻流,需要在初始化時設定一系列引數檔案名,編解碼器,幀率,寬度,高度等等
CV_FOURCC可以獲取的編碼格式
CV_FOURCC(‘P’, ‘I’, ‘M’, ‘1’) = MPEG-1 code
CV_FOURCC(‘M’, ‘J’, ‘P’, ‘G’) = motion-jpeg codec
CV_FOURCC(‘M’, ‘P’, ‘4’, ‘2’) = MPEG-4.2 codec
CV_FOURCC(‘D’, ‘I’, ‘V’, ‘3’) = MPEG-4.3 codec
CV_FOURCC(‘D’, ‘I’, ‘V’, ‘X’) = MPEG-4 codec
CV_FOURCC(‘U’, ‘2’, ‘6’, ‘3’) = H263 codec
CV_FOURCC(‘I’, ‘2’, ‘6’, ‘3’) = H263I codec
CV_FOURCC(‘F’, ‘L’, ‘V’, ‘1’) = FLV1 codec

MPEG-1是為CD光碟介質定制的視頻和音頻壓縮格式.
Motion JPEG是一種視頻壓縮格式,其中每一幀影像都分別使用JPEG編碼.
MPEG-4利用很窄的帶寬,通過幀重建技術,壓縮和傳輸資料,求出以最少的資料獲得最佳的影像質量.

VideoWriter::open()函式原型CV_WRAP virtual bool open(const String& filename, int fourcc, double fps, Size frameSize, bool isColor = true);

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

using namespace std;
using namespace cv;

int main() {
    VideoCapture capture;
    capture.open("D:/test_video.mp4");
    if (!capture.isOpened()) {
        cout << "can not open the video" << endl;
    }
    double fps = capture.get(CAP_PROP_FPS);
    int width = int(capture.get(CAP_PROP_FRAME_WIDTH));
    int height = int(capture.get(CAP_PROP_FRAME_HEIGHT));
    int fourcc_type = VideoWriter::fourcc('M', 'P', '4', '2');
    VideoWriter videoWriter;
    videoWriter.open("D:/write_video.avi", fourcc_type, fps,
                     Size(width, height), true);
    if (!videoWriter.isOpened()) {
        cout << "video_writer can not open" << endl;
        return -1;
    }
    Mat frame;
    while (capture.read(frame)) {
        //videoWriter << frame;這種寫法也是正確的,使用VideoWriter的輸出流進行寫入
        videoWriter.write(frame);

    }
    return 0;
}
}

效果顯示

使用cv::VideoCapture讀取攝像頭

#include <iostream>
#include <opencv2/imgcodecs.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/opencv.hpp>
#include <thread>

using namespace std;
using namespace cv;
thread::id main_thread_id = this_thread::get_id();

void open_video(VideoCapture &capture_usb, const string &filename, Mat &frame) {
    capture_usb.open(filename);
    if (!capture_usb.isOpened()) {
        cerr << "url not exit" << endl;
    }
    if (this_thread::get_id() == main_thread_id) {
        cout << "this is the main thread" << endl;
    } else {
        cout << "this is not the main thread!" << endl;
    }
    capture_usb.set(CAP_PROP_FOURCC, VideoWriter::fourcc('M', 'J', 'P', 'G'));
    capture_usb.set(CAP_PROP_FRAME_HEIGHT, 500);
    capture_usb.set(CAP_PROP_FRAME_WIDTH, 500);
    capture_usb.set(CAP_PROP_FPS, 30);
    namedWindow("capture", WINDOW_AUTOSIZE);
    while (capture_usb.read(frame)) {
        imshow("capture", frame);
        waitKey(1000);
    }
}

void open_video1(VideoCapture &capture_usb, const string &filename, Mat &frame) {
    capture_usb.open(filename);
    if (!capture_usb.isOpened()) {
        cerr << "url not exit" << endl;
    }
    if (this_thread::get_id() == main_thread_id) {
        cout << "this is the main thread" << endl;
    } else {
        cout << "this is not the main thread!" << endl;
    }
    //設定采集格式
    capture_usb.set(CAP_PROP_FOURCC, VideoWriter::fourcc('M', 'J', 'P', 'G'));
    //設定解析度
    capture_usb.set(CAP_PROP_FRAME_HEIGHT, 500);
    capture_usb.set(CAP_PROP_FRAME_WIDTH, 500);
    //設定每秒讀取的圖片數量
    capture_usb.set(CAP_PROP_FPS, 30);
    namedWindow("capture1", WINDOW_AUTOSIZE);
    while (capture_usb.read(frame)) {
        imshow("capture1", frame);
        waitKey(1000);
    }
}

int main() {
    VideoCapture capture_usb, local_video_capture, capture_usb1, capture_usb2;
    Mat frame, frame_video, frame1, frame2;
    //url為讀取的視頻流地址(介面)
    const string filename = url;
    const string local_file = "D:/test_video.mp4";
    const string filename1 = url;
    thread thread_1(open_video, ref(capture_usb), filename, ref(frame));
    thread thread_2(open_video1, ref(capture_usb1), filename1, ref(frame));
    thread_2.join();
    thread_1.detach();

    for (;;) {
        if (!capture_usb.isOpened()) {
            break;
        }
        if (!capture_usb1.isOpened()) {
            break;
        }
       
    capture_usb.release();
    capture_usb1.release();
    return 0;
}

capture_usb.set()引數簡介

使用互斥量、鎖、多執行緒進行資料讀取和顯示

#include <iostream>
#include <opencv2/imgcodecs.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/opencv.hpp>
#include <thread>
#include <deque>
#include <mutex>
#include <Windows.h>
#include <ctime>
using namespace std;
using namespace cv;
thread::id main_thread_id = this_thread::get_id();
deque<cv::Mat> mat_deque;
mutex mut;

void open_video(VideoCapture &capture_usb, const string &filename, Mat &frame) {
    capture_usb.open(filename);
    if (!capture_usb.isOpened()) {
        cerr << "url not exit" << endl;
    }
    if (this_thread::get_id() == main_thread_id) {
        cout << "this is the main thread" << endl;
    } else {
        cout << "this is not the main thread!" << endl;
    }
    capture_usb.set(CAP_PROP_FOURCC, VideoWriter::fourcc('M', 'J', 'P', 'G'));
    capture_usb.set(CAP_PROP_FRAME_HEIGHT, 200);
    capture_usb.set(CAP_PROP_FRAME_WIDTH, 200);
    capture_usb.set(CAP_PROP_FPS, 300);
    capture_usb.set(CAP_PROP_BUFFERSIZE, 3);
    while (capture_usb.read(frame)) {
    	long start_time = clock();
        if (waitKey(20) >= 0) { break; }
        lock_guard<mutex> lk(mut);
        mat_deque.push_back(frame);
        long end_time = clock();
        cout<<"read_total_time="<<end_time-start_time<<endl;
    }
}

void get_video_data(Mat &frame) {
    while (true) {
        if (!mat_deque.empty()) {
        	long start_time = clock();
            unique_lock<mutex> lk(mut);
            frame = mat_deque.back();
            long end_time = clock();
            cout<<"get_total_time="<<end_time-start_time<<endl;
            lk.unlock();
            if (waitKey(20) >= 0) { break; }
            imshow("capture", frame);
            waitKey(1000);
            mat_deque.pop_back();
        }
    }
}

int main() {
    VideoCapture capture_usb;
    Mat frame, frame_video, frame1, frame2;
    const string filename = url;
    const string filename1 = url;
    const string filename_123 = url;
    thread thread_1(open_video, ref(capture_usb), filename_123, ref(frame));
    thread thread3(get_video_data,ref(frame_video));
    thread_1.join();
    thread3.join();

    for (int i = 0; i < 5; ++i) {
        thisthread::sleepfor(2000ms);
    }
    return 0;
}