opencv：了解 cvsetcaptureproperty 函数和参数

Question

我开始学习 openCv 但我在理解这个函数的最后一个参数（双精度值）时遇到了一些困难

int cvSetCaptureProperty(CvCapture* capture, int property_id, double value)

我知道这是 c++ 中使用的属性，但它在下面的代码中如何工作？

void onTrackSlide(int pos)
{
    cvSetCaptureProperty(cap, CV_CAP_PROP_POS_FRAMES, pos);
}

然后在主程序中这样调用：

cvCreateTrackbar("position", "example3", &slider, frames, onTrackSlide);

我不明白为什么在 onTrackSlide 函数的参数中从不填充或使用 pos。

这是完整的程序：

#include<opencv\cv.h>
#include<opencv\highgui.h>
#include "opencv2/opencv.hpp"
//#include <iostream>

using namespace cv;

int slider=0;
CvCapture* cap = NULL;

void onTrackSlide(int pos)
{

    cvSetCaptureProperty(cap, CV_CAP_PROP_POS_FRAMES, pos);
}


int main(int argc, char ** argv)
{

    cvNamedWindow("example3", CV_WINDOW_AUTOSIZE);
    cap = cvCreateFileCapture(argv[1]);
    int frames = (int)cvGetCaptureProperty(cap, CV_CAP_PROP_FRAME_COUNT);

    if (frames != 0)
    {
        cvCreateTrackbar("position", "example3", &slider, frames, onTrackSlide);

    }

    IplImage* frame;
    while (1)

    {
        frame = cvQueryFrame(cap);
        if (!frame)break;
        cvShowImage("example3", frame);
        char c = cvWaitKey(33);
        if (c == 27)  break;

    }

    cvReleaseCapture(&cap);
    cvDestroyWindow("example3");


}

Answer 1

createTrackbar 是 OpenCV 用户界面 (Highgui) 的一个函数，它接受一个回调函数 onChange:

C++: int createTrackbar(const string& trackbarname, const string& winname, int* value, int count, TrackbarCallback onChange=0, void* userdata=0)

C: int cvCreateTrackbar(const char* trackbar_name, const char* window_name, int* value, int count, CvTrackbarCallback on_change=NULL )

其中

onChange – Pointer to the function to be called every time the slider changes position. This function should be prototyped as void Foo(int,void*); , where the first parameter is the trackbar position

您定义了回调 onChange，例如：

void onTrackSlide(int pos)
{ 
    // pos is the current trackbar position
    ...
}

由 Highgui 内部回调系统调用。每次移动轨迹栏时，都会使用新的 pos 值调用回调。

根据评论，您定义了一个回调函数，例如：

void onTrackSlide() 
{ 
    int pos; 
    cvSetCaptureProperty(cap, CV_CAP_PROP_POS_FRAMES, pos); 
}

而这个不会工作，因为变量 pos 永远不会被初始化。

您也可以使用 getTrackbarPos 获取轨迹栏的当前值。

Answer 2

此代码读取带有跟随阅读过程的轨迹栏的视频文件。

#include <opencv2/core.hpp>
#include <opencv2/imgcodecs.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/videoio/videoio_c.h>
#include <iostream>
using namespace cv;


//First we defi ne a global variable for the slider position.
int g_slider_position = 0;


/*

    cv::VideoCapture::VideoCapture  (   const String &  filename    )

    Open video file or a capturing device or a IP video stream for video capturing with API Preference.
    Parameters: filename    it can be:
    1- name of video file (eg. video.avi)
    2- or image sequence (eg. img_%02d.jpg, which will read samples like img_00.jpg, img_01.jpg, img_02.jpg, ...)
    3- or URL of video stream (eg. protocol://host:port/script_name?script_params|auth)
    4- or GStreamer pipeline string in gst-launch tool format in case if GStreamer is used as backend Note that each video stream
       or IP camera feed has its own URL scheme. Please refer to the documentation of source stream to know the right URL.
*/
VideoCapture capture("../../videos/Mahrez.mp4"); //put your own video path.

Mat img;

/*
Now we defi ne a callback routine to be used when the user pokes the slider.
function to be called every time the slider changes position.This function should be prototyped as void Foo(int, void*);, 
where the first parameter is the trackbar position and the second parameter is the user data.
*/
void onTrackbarSlide(int position, void* data)
{
    /*
    CV_CAP_PROP_POS_FRAMES indicates that we would like to set the read position
    in units of frames. (We can use AVI_RATIO instead of FRAMES if we want to set the position as a fraction of the overall video length).
    Finally, we pass in the new value of the position.
    */
    capture.set(CAP_PROP_POS_FRAMES, position);
}

int main()
{
    namedWindow("Video_Slider", WINDOW_AUTOSIZE);
    
    /*
    cv::VideoCapture::get (int propId)  In this case, we want to find out how many frames are in the video
    so that we can calibrate the slider (in the next step).
    */
    int frames = (int) capture.get(CAP_PROP_FRAME_COUNT);

    if (frames != 0)
    {
        createTrackbar("Trackbar", "Video_Slider", &g_slider_position, frames, onTrackbarSlide);
    }

    while (true)
    {
        /*
        We then wait for 25 ms.* If the user hits a key, then c will be set to the ASCII value of that key;
        if not, then it will be set to –1. If the user hits the Esc key (ASCII 27), then we will exit the read loop.
        Otherwise, 25 ms will pass and we will just execute the loop again.

        We can control the speed of the video by adjusting the waitKey(25). It waits 25 ms to read the next frame.
        */
        char c = waitKey(25);
        if (c == 27) break;

        bool bSuccess = capture.read(img); // read a new frame from video 
        //Breaking the while loop at the end of the video
        if (bSuccess == false) break;

        /*
         The imshow() function requires that a named window already exist(created by cvNamedWindow()).
         */
        imshow("Video_Slider", img);

        /*
        void cv::setTrackbarPos (const String & trackbarname, const String & winname, int position) 
        This function sets the position of the specified trackbar in the specified window.
        */
        setTrackbarPos("Trackbar", "Video_Slider", g_slider_position);
        // Increment the slider position after reading each frame.
        g_slider_position++;

    }

    capture.release();
    destroyAllWindows();
    return 0;

}

opencv：了解 cvsetcaptureproperty 函数和参数

opencv: understanding cvsetcaptureproperty function and argument

c++

opencv

visual-studio

opencv3.0