Take screenshot of webcam feed on Keypress using OpenCV and C++ - c++

Question

I'm currently getting the webcam feed of my laptop using VideoCapture cap(0) function and then display it in a Mat frame. What I want to do next is whenever I press a key 'c' for example, it takes the screenshot of the frame and save it into a folder as a JPEG image. However I have no idea on how to do so. Help is much needed, thank you.

Answer 1

I have spent several days searching the internet for the right solution with simple keyboard input. Ther was allways some leg / delay while using cv::waitKey.
The solution i have found is with adding Sleep(5) just after the capturing the frame from webcam.
The below example is a combination of different forum threads.
It works without any leg / delay. Windows OS.
Press "q" to capture and save the frame.
There is a webcam feed always present. You can change the sequence to show the captured frame / image.
PS "tipka" - means "key" on the keyboard.
Regards, Andrej
#include <opencv2/opencv.hpp>
#include <iostream>
#include <stdio.h>
#include <windows.h> // For Sleep
using namespace cv;
using namespace std;
int ct = 0;
char tipka;
char filename[100]; // For filename
int c = 1; // For filename
int main(int, char**)
{
Mat frame;
//--- INITIALIZE VIDEOCAPTURE
VideoCapture cap;
// open the default camera using default API
cap.open(0);
// OR advance usage: select any API backend
int deviceID = 0; // 0 = open default camera
int apiID = cv::CAP_ANY; // 0 = autodetect default API
// open selected camera using selected API
cap.open(deviceID + apiID);
// check if we succeeded
if (!cap.isOpened()) {
cerr << "ERROR! Unable to open camera\n";
return -1;
}
//--- GRAB AND WRITE LOOP
cout << "Start grabbing" << endl
<< "Press a to terminate" << endl;
for (;;)
{
// wait for a new frame from camera and store it into 'frame'
cap.read(frame);
if (frame.empty()) {
cerr << "ERROR! blank frame grabbed\n";
break;
}
Sleep(5); // Sleep is mandatory - for no leg!
// show live and wait for a key with timeout long enough to show images
imshow("CAMERA 1", frame); // Window name
tipka = cv::waitKey(30);
if (tipka == 'q') {
sprintf_s(filename, "C:/Images/Frame_%d.jpg", c); // select your folder - filename is "Frame_n"
cv::waitKey(10);
imshow("CAMERA 1", frame);
imwrite(filename, frame);
cout << "Frame_" << c << endl;
c++;
}
if (tipka == 'a') {
cout << "Terminating..." << endl;
Sleep(2000);
break;
}
}
// the camera will be deinitialized automatically in VideoCapture destructor
return 0;
}