尝试使用 VideoCapture 和 imshow()，引发 Assertion failed (size.width>0 && size.height>0) in cv::imshow

Question

I'm using Visual Studio Express 2013 with OpenCV 2.4.7 , following this tutorial .

I have spent hours searching the web for solutions, including all of the relevant SO questions. I have tried:

• the return value of VideoCapture::open is 1

• extending the waitKey() delay to 50ms and later 500ms

• setting the dimensions of the window

• creating another project on Visual C++

• opening an existing image instead of reading from camera (same error)

but no luck, please help!

Here's my code:

#include <opencv/cv.h>
#include <opencv/highgui.h>
#include <iostream>

using namespace std;
using namespace cv;

int main() {
    Mat image;

    VideoCapture cap;
    int camOpen = cap.open(CV_CAP_ANY);

    namedWindow("window", CV_WINDOW_AUTOSIZE);

    while (true) {
        cap >> image;

        imshow("window", image);

    // delay 33ms
    waitKey(33);        
    }

}

As I compiled and ran it, I got the following error:

OpenCV Error: Assertion failed (size.width>0 && size.height>0) in cv::imshow, file........\opencv\modules\highgui\src\window.cpp, line 261

Error occurs at the line imshow("window", image); . When I commented it out, there are no complaints.

---

UPDATES:

A plausible explanation of why this error occured was that my webcam takes time to start, which is why image.empty() is true initially, hence the abort() function was called to exit the program.

With the code

if (!image.empty()) {
    imshow("window", image);
}

we can wait for the camera to start

Answer 1

I tried your code and for me it works (it visualizes the current webcam input)!
I ran it on Visual Studio 2012 Ultimate with OpenCV 2.4.7.
...
The error occurs because the image is empty, so try this:

while (true) {
    cap >> image;

    if(!image.empty()){
        imshow("window", image);
    }

// delay 33ms
waitKey(33);        
}

Maybe the first image you receive from your webcam is empty. In this case imshow will not throw an error. So hopefully the next input images are not empty.

Answer 2

Do this:

VideoCapture cap;
cap.set(CV_CAP_PROP_FRAME_WIDTH, 640);
cap.set(CV_CAP_PROP_FRAME_HEIGHT, 480);
int camOpen = cap.open(CV_CAP_ANY);

Or you can try changing this:

while (true) {
        cap >> image;

        imshow("window", image);

    // delay 33ms
    waitKey(33);        
    }

to

try
{
     cap >> image;
     imshow("window", image);
     waitKey(33); 
}
catch (Exception& e)
{
    const char* err_msg = e.what();
    std::cout << "exception caught: imshow:\n" << err_msg << std::endl;
}

Answer 3

int i=0;

while(i<4)

{

VideoCapture cap(0); // force camera to open 4 tiMEs

i++;

}

waitKey(5000); 
VideoCapture cap(0);
int camOpen = cap.open(CV_CAP_ANY);

namedWindow("window", CV_WINDOW_AUTOSIZE);

while (true) {
    cap >> image;

    imshow("window", image);
waitKey(33);        
}

Do this it will work for you for sure.

Answer 4

Always check for errors.

You can either enable exception throwing from the VideoCapture instance, or you will have to check manually.

If checking manually, there are two places you need to check:

1. assert(cap.isOpened()); after creating/opening the VideoCapture instance. If it could not be opened, there is no exception being raised by that. You have to check and handle this. An assertion suffices but you can handle this more gracefully if you want.

2. if (image.empty()) break; after each cap >> image; , or if (.cap;read(image)) break; instead of cap.read(image); , because the video stream may have ended or there's some issue with the camera. Again, no automatic exception being thrown. You need to check and handle this. The read() may fail intermittently (ie next read() works again), but that's rare. Usually, once a read() fails, all following read() calls on that VideoCapture instance will also fail.

There is VideoCapture::setExceptionMode() , which allows read() and open() calls to actually throw exceptions on their own instead of failing silently. In that case, you still have to catch the exception and inspect it ( e.what() ). If you don't catch and inspect the exception explicitly, the runtime may just print to stderr that some exception occurred, but no details on it.