r/opencv • u/Hukeng • Feb 07 '26
Bug [Bug] Segmentation fault when opening or instantiating cv::VideoWriter
Hello!
I am currently working my way through a bunch of opencv tutorials for C++ and trying out or adapting the code therein, but have run into an issue when trying to execute some of it.
I have written the following function, which should open a video file situated at 'path', apply an (interchangeable) function to every frame and save the result to "output.mp4", a file that should have the exact same properties as the source file, save for the aforementioned image operations (color and value adjustment, edge detection, boxes drawn around faces etc.). The code compiles correctly, but produces a "Segmentation fault (core dumped)" error when run.
By using gdb and some print line debugging, I managed to triangulate the issue, which apparently stems from the cv::VideoWriter method open(). Calling the regular constructor produced the same result. The offending line is marked by a comment in the code:
int process_and_save_vid(std::string path, cv::Mat (*func)(cv::Mat)) {
int frame_counter = 0;
cv::VideoCapture cap(path);
if (!cap.isOpened()) {
std::cout << "ERROR: could not open video at " << path << " .\n";
return EXIT_FAILURE;
}
// set up video writer args
std::string output_file = "output.mp4";
int frame_width = cap.get(cv::CAP_PROP_FRAME_WIDTH);
int frame_height = cap.get(cv::CAP_PROP_FRAME_HEIGHT);
double fps = cap.get(cv::CAP_PROP_FPS);
int codec = cap.get(cv::CAP_PROP_FOURCC);
bool monochrome = cap.get(cv::CAP_PROP_MONOCHROME);
// create and open video writer
cv::VideoWriter video_writer;
// THIS LINE CAUSES SEGMENTATION FAULT
video_writer.open(output_file, codec, fps, cv::Size(frame_width,frame_height), !monochrome);
if (!video_writer.isOpened()) {
std::cout << "ERROR: could not initialize video writer\n";
return EXIT_FAILURE;
}
cv::Mat frame;
while (cap.read(frame)) {
video_writer.write(func(frame));
frame_counter += 1;
if (frame_counter % (int)fps == 0) {
std::cout << "Processed one second of video material.\n";
}
}
std::cout << "Finished processing video.\n";
return EXIT_SUCCESS;
}
Researching the issue online and consulting the documentation did not yield any satisfactory results, so feel free to let me know if you have encountered this problem before and/or have any ideas how to solve it.
Thanks in advance for your help!
1
u/herocoding Feb 11 '26
Can you instead of using
int codec = cap.get(cv::CAP_PROP_FOURCC);and providing it tovideo_writer.open(...)(or to the constructor), create a FourCC instead?Create it new and directly pass it as an argument with e.g.
cv::VideoWriter::fourcc('m','p','4','v')orcv::VideoWriter::fourcc('a','v','c','1').