OpenCV QT, Displaying Frames of a Video (Not using a While Loop) - c++

Question

I have a simple personal project which uses QT and OpenCV frameworks to display a video in a QLabel. I know how to do the conversions into QImage and setting the Pixmap.
However, the video is running too fast under the while loop and when I checked the fps is either 29 or 30 no matter which video I load.
To counter this, I have also implemented a QTimer to start when the video is loaded.
I am not sure how to use that to display the frames with an appropriate framerate that I need to set.
Any Idea how i can implement this?

Answer 1

I have done mat to QImage conversion in my project earlier.
static QImage Mat2QImage(const cv::Mat3b &src) {
QImage dest(src.cols, src.rows, QImage::Format_ARGB32);
for (int y = 0; y < src.rows; ++y) {
const cv::Vec3b *srcrow = src[y];
QRgb *destrow = (QRgb*)dest.scanLine(y);
for (int x = 0; x < src.cols; ++x) {
destrow[x] = qRgba(srcrow[x][2], srcrow[x][1], srcrow[x][0], 255);
}
}
return dest;
}
usage might be like this
void foo::timeout() // A slot which QTimer's timeout signal is connected to
{
// I didn't tested the code but it should work
Mat frame;
m_cap >> frame;
QImage img = Mat2QImage(frame);
QPixmap pixmap = QPixmap::fromImage(img);
ui->streamDisplay->setPixmap(pixmap);
}
As far as i remember, Mat image should be ARGB32. It had been worked at 30 fps smoothly.
I heard that the best performant solution is to use QOpenglWidget but i dont know how to implement the same functionality. Maybe you can take a look.
my older repo link
display-code-cpp
image-conversion-cpp

Answer 2

There is nothing wrong with using while-loop, you just need to calculate the frame rate correctly and then sleep for each loop, just do it like this:
int FPS = static_cast<int>(capture.get(cv::CAP_PROP_FPS));
uint delayTime = static_cast<uint>(1000 / FPS);
while(capture.read(frame))
{
// process the frame
...
// now sleep (milli secconds)
QThread::msleep(delayTime);
}
Now the video will be displayed at a normal rate.