Qt 5 with USB video grabber - qt

Question

My demo code is to choose a camera from the integrated camera of my laptop, and a USB video grabber (STK1160). My code is attached.
main.cpp:
#include "mainwindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}
mainwindow.h:
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QCamera>
#include <QCameraInfo>
#include <QCameraImageCapture>
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow {
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
private:
Ui::MainWindow *ui;
QList <QCameraInfo> camList;
QCamera *camera;
private slots:
void onCameraChanged(int);
};
#endif // MAINWINDOW_H
mainwindow.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent), ui(new Ui::MainWindow) {
ui->setupUi(this);
camera = NULL;
connect(ui->cameraComboBox,static_cast<void (QComboBox::*)(int)>(&QComboBox::currentIndexChanged),this,
&MainWindow::onCameraChanged);
// find all available cameras and put them in the combo box
camList = QCameraInfo::availableCameras();
for(QList <QCameraInfo>::iterator it = camList.begin();it!=camList.end();++it) {
ui->cameraComboBox->addItem(it->description());
}
}
MainWindow::~MainWindow() {
delete ui;
}
void MainWindow::onCameraChanged(int idx) {
if(camera != NULL) {
camera->stop();
}
camera = new QCamera(camList.at(idx),this);
camera->setViewfinder(ui->viewfinder);
camera->setCaptureMode(QCamera::CaptureStillImage);
camera->start();
}
My problem is that when I choose the USB grabber from the combo box, I got the following error message:
libv4l2: error turning on stream: Message too long
CameraBin error: "Error starting streaming on device '/dev/video1'."
CameraBin error: "Could not negotiate format"
and the camera view is all black. Anyone has any idea? I have tested my video input on an AV screen, it works well.

Answer 1

I don't have a very concrete answer, but a more general one about camera & media APIs inside Qt5. In my experience some features only work on some platforms while other work on other platforms. For example I am currently struggling with having QVideoProbe work on Ubuntu even if it works fine on Android.
And development focus by Qt developers seems to be 80% on the mobile platforms recently. Also on Linux platforms the "backend" for video is gstreamer, which means that most errors originate there. My best tip is to upgrade to Qt 5.6 which relies on gstreamer1.0 instead of the ancient gstreamer0.1. Also make sure to install all the gstreamer plugins etc for your platform, as this may have a big impact on how well media works.
Also if you can reproduce the error directly in gstreamer, you might be able to find a fix for it there and this fix will then be available from Qt as well. For example if you are missing a codec or driver, using the gstreamer tools to add the support you need might solve the problem
I find the media APIs in Qt to be solid and I know that work is continuously being made to fill in the massing features in each platform's media back-end, so for each update of Qt more features should be added and bugs should be fixed.
I hope this helps, even if it does not address your question directly (which probably is because very few have the exact experience that you had).