How to start and stop QCamera

[mit_cruze]

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "unistd.h"
#include <qdebug.h>
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
m_scene = new QGraphicsScene;
m_item = new QGraphicsVideoItem;

ui->graphicsView->setScene(m_scene);
ui->graphicsView->scene()->addItem(m_item);

camera = new QCamera(QCameraInfo::defaultCamera());
camera->setViewfinder(m_item);
camera->setCaptureMode(QCamera::CaptureVideo);
}

MainWindow::~MainWindow()
{
delete ui;
}
// On Start Button
void MainWindow::on_pushButton_clicked()
{
camera->start();
}
// On Stop Button
void MainWindow::on_pushButton_2_clicked()
{
camera->stop();
}

I have above code to stream USB camera. Camera stream does not start on pressing "Start Button". Camera stream is displayed when I first press Stop button and then Start button. Multiple USB cameras are connected to my Linux system. I want select any one camera and start or stop stream. I do not want to record.

How to make it work correctly?

[Pradeep P N]

Hi @mit_cruze

The above code works fine, behavior is also as expected.

void MyCamera_Forum::on_pushButton_clicked()
{
    camera->start(); // Start Camera & Streaming 
}

void MyCamera_Forum::on_pushButton_2_clicked()
{
    camera->stop(); // Stop Streaming 
}

Do you have any other warnings ?
And you can check for availableCameras under QCameraInfo
With the available devises you can specify which Camera you want to use.

All the best.

[Kent-Dorfman]

Check the error() condition and state() to determine whether an operation is valid or if an error occured when you try to do something with your camera. Don't make assumptions about the state of the device.

[mit_cruze]

@Pradeep-P-N On what Qt version did you test?

[Pradeep P N]

@mit_cruze
Qt 5.12.1 (Creator 4.10.1) & Qt 5.9.2 (Creator 4.4.1)

And as said by @Kent-Dorfman
Did you check the error() QCamera::Error and state() QCamera::State of the camera ?

[mit_cruze]

@Pradeep-P-N Hi Pradeep Thanks for reply.

I have added debugs for Camera state, status and error. Also added output.
My pushbutton press sequence is 1) Start 2) Stop 3 ) Start

Debugs:

// On Start Button
void MainWindow::on_pushButton_clicked()
{
    qDebug() << "before start camera->status(): " << camera->status();
    qDebug() << "before start camera->state(): " << camera->state();
    qDebug() << "before start camera->error(): " << camera->error();
    qDebug() << "before start camera->errorString():" << camera->errorString() ;
    camera->start();
    qDebug() << "after start camera->status(): " << camera->status();
    qDebug() << "after start camera->state(): " << camera->state();
    qDebug() << "after start camera->error(): " << camera->error();
    qDebug() << "after start camera->errorString():" << camera->errorString() << "\n";
}
// On Stop Button
void MainWindow::on_pushButton_2_clicked()
{
    qDebug() << "before stop camera->status(): " << camera->status();
    qDebug() << "before stop camera->state(): " << camera->state();
    qDebug() << "before stop camera->error(): " << camera->error();
    qDebug() << "before stop camera->errorString():" << camera->errorString() ;

    camera->stop();
    qDebug() << "after stop camera->status(): " << camera->status();
    qDebug() << "after stop camera->state(): " << camera->state();
    qDebug() << "after stop camera->error(): " << camera->error();
    qDebug() << "after stop camera->errorString():" << camera->errorString()  << "\n";

}

Outputs:

before start camera->status():  QCamera::UnloadedStatus
before start camera->state():  QCamera::UnloadedState
before start camera->error():  QCamera::NoError
before start camera->errorString(): ""
after start camera->status():  QCamera::LoadingStatus
after start camera->state():  QCamera::ActiveState
after start camera->error():  QCamera::NoError
after start camera->errorString(): "" 

before stop camera->status():  QCamera::LoadedStatus
before stop camera->state():  QCamera::ActiveState
before stop camera->error():  QCamera::NoError
before stop camera->errorString(): ""
after stop camera->status():  QCamera::LoadedStatus
after stop camera->state():  QCamera::LoadedState
after stop camera->error():  QCamera::NoError
after stop camera->errorString(): "" 

before start camera->status():  QCamera::LoadedStatus
before start camera->state():  QCamera::LoadedState
before start camera->error():  QCamera::NoError
before start camera->errorString(): ""
after start camera->status():  QCamera::StartingStatus
after start camera->state():  QCamera::ActiveState
after start camera->error():  QCamera::NoError
after start camera->errorString(): "" 

I have Qt 5.9.5 and Qt creator 4.5.2

[SURYA P]

Let you check with like this,
ui->setupUi(this);
mCamera=new QCamera(this);
mCameraViewfinder=new QCameraViewfinder(this);
mCameraImageCapture=new QCameraImageCapture(mCamera,this);
mLayout=new QVBoxLayout;
mOpenMenu=new QMenu("OPEN",this);
mEncoderAction=new QAction("ENCODER",this);
mAction=new QAction("Capture",this);
mCaptureAction=new QAction("Save As",this);
mOpenMenu->addActions({mEncoderAction,mAction,mCaptureAction});
mCamera->setViewfinder(mCameraViewfinder);
mLayout->setMargin(0);
ui->pushButton->setMenu(mOpenMenu);
mLayout->addWidget(mCameraViewfinder);
ui->scrollArea->setLayout(mLayout);

connect(mEncoderAction,&QAction::triggered,[&](){
    mCamera->start();
});
connect(mAction,&QAction::triggered,[&](){
    mCamera->stop();
});
connect(mCaptureAction,&QAction::triggered,[&](){
   auto filename= QFileDialog::getSaveFileName(this,"Capture","/","Image(*.jpg,*.jpeg)");
   if(filename.isEmpty()){
       return;
   }
    mCameraImageCapture->setCaptureDestination(QCameraImageCapture::CaptureToFile);
    QImageEncoderSettings imageEncodersettings;
    imageEncodersettings.setCodec("image/jpeg");
    imageEncodersettings.setResolution(1600,1200);
    mCameraImageCapture->setEncodingSettings(imageEncodersettings);
    mCamera->setCaptureMode(QCamera::CaptureStillImage);
    mCamera->start();
    mCamera->searchAndLock();
    mCameraImageCapture->capture(filename);
    mCamera->unlock();
});

[mit_cruze]

@SURYA-P Hi, I have tried your code. I have to press "Capture" and then "ENCODER" to start camera stream. Only pressing ENDCODER camera does not start streaming.

[SURYA P]

Hi i think you want import on your .pro file this
QT += core gui multimedia multimediawidgets