使用QPainter显示实时摄像头

Question

I am just curious if we can use QPainter to display the live camera output. If so can anyone tell me how will be the implementation. I already used it with QLabel like this:

cM2 = new QCamera(this);
cV2 = new QCameraViewfinder(this);
cM2->setViewfinder(cV2);
cBox2 = new QVBoxLayout();
cBox2->addWidget(cV2);
ui->label_2->setLayout(cBox2);
cM2->start();

Answer 1

I'm not sure how you propose to use the QPainter so I can't really comment on that.

However, one option might be to repackage the QCameraViewfinder widget up in a QGraphicsView and corresponding QGraphicsScene and rotate the item within the scene.

QGraphicsView view;
QGraphicsScene scene;

/*
 * Add the view finder widget to the scene and keep hold of the
 * proxy returned.
 */
auto *proxy = scene.addWidget(my_camera_view_finder);

/*
 * Use the proxy to rotate the view finder by 90 degrees.
 */
proxy->setRotation(90.0);
view.setScene(&scene);
view.show();

Note that there will probably be performance implications due to painting via the proxy. This method also assumes that the QCameraViewfinder uses Qt to perform its painting logic rather than something more direct such as OpenGL -- otherwise it won't work as expected.

The above logic could probably be packaged up as something fairly self-contained such as...

class rotated_widget: public QGraphicsView {
  using super = QGraphicsView;
public:
  explicit rotated_widget (QWidget *parent = nullptr)
    : super(parent)
    , m_widget(nullptr)
    , m_proxy(nullptr)
    , m_degrees(0.0)
    {
      setScene(new QGraphicsScene);
    }
  virtual void set_widget (QWidget *widget)
    {
      scene()->clear();
      m_proxy = nullptr;
      if ((m_widget = widget)) {
        m_proxy = scene()->addWidget(m_widget);
        m_proxy->setRotation(m_degrees);
        fixup();
      }
    }
  virtual void set_angle (double degrees)
    {
      m_degrees = degrees;
      if (m_proxy) {
        m_proxy->setRotation(m_degrees);
        fixup();
      }
    }
  virtual QSize sizeHint () const override
    {
      return(sceneRect().size().toSize());
    }
private:
  void fixup ()
    {
      setSceneRect(scene()->itemsBoundingRect());
    }
  QWidget              *m_widget;
  QGraphicsProxyWidget *m_proxy;
  double                m_degrees;
};

Then use as...

cV2 = new QCameraViewfinder;
auto *rotated = new rotated_widget;
rotated->set_widget(cV2);
rotated->set_angle(90.0);