Moving QLabel on QCameraViewfinder will blink

[CYLee]

Dear Qt experts,

I used QT example "camera" to add a QLabel on the QCameraViewfinder.
And I used the Horizontal Slider and setGeometry to change the QLabel position.
The problem is, when the video active and I operate the slider.
The QLabel will blink when it moving.
Can someone tell me how to improve it?
Thanks.

[mrjj]

Hi
You can try subclass QCameraViewfinder and override its paintEvent.
Then in paintevent call base paint
and then draw your text.
That should prevent flickering.
Sadly i dont have any camera so i cant tell if it will work.

[CYLee]

@mrjj thank you for your reply.
I'm new in QT and C++.
So...could you be a bit more specific about implementation?
And I found the same problem when I import a png image on the QCameraViewfinder.
Should I use the same way to improve it?
Thank you.

[mrjj]

@CYLee

Hi
I wish i had a camera so i could test it fast but lets talk about the terms

Subclassing:
Make your own class based on other class

class MyCameraViewfinder : public QCameraViewfinder {
  Q_OBJECT
public:
    explicit MyCameraViewfinder(QWidget* parent = nullptr) : QCameraViewfinder(parent) {     
    }
protected:
  virtual void paintEvent(QPaintEvent* event) override { // override a base function with own
    // Default rendered -> call base class
    QCameraViewfinder::paintEvent(event);
   // draw some text
     QPainter painter(this);
      painter.drawText(100,100,"text");
   };
};

You then use your instead of the default one.
You can new it and place it yourself or use promotion feature
http://doc.qt.io/qt-5/designer-using-custom-widgets.html
which makes it easy to use in Designer / the camera sample.
(it already use promotion that sample)

Anyway, i test with camera sample and i think it will work but
since i cant see with camera on, i cant not know 100%

Changed project with custom widget.
https://www.dropbox.com/s/r25lqvmj3lqimhg/camdraw.zip?dl=0

To move it around, you just make the x,y of the drawtext variable/properties and
then change those.

[CYLee]

@mrjj thank you for your reply again.

How can I change drawtext variable/properties via slider or button?

I declared a variable and tried to change that value via Horizontal Slider.

But it doesn't work.

[mrjj]

@CYLee
you the
100,100 in drawText need to be changed.

Can you show what you tried and "didnt work"?

[CYLee]

@mrjj

I'm sorry that I did not describe it well.

I declared a variable drawText_x and change that value via Horizontal Slider.

mycameraviewfinder.h

class MyCameraViewfinder : public QCameraViewfinder {
    Q_OBJECT
public:
    explicit MyCameraViewfinder(QWidget* parent = nullptr) : QCameraViewfinder(parent) {
    }

    int drawText_x;

protected:
    virtual void paintEvent(QPaintEvent* event) override { // override a base function with own
        // Default rendered -> call base class
        QCameraViewfinder::paintEvent(event);
        // draw some text
        QPainter painter(this);
        painter.drawText(drawText_x, 100, "this is my text");
    }
};

camera.cpp
void Camera::on_horizontalSlider_valueChanged(int value)
{
    MyCameraViewfinder m;
    m.drawText_x = value;
}

As you can see, the string disappeared.

I set a breakpoint to trace drawText_x, the value didn't change.

[mrjj]

Hi
with 100,100 fixed value, is the string there ?
Just checking the idea works at all :)

Regardless
void Camera::on_horizontalSlider_valueChanged(int value)
{
MyCameraViewfinder m; <<< local value, and other copy. not the one on screen
m.drawText_x = value;
} // dead here also

the one in sample is changed to MyCameraViewfinder

so it should be
void Camera::on_horizontalSlider_valueChanged(int value)
{
ui->viewfinder-> drawText_x = value; // drawText_x is public.
}

[CYLee]

@mrjj

Problem solved!

You really helped me a lot.

Thank you for the detailed reply.

[mrjj]

@CYLee
You are welcome. Glad it did work as i hoped and camera feed not drawn
as overlay on widget :)