I show image firstly as scaled after i read it. My codes:

/*HEADER*/
    QGraphicsScene *scene = new QGraphicsScene;
    QImage qimg;
    QImage scaledQimg;
    QPixmap pixmap;
    cv::Mat img;
    cv::Mat imgProcessed;
    double brightness = 0;
    double contrast = 1;
    bool isImageScaled;
    QGraphicsPixmapItem *pix;

/*SOURCE*/
//When image readed from device, signal emitted and that slot runs
void MainWidget::ImageSlot(cv::Mat *image)
{
    img = *image;
    isImageScaled = true;

    DrawImage();
}

/*Other slots*/
void MainWidget::sliderBrightness_valueChanged(int value)
{
    brightness = value;
    DrawImage();
}

void MainWidget::sliderContrast_valueChanged(int value)
{
    contrast = value;
    DrawImage();
}

void MainWidget::buttonFIT_clicked()
{
    isImageScaled = true;
    DrawImage();
}

void MainWidget::button11_clicked()
{
    isImageScaled = false;
    DrawImage();
}

void MainWidget::DrawImage()
{
    scene->clear();

    if(!img.empty()){
        img.convertTo(imgProcessed, -1, contrast, brightness);

        qimg = QImage(imgProcessed.data, imgProcessed.cols, imgProcessed.rows, imgProcessed.step, QImage::Format_Grayscale8);
        scaledQimg = qimg.scaled(ui->graphicsView->width() - 10, ui->graphicsView->height() - 10, Qt::KeepAspectRatio);

        if(isImageScaled)
            pixmap = QPixmap::fromImage(scaledQimg);
        else
            pixmap = QPixmap::fromImage(qimg);

        pix = scene->addPixmap(pixmap);
        ui->graphicsView->setScene(scene);
    }
}

Now I have multiple problems.
1-When i read another image from device, the old image in graphicsview still stays there and new image overwrited to this, so sometimes i can see old image's part at left side of graphicsview. I added scene->clear(); to fix issue but nothing changed. How to solve this issue?
2-1:1 button show image at original size and activate scrollbars, but after i click FIT, scrollbars are still there. But I dont want any scrollbars in FIT mode because image is already scaled to available ui in this mode. How to do this?
3- Dealing with multiple qimage and isImageScaled variable are making program compilacted, are there any better approach for it? So it automatically activate scrollbars etc.
BONUS: Any advice for creating zoom in/out functionality to graphicsview? On touchscren with 2 fingers, on pc with mouse whell or maybe with buttons.

I hope someone will read and give some solutions, thanks in advance.