All of your image manipulation is being done with OpenCV, which has nothing to do with Qt. If you want OpenCV to draw a rotated rectangle on top of the image you load from a file then you need to refer to the OpenCV documentation as to how you do that.

If you want to rotate the entire image after you place it into the QGraphicsScene then you use QGraphicsItem::setRotation() on the QGraphicsPixmapItem you create as vidar suggested.

All of this can be done without OpenCV.

/////////////////////////////////////////////////

void MainWindow::on_btnSetROI_clicked()
{
CvRect rect,rectb,rectn;

cv::Mat img;
QGraphicsScene* scene = new QGraphicsScene();
scene->clear(); //for cleaning the qgraphicsview and loading the image again

img = imread(filename.toAscii().data());

//Drawing ROIs

rect.x=ui->txtx0->toPlainText().toInt();

rect.y=ui->txty0->toPlainText().toInt();

rect.height=ui->txtheight->toPlainText().toInt();

rect.width=ui->txtwidth->toPlainText().toInt();

cv::rectangle(img,rect,COLOR_RGB2RGBA,1,8,0);

//ROI white

rectb.x=ui->txtxb1->toPlainText().toInt();

rectb.y=ui->txtyb1->toPlainText().toInt();

rectb.height=ui->txtheightwhite->toPlainText().toInt();

rectb.width=ui->txtwidthWhite->toPlainText().toInt();

cv::rectangle(img,rectb,COLOR_RGB2RGBA,1,8,0);

//ROI black

rectn.x=ui->txtxn1->toPlainText().toInt();

rectn.y=ui->txtyn1->toPlainText().toInt();

rectn.height=ui->txtheightBlack->toPlainText().toInt();

rectn.width=ui->txtwidthBlack->toPlainText().toInt();

cv::rectangle(img,rectn,COLOR_RGB2RGBA,1,8,0);
/////////////////////////////////
QImage img1 = QImage((const unsigned char*)(img.data), img.cols, img.rows, img.step, QImage::Format_RGB888);
QGraphicsPixmapItem* item = new QGraphicsPixmapItem(QPixmap::fromImage(img1));

QGraphicsItem* i=new QGraphicsItem() ;
ui->View->setScene(scene); //"View" is the name of object that I have on my .ui file(form)
--}