C++ – Setting a plain background color for a QGraphicsView widget

The best and recommended way is to use Qt Style Sheet.

To change the text color and background color of a QLabel, here is what I would do :

QLabel* pLabel = new QLabel;
pLabel->setStyleSheet("QLabel { background-color : red; color : blue; }");

You could also avoid using Qt Style Sheets and change the QPalette colors of your QLabel, but you might get different results on different platforms and/or styles.

As Qt documentation states :

Using a QPalette isn't guaranteed to work for all styles, because style authors are restricted by the different platforms' guidelines and by the native theme engine.

But you could do something like this :

 QPalette palette = ui->pLabel->palette();
 palette.setColor(ui->pLabel->backgroundRole(), Qt::yellow);
 palette.setColor(ui->pLabel->foregroundRole(), Qt::yellow);
 ui->pLabel->setPalette(palette);

But as I said, I strongly suggest not to use the palette and go for Qt Style Sheet.

An alternative approach would be to extract features (keypoints) using the scale-invariant feature transform (SIFT) or Speeded Up Robust Features (SURF).

You can find a nice OpenCV code example in Java, C++, and Python on this page: Features2D + Homography to find a known object

Both algorithms are invariant to scaling and rotation. Since they work with features, you can also handle occlusion (as long as enough keypoints are visible).

Image source: tutorial example

The processing takes a few hundred ms for SIFT, SURF is bit faster, but it not suitable for real-time applications. ORB uses FAST which is weaker regarding rotation invariance.

The original papers

• SURF: Speeded Up Robust Features
• Distinctive Image Features from Scale-Invariant Keypoints
• ORB: an efficient alternative to SIFT or SURF