So, I am reading a paper about [ORB: An Efficient Alternative to SIFT or SURF](http://www.willowgarage.com/sites/default/files/orb_final.pdf) and it happens to calculate the direction of corner from it center . According to Rosin the author of the paper;moments of a patch is given by 

![moment](https://i.stack.imgur.com/Kz9Ny.jpg)

and the centroid is of the patch from the center is calculated using:

![centroid](https://i.stack.imgur.com/15dtR.jpg)

The direction of the patch is computed from

![enter image description here](https://i.stack.imgur.com/pU1cy.jpg)

My question is what are these value of `p` and `q` and why is it out there?