@muzammil360 in terms of the location of pixel index (0, 0) for the fisheye correction glancing at the Pix4d document again it looks to me like it's at the lower left corner.
Take a look at section 2.1.1 and take a look at figure 2 and the following comment:
"the origin of the image coordinate system is at the lower left corner of the image."
Although the image in section 2.2 showing an example for the Sigma lens is confusing since the origin appears to be the top left given the x y axes labels.
Also I don't follow the use of the minus sign in equation (3).
The following diagram I think makes things much clearer. In this diagram the origin is the top left corner and the Yc axis is positive downwards.
So to match the comment about the origin being the lower left corner you just need to flip the direction of Yc and remove the minus sign from equation (3).
In terms of the origin for the vignetting case looking at the application note again it's not stated at all where the origin is so Parrot need to clear that up.
Parrot and Pix4d haven't really been clear enough.