I am rotating a group of sprites in XNA, which I developed with the help of the following example: http://msdn.microsoft.com/en-us/library/bb194912%28v=xnagamestudio.10%29.aspx
It all works very well, however, the rotations rely on a point of origin, which it will rotate around. My problem is that I do not know how to calculate the point of origin, so that it is exactly in the center of the sprite group.
I am able to set the point of origin to where I want it, but as soon as the sprite group changes, the origin is typically off, and it doesn't rotate from the center of the group.
My main question, is there a way to calculate the center of a group of sprites? My original idea was to calc the boundaries (left, right, top, bottom), and place the origin the the center, but this doesn't work properly.