Meshlab's code implementation for trackball orientation operation of the object

[WillyTuring]

Hi you guys,
I am trying to implement the trackball orientation for the object in Qt5 similar to how we could drag the left-button of the mouse and having the object rotate with different orientation while keep the center of the "spherical ball" stand-still not changing at all.

My current problem is that with my implementation, the object will appear rotate away from or toward to us while we drag the left-button of the mouse around, meaning that the center of the "trackball" is actually changing. I know the description is rather vague, but i am hoping that some people could give me some guidance on the procedures/thinkings behind it or pointing me to some resources to study. Thanks in advance for any kind help.

Edit: My current situation is like when we rotate the "trackball", the ball behaves as if it is like the earth rotates around the sun rather than behaving as self-rotates which is what we want.