Question

How can I detect mouse passive motion with opengl ? In other words, how can I understand whether it is moving forward, backward, left, right ?

I have done

glutPassiveMotionFunc ( func ) 

void func ( int x, int y ) {

   // x and y always positive, I wait it should be negative if it goes left 
   //   acc. to  my coordinate system determined in glLookAt.
}

Answer 1

From the docs:

The x and y callback parameters indicate the mouse location in window relative coordinates.

If you’re interested in how the cursor moved between two frames (a delta), store the cursor position every frame and calculate the difference between the “current” position and the “last seen” position.