Move the camera along a different orbit with ModelViewer

[pechiang66]

Hi.

I am using the ModelViewer to show a model and take advantage of the gesture detection so that the camera can move around the model.

Below is a sample model. The pivot is around the center of the model. In other words, the camera would move on the surface of a sphere where the origin of the sphere is the pivot:

The Manipulator for the ModelViewer is instantiated as below:

        val manipulator = Manipulator.Builder()
            .mapMinDistance(-10f)
            .targetPosition(0.0f, 0.0f, -4.0f)
            .orbitHomePosition(0.00f, 2.0f, 0.5f)
            .viewport(surfaceView.width, surfaceView.height)
            .zoomSpeed(0.03f)
            .orbitSpeed(0.004f, 0.004f)
            .build(Manipulator.Mode.ORBIT)

What I am trying to do is embedding a point (called a hotspot) within the model and when the user clicks on that point, the camera would move on an orbit to a position and look at the hotspot. Please see the video below:

Screen_recording_20240410_140048.mp4

I am doing this by:

1. Create a custome model viewer based on ModelViewer
2. Read the hotspot coordinates and convert that to the corresponding polar and azimuthal angles where the pivot is (0.0f, 0.0f, -4.0f), i.e. the targetPosition specified when the manipulator was created. The angles are calculated by doing the inverse of what is done here
3. Calculate the same angles for the current camera position.
4. Interpolate the angles between what we got from steps 2 and 3 and then call cameraManipulator.grabBegin and some calculated number of cameraManipulator.grabUpdate. I am doing this because the ModelViewer reads the paramters for the camera.lookAt calls from the cameraManipulator. Thus, even if different values are passed to camera.lookAt outside the ModelViewer, camera would be re-positioned according to what cameraManipulator.getLookAt returns after every frame. Essentially, we are simulating a series of gesture movements.
5. Finally, the camera would be positioned in such a way that the pivot, the hot spot, and the camera itself are on a straight line.

Question:
This looks ok so far. Because hotspot1 is above the pivot in the y-axis, the camera would be looking down the hot spot at an angle. Now, instead of this angle, we want the camera to move along a different orbit so that the end state would look something like this:

It looks like to achieve this, we will need to somehow move the pivot to somewhere else. I know the panning motion can achieve this, but unlike rotation or zooming, it's a little difficult to simulate and animate the panning motion. Is there a way, we can achieve this without touching the pivot?

Thanks!