3D (iOS SceneKit), switch transforms of camera and another object

Question

I have a scene with a camera and one other object: a 3D character model.

Using orientation information from a pose estimation algorithm (a vec3 for translation, and a vec4 quaternion for rotation) my camera's transform is being translated and rotated to focus on this character, which is at (0,0,0) and with 0 rotation . This works correctly and precisely, but I want to 'reverse' these values so that I'm influencing the character object with the input, having the camera placed at 0,0,0 with 0 rotation instead.

I feel like I should be rotating the supplied camera transformation matrix about the origin to create myObjectTransform , then zeroing the camera values and simply applying myObjectTransform as the transform of the character object. If I do this, though, my input translation and rotation values don't influence the scene as I'd like - rotation seems to go in the wrong directions, and even the translation position seems off.

This sounds like it should be easy, but I'm obviously missing something. Can anyone help?

Answer 1

I solved this using SCNMatrix4Invert - the inverse of the camera's matrix is exactly what I was looking for. I forgot, though, that I was applying an extra X rotation of 180 degrees upon the initial given translation and rotation.

So my process, albeit a little untidy-looking, is:

1. Create an SCNMatrix4, and set its position and rotation from the pose estimation input
2. Rotate the matrix in the X axis by 180 degrees (my custom step)
3. Invert the matrix
4. I also arbitrarily scale my character object up, so I take a note of its scale vector.
5. Apply the matrix as the 'transform' property of my character object's SCNNode
6. Set the scale of the SCNNode obtained in #4. There are many ways I could do this, like applying the same scale to the SCNMatrix4 - that would probably better practice.