How to decode surface normal colormap to know normal?

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

Hello,

For a mesh model, I can get the colormap of surface normal by setting the mesh rendering options. But the surface color keeps changing when toggling the view. I know probably the coordinates of surface normal is with respect to current view.

So the question are,
(a) are we able to decode the colormap to obtain surface normal or view vector? for example, how to convert the rgb value so we can know the view angle?
(b) or how to make the colormap consistent when toggling the view? It means the surface normals should be under scene coordinates.

Hope my question make sense. Thanks for your reply.

Best,
Houhao