Type hints for TransformManager

[AlexanderFabisch]

@erikmannerfelt I checked how you use the TransformManager in xdem. It seems like you are using integers as frame names. pytransform3d 1.9 will have type hints and my idea was to require frame names to be strings. Would that be a problem? Would it be better for you if it could be str or int?

[erikmannerfelt]

Thanks @AlexanderFabisch for the consideration! For that particular case, I can just use string representations of the numbers, which is completely fine. I do however kind of like the integer indexing version, but it's nothing that can't be circumvented!

Alternatively, do you have a recommendation on how to make this more nicely?

[AlexanderFabisch]

I come more from a robotics background and we often have similar cases (forward kinematics), but we typically have names for all the frames (links) in between.

[erikmannerfelt]

The use case there is for merging the transforms of a coregistration pipeline. Since there may be an arbitrary amount of steps in the pipeline, I opted for a single integer index. An example is:

1. A pipeline of elevation bias correction, followed by iterative closest point registration (two steps)
2. Fit the bias on the initial DEM (creates a vertical transform: A-B)
3. FIt the ICP transform on the bias-corrected DEM (creates a rigid 3D transform: B-C)
4. Export or apply the A-C transform.

Since there is no theoretical limit to the amount of steps involved, integer indices sounded good. But it could just as well be anything else unique (like the string representation of an integer). The only thing we're interested in there is the A-X transform, where X is the last step in the pipeline.

[AlexanderFabisch]

OK, I think it is best to allow Union[str, int] in this case to avoid unnecessary conversion between str and int. I will change that.

[AlexanderFabisch]

FYI, I will allow any hashable to be used as a frame name: #137

[erikmannerfelt]

Sounds great! Thanks for your work!