How to fix a child entity's transform fixed while parent entity is scaling/rotating

[onyx palm]

I have a parent entity that will move around and rotate or scale based on external forces. I want to fixate its child entity to a relative translation to the parent's translation, but without it's scale or rotation.

I'm thinking about updating the child entity's transform based on the parent's global transform inverse. But I found I suck badly at the maths and can't figure out the correct operation to do. Also I don't know what schedule to run the update system to make sure everything is aligned properly.

[upbeat lotus]

I would consider whether they really need to be parent/child. What if you just figure out the relative position you need (by using the "parent's" GlobalTransform::transform_point, or maybe the inverse of that, not sure), then just set that child's position to that?

Note you would probably need to schedule it in PostUpdate before the TransformSystem::TransformPropagate and use TransformHelper instead of GlobalTransform directly

If you're dead set on having the child actually be a child, I think you should just be able to set the child to have its rotation be the inverse of the parent's GlobalTransform rotation and same for the scale

But that would still require you to use TransformHelper, so it stays in sync

[onyx palm]

Thank you @upbeat lotus ! I ended up not spawning the entity as child because it's much simpler to deal with transform configuration. I have to manually despawn the entity when the other entity despawn, which isn't too bad.