Child's Transform component ignored when parent does not have the GlobalTransform/Transform components

[allsey87]

Bevy version

9f94f7e

Operating system & version

Arch Linux

What you did

1. Create an entity using just spawn, i.e., without Transform and GlobalTransform components.
2. Add a child using spawn_bundle and PbrBundle, setting the mesh, material, and transform fields (use ..Default::default() for the other fields)

What you expected to happen

Since the parent entity didn't have either a Transform or a GlobalTransform component, I would expect that the transform component in the child entity would be relative to the reference frame. If this had been the case, the transform_propagate_system should have set the GlobalTransform component according to the Transform component and the mesh should have been rendered at the offset provided in the PbrBundle.

What actually happened

The mesh renders at the origin (i.e., the transform field from the PbrBundle appears to have been ignored).

Additional information

From the query in the transform_propagate_system, it appears that this system does not handle the case where the parent entity doesn't have a GlobalTransform/Transform component, but it's children do. Is this behavior intentional?

[allsey87]

@alice-i-cecile I think this issue is more about bevy's transform and its associated systems. Perhaps ecs and usability are the most suitable tags here.

[alice-i-cecile]

Yeah; this is... kind of at the intersection of a number of concerns, due to how heavily Transforms are used. @NathanSWard what would you classify this as under the proposed new labelling scheme?

[NathanSWard]

what would you classify this as under the proposed new labelling scheme

I wouldn't say ecs since the problem isn't due to ECS, but rather the transform systems that are implemented using ECS.
So
T: bug
S: needs investigation
C: ???

it doesn't really fit nicely into a single category, so does it deserve it's own? I mean it already has it's own crate?

C: Transform

[alice-i-cecile]

Yeah, I think that if we were to use content labels based on crates it would work well.

[djeedai]

As of 0.5 release, my observation is that the children collapse at origin not if the parent is missing Transform or GlobalTransform, but if it's missing Transform and GlobalTransform (both). That is, you can add one of those two only to the parent, and the bug still repro. Only when you add both Transform and GlobalTransform then the children are positioned correctly relative to the parent. I suppose this makes sense given how I imagine things work internally, but this is extremely confusing for new users like me.