Add `ZIndex`, `YSort`, and `SortBias` 2d components

[tychedelia]

Objective

Using Z transform for sprites and 2d meshes is gross! Users expect familiar sorting strategies.

Solution

Adds 3 new sorting components for 2d:

• ZIndex: sorts entities into layers and always takes priority.
• YSort: sorts entities in each ZIndex layer by their Y transform.
• SortBias: an arbitrary bias that is either added to Y transform or used as a secondary sort value.

TODO:

• What should we do with the name conflict with the existing ui ZIndex.
• How should 2d mesh opaque behave? Right now, using any of these components opts you into transparent sorting on the CPU, but ideally we'd be able to do GPU based fragment sorting as well. This is an advantage that using the transform Z has that we should take into consideration.

Testing

Tested a bunch of examples, might be more broken.

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[tychedelia]

What about propagation? It feels like this needs some sort of local ordering support so children are drawn above their ancestors and inherit their parent's z-ordering.

We just merged https://github.com/bevyengine/bevy/pull/17575 which could help. I'm the wrong person to ask about 2d design patterns, but it's not clear to me that children should always be drawn above their ancestors, although I realize this is a pattern that using z position plus transform propagation makes trivial and we are breaking. Would appreciate input here from others.

[NthTensor]

it's not clear to me that children should always be drawn above their ancestors

I don't think we can assume this. We need to make it possible for children to be drawn in front of behind their ancestors, depending on artist intent.

[alice-i-cecile]

I don't think we can assume it, but I do think it's a good default :)

[tychedelia]

I don't think we can assume it, but I do think it's a good default :)

I guess the question is, what does this look like? Do we have a GlobalZIndex component? Or do we just have some kind of mechanism that silently updates ZIndex += 1 behind the scenes? What if you don't want this behavior?

[hymm]

It feels to me like y-sorting is more a property of an individual layer rather than individual sprites. Wouldn't you want all sprites in the same layer to be y sorted?

Does the y sort take into account the anchor point at all?

[tychedelia]

Wouldn't you want all sprites in the same layer to be y sorted?

Probably? But again I think this is more a function of questions around patterns for inheritance/propagation. At the end of the day, they will be sorted one by one and so I think it still makes sense for the components to apply to individual entities if you want. I wouldn't want to take away this fine grained control just add other tools for making common use cases easier.

[tomara-x]

mechanism that silently updates ZIndex += 1 behind the scenes? What if you don't want this behavior?

bevy_lunex does this. by default every child is one z step above its parent. and there's an override component (with set and add variants). i like that those can also be propagated now :)

i wonder if different variants like that can be useful here

[alice-i-cecile]

I fundamentally like this design, but there's too many loose ends and moving parts for me to be comfortable merging this, even as an iterative step. We need a real design doc for our transform overhaul which tackles these details and lays out a sequence of PRs to allow us to migrate.