Overlapping tiles in TileMaps with Y Sort enabled under a Y Sort node randomly changes rendering order

[AtomaFajrovulpo]

Godot version:

3.1 stable and 3.2 alpha 3

OS/device including version:

All

Issue description:

Having two or more TileMaps under a Y Sort node with overlapping tiles and when both have Y Sort enabled results in them randomly changing their rendering order, in the editor or whilst playing the game.

Steps to reproduce:

Create a V Sort node, add two TileMaps, both with V Sort enabled, create overlapping tiles.

Minimal reproduction project:

TileMap Y Sort Overlap DEMO.zip

[bojidar-bg]

Discussed this with reduz on IRC:

[13:40:01] <bojidar_bg> reduz: sorry for pinging you, I was wondering if https://github.com/godotengine/godot/issues/33230 needs fixing, or is it too much of a performance hit to have a stable sort order on YSorted nodes with equal Y coordinates
[13:40:54] <reduz> bojidar_bg: yeah i guess that should be fixed, maybe comparing X, then Y?
[13:41:20] <reduz> bojidar_bg: i think its a more memory bound operation than CPU, so comparing X and Y should have no cost imo. You can make a small benchmark to test if you want I guess
[13:41:34] <reduz> bojidar_bg: sorry I ment comparing Y, then X if they are the same
[13:52:05] <bojidar_bg> yeah, that would be a good solution
[13:52:34] <bojidar_bg> reduz: maybe add custom sorting axis at the same time? ^^
[13:53:07] <reduz> bojidar_bg: i think that could workm but then that means you would need to convert the sort function to a template
[13:54:09] <reduz> bojidar_bg: but I would do something like this only if we see there is a significant performance hit to adding X

[Pennycook]

I've just encountered this issue in a project I'm working on, and I'm not sure if the stable sort suggested is sufficient for the behavior that I expected. I've modified the original reproducer (by swapping the order of the layers) to show what I'm talking about more easily: TileMap.Y.Sort.Overlap.Layers.DEMO.zip

With Y Sort disabled, all of the red tiles render above the green tiles, reflecting their order in the scene tree:

With Y Sort enabled, the order is random, as reported by @AtomaFajrovulpo:

Would it make sense to check the index or Z-index instead of the X coordinate when the Y coordinates are equal? I think that would result in more predictable behavior, and would allow for different tile maps to act more like layers.

[Pennycook]

After playing with this some, I realized that my original suggestion to use Z-index as a tiebreaker doesn't make sense. But tracking the order in which YSort nodes collect their children seems to work: https://github.com/Pennycook/godot/tree/3.2-stable-ysort.

@bojidar-bg: Does this look reasonable to you? Should I open a pull request?

[akien-mga]

After playing with this some, I realized that my original suggestion to use Z-index as a tiebreaker doesn't make sense. But tracking the order in which YSort nodes collect their children seems to work: https://github.com/Pennycook/godot/tree/3.2-stable-ysort.

As discussed on IRC with @reduz, this seems to be a good solution indeed. Please open a PR against the master branch :)