Implement gpu based depth sorting

[CoolDotty]

Currently blocked by this.

At least without rebuilding the engine or porting everything back to 3D (and being able to access it via spatial shader).

[CoolDotty]

After talking with one of the contributors, this wont be happening any time soon in the 2D editor. Even if gl_FragDepth was enabled, depth sorting is disabled as a whole and certain optimizations in the 2D editor don't even guarantee it would work.

Best bet is to use the 3D editor, or stick with painter's algo sorting on the cpu.

[CoolDotty]

I've managed to create a 3D POC that renders sprites accurately and a spatial shader that does depth sorting per pixel. Will share once it's done.

[adamscoble]

Hey! Came across this from the linked Godot issue, did you end up finding a solution? I'd love a look-see if you're so inclined!

[CoolDotty]

Hi @adamscoble!

I messed around a bit with putting a bunch meshes on the X and Y axis (z is zero) and putting an orthogonal camera in front of it. I made two shaders. Both shaders assign a depth value to each fragment based on the Y position in "screen space". One that assigns the same depth based on the node position (sprite / billboard) and another one where the depth value is based on the pixel position (the spritestack one). It looks something like this

Here's a very confusing gif showing it working lol

I'm undecided how much more I want to work on the code for my use. Getting pixel perfect snapping in 3D is a PITA but I think I'm gonna try. Anyway, here's the code for what I've made so far.

[adamscoble]

This is great, thanks very much @KarlTheCool! I'm hoping to combine this with pre-rendered depth-textures, though my inexperience with shaders means I don't even know if that's possible and it might be all for nought, haha.

[CoolDotty]

This issue is effectively fixed by this. Not sure when I'll package it into the repo but linking it here for visibility.