Render a 3D pass on top of another

[HackerFoo]

What problem does this solve or what need does it fill?

When building a 3D UI, it's desirable to always render object in the UI on top regardless of depth.

What solution would you like?

Add a renders_on_top flag to Camera that causes the depth texture to clear at the start of a 3D render pass.

What alternative(s) have you considered?

I considered creating a depth-only clear pass, which could be done without modifying Bevy, but this would be an extra pass and a lot of code in the application.

Additional context

[alice-i-cecile]

This ties into some of my thoughts on "UI is a collection of properties". This is a solid idea, and attaching it to the camera is a reasonable strategy.

How do we determine which entities are rendered by each camera?

[HackerFoo]

The RenderLayers component is a 32-bit mask such that each object is visible to a camera if the bitwise-and of the camera and object are nonzero.

[HackerFoo]

A higher-level API would be to assign a layering order to each camera, and each unique layer would require a pass after clearing the depth buffer. I'd wait on this, though, because it's somewhat complicated and might not be useful.

[HackerFoo]

Another reason to render on top of another pass is to implement a skybox, since the skybox should be behind everything else.

[aevyrie]

Some relevant discussion from cart here: https://discord.com/channels/691052431525675048/931614194229477417/931974884534415380

[superdump]

@HackerFoo I assume this can be closed now that camera-driven rendering was merged?