LineMaterial: Write line segments with capsule depth

[gkjohnson]

Related issue: #27349, #26916

Description

A quick try at enabling the Line2 to write the line segments depth buffer as capsule shapes so they more sensibly intersect with other scene objects. Unfortunately I'm not sure how to make this work cleanly with alpha-to-coverage to make sure the edges have clean AA without calling the previous intersect function along with the new depth function which would have performance implications. We could add a separate toggle, though, but I don't love that. I'll leave this here for now in case someone else has some ideas.

Demo here. You can toggle the new depth write with the "capsule depth" option.

Before	After

[mrdoob]

Looks great! The problem is that we lose antialiasing?

[mrdoob]

/ping @WestLangley

[gkjohnson]

The problem is that we lose antialiasing?

The problem is that when using the function to determine capsule intersection it doesn't give the right information needed to determine alpha-to-coverage anti aliasing. The solution that came to mind was to run both capsule intersection functions but that felt messy / slow.

It needs a bit more work but it's possible there's another way to determine the AA value - I just need to think about it more. Here is what I was thinking for a more fully featured version of this if other are interested in picking it up:

• AA + capsule depth write for world-space lines
• AA + capsule depth write for screen space lines
• Ensure capsule depth write is provided as an optional field on the material
• Update raycasting to report the capsule surface as the intersection point