Add vertex color alpha channel support to LineMaterial

[jhurliman]

Is your feature request related to a problem? Please describe.

I am using Line2 with vertex colors and my data source provides RGBA values, but I cannot support the alpha channel without creating my own copy of LineMaterial.js.

Describe the solution you'd like

#20975 added support for vertex color alpha, but the LineMaterial class assumes vec3 colors only. I would like to see the LineMaterial class natively support vec4 vertex colors.

Describe alternatives you've considered

I've created my own copy of LineMaterial that uses vec4, but automatically doing the correct thing for vec3 and vec4 colors and upstreaming this change would be preferable.

[WestLangley]

Related: #16570

[Mugen87]

It was stated in #16570 to not support transparency because of the resulting artifacts.

@jhurliman Don't you experience these in your custom version of LineMaterial?

[jhurliman]

@Mugen87 the artifacts are acceptable for my use case (user-generated content).

[WestLangley]

@Mugen87 if @mrdoob and @gkjohnson agree, we could add opacity and alpha-channel support to Fat Lines.

Some users will be happy... others will complain.

[gkjohnson]

With two draws (one depth only, one with stencil operations) it's possible to alleviate the corner artifacts though you sacrifice any line overlap rendering. But it does look cleaner:

https://gkjohnson.github.io/threejs-sandbox/fat-line-opacity/webgl_lines_fat.html

It may look odd if a very transparent line is in front of an opaque line, though. I don't have strong opinions either way. Transparent thick lines seem to be something a lot of users ask for.

[jhurliman]

@gkjohnson thanks for the pointer, we are using this technique in Foxglove Studio (see https://github.com/foxglove/studio/blob/main/packages/studio-base/src/panels/ThreeDeeRender/renderables/markers/RenderableLineList.ts#L39)

[vincerubinetti]

@jhurliman Your link is now broken. I'm guessing that link contains an example where you modified LineMaterial to accept vertex alphas?

I've created my own copy of LineMaterial that uses vec4

On a related note, in my case the overlapping thing isn't an issue because I'm rendering disconnected line segments.