Option to disable two-pass rendering of transparent objects

[dmikis]

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

There's a hardcoded path for rendering transparent objects with doublesided materials. However it's not particularly suitable for our use-case since our models are optimized for a single blended pass with backface fulling disabled. That leads to artifacts.

Describe the solution you'd like

An option for materials to override this behaviour.

Describe alternatives you've considered

Right now we're forced to patch by hand and commit into our repo a copy of threejs build.

[Mugen87]

Related #21967.

[Mugen87]

Since this feature request came up multiple times in the past, I'm inclined to support this request.

Finding a nice and compact property name might be difficult though since only double-sided, transparent materials are affected.

[dmikis]

twoPassTransparentRendering? separateFrontAndBackSide? Both defaulting to true to preserve compatibility. I think a descriptive name would be better that a short one TBH :)

[Mugen87]

twoPassTransparentRendering sounds good to me. Would you be interested in making a PR?

[dmikis]

Will do.

[WestLangley]

Adding this feature is up to @mrdoob.

But if it is added, I think it should be a property of the object, as is .renderOrder and .frustumCulled.

Double-sided, transparent, planar objects are a use case.

[dmikis]

@WestLangley TBH I don't have any preference in the matter, I just want the option to exist and ready to submit a patch with any agreed upon solution (well, assuming, it won't take me a week to implement it :) ).

[Mugen87]

I'm okay with making this a property of Object3D. But let's leave #24719 open until @mrdoob tells his preference.

[dwoodruffsf]

I'd like to vote for this too. We are using the threejs runtime for the spine animation library, which exposes this problem. Spine uses 2-sided, transparent triangle batches, and it relies on those triangle batches being rendered in exactly the order they are submitted. But the winding-order of triangles in the list is heterogeneous (because it's driven by animations which can do funny things like scaling by -1). This creates artifacts where some parts of a character sort out of order.

[Mugen87]

@mrdoob Given that the two-pass rendering introduces performance issues for certain users and an issue in context of gl_FrontFacing, I think we should make it optional. Are you okay with @WestLangley's suggestion in #24711 (comment) and introduce a new property for Object3D?

[mrdoob]

How about adding a new material.side constant named THREE.TwoPassDoubleSide?

[Mugen87]

Sounds good to me, too! I'll give it a try.