ShaderChunks: Ignore alpha in output_fragment when transparent = false

[mrdoob]

Related issue: #15483 #18631 #21744 #22196

Description

When rendering with alpha: true opaque materials with textures with alpha channel were "carving out" pixels from the output render.

Before	After

/cc @elalish

[mrdoob]

Crazy... Totally forgot I tried doing this last year... #18631 (What a year...)

The implementation is very similar and I suspect this comment by @WestLangley is relevant here too:

Confirmed. This breaks AdditiveBlending when texture.a < 1 and material.transparent is false.

And this other comment:

I don't think this can be retained. I think you will have to implement something different -- something specific to the glTF alphaMode setting.

So maybe the solution is to introduce OpaqueBlending?

/cc @donmccurdy

[elalish]

It'll be nice to remove my hack for fixing this with glTF.

[donmccurdy]

So maybe the solution is to introduce OpaqueBlending?

I'm not sure I understand the suggestion, when would this be used?

I have trouble wrapping my head around this because .transparent=true flag has compound effects... I wonder if we could make those effects separately configurable, and decouple this? Something like this, where .blend is a new boolean property:

class Material {
  set transparent ( transparent ) {
    this.blend = transparent;
    this.depthWrite = ! transparent;
    this.renderPass = transparent ? TransparentPass : OpaquePass;
  }
  get transparent () {
    return this.blend && ! this.depthWrite && this.renderPass === TransparentPass;
  }
  ...
}

Then to enable .blending=AdditiveBlending + .transparent=false the user would set .blend = true manually.

I've also suggested a more direct copy of glTF's .alphaBlend = OPAQUE | BLEND | MASK | ... property in some other thread, which has certain advantages over a boolean .blend = true | false (e.g. forward-compatible with alpha hash?) but it isn't necessary here.

[mrdoob]

Hmm, let me try the OpaqueBlending approach... 🧐

Edit: Maybe we can just use NoBlending for this...