ShaderChunks: Ignore alpha in output_fragment when using NoBlending

[mrdoob]

Related issue: #15483 #18631 #21744 #22196 #22414

Description

This makes NoBlending behave as expected. Before this PR NoBlending was writing alpha pixels when the framebuffer had an alpha channel.

We can not use NoBlending to support GLTF's OPAQUE and MASK alpha modes which was previously broken when setting alpha: true in WebGLRenderer.

Here is an example with alpha: true and renderer.domElement.style.backgroundColor = 'red':

Before	After

Note: I'm following a similar implementation to Khronos' GLTF Viewer:

• ALPHAMODE_OPAQUE
• ALPHAMODE_MASK

/cc @elalish @donmccurdy

[WestLangley]

Why are you overwriting diffuseColor.a to 1 at the end, rather than at the beginning?

OPAQUE - The rendered output is fully opaque and any alpha value is ignored.

That means it must be ignored from the start of the shader, no?

[mrdoob]

Just in case there is also a alphaMap:
https://github.com/mrdoob/three.js/blob/dev/src/renderers/shaders/ShaderChunk/alphamap_fragment.glsl.js

[WestLangley]

But that is not "ignoring any alpha value" if you are honoring it.

The alphaMode property defines how the alpha value of the main factor and texture should be interpreted.
OPAQUE - The rendered output is fully opaque and any alpha value is ignored.

What you are doing is reasonable, it seems... I'm just not sure if that is what the spec calls for...

[mrdoob]

I've just had a chat with @elalish about this and made me unsure about alphaMap but...

The GLTF spec says this:

https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#alpha-coverage

• OPAQUE - The rendered output is fully opaque and any alpha value is ignored.
• MASK - The rendered output is either fully opaque or fully transparent depending on the alpha value and the specified alpha cutoff value. This mode is used to simulate geometry such as tree leaves or wire fences.

Which makes me think the shader part of this PR is fine. But things get complicated when trying to get transmission working with alpha: #22425

I guess the GLTF spec doesn't say anything about disabling gl blend, so it's not possible in the current three.js API to do that. In the meantime, I'm going to try solving this by creating a OpaqueBlending (we can rename later).