SSRPass: Ensure metalness pass is not affected by fog and background.

[Mugen87]

Related issue: -

Description

The metalness pass of SSRPass should save the metalness of the scene's objects so it's possible to decide what objects are affected by SSR. However, the pass is not correctly configured since the scene's background and fog currently affects the metalness data which is wrong.

[Mugen87]

@sunag This is also an issue with WebGPURenderer. When I configure a background color and use this MRT setup:

scene.background = new THREE.Color( 0x443333 );
scene.fog = new THREE.Fog( 0x443333, 1, 4 );

const scenePass = pass( scene, camera );
scenePass.setMRT( mrt( {
	output: output,
	normal: transformedNormalView,
	metalness: metalness
} ) );

The background of the normal and metalness output is affected as well which is not correct.

The following screenshot shows the metalness output of the SSR example (without ground).

The background should be black though.

I guess we need the ability in PassNode to define different clear colors per MRT target. The scene's background should only apply for the beauty pass by default, imo.

[Mugen87]

I'm currently develop SSRNode in https://github.com/Mugen87/three.js/commits/dev3/.

There is already webgpu_postprocessing_ssr for testing. Just replace the WIP SSRNode instance in postProcessing.outputNode = ssrPass; with the output node you want to debug.