MaterialXLoader: Unlit Material

[sunag]

Related issue: Closes #27494, #27496

Description

Initial support for NPR (non-photorealistic rendering) and viewDirectionWorld.

I was unable to reproduce different types of spaces in the official MaterialX Viewer for viewdirection, that's why the viewDirectionWorld is fixed for now.

[WestLangley]

@sunag

In the shaders, we use this pattern consistently:

vec4 mvPosition = vec4( transformed, 1.0 );

vViewPosition = - mvPosition.xyz

vec3 viewDir = normalize( vViewPosition );

So the view direction is a unit vector from the vertex toward the camera.

Just making sure you are aware. :-)

[sunag]

I added this pattern through positionViewDirection and it is widely used in lighting models.

three.js/examples/jsm/nodes/accessors/PositionNode.js

Lines 54 to 58 in b8b82d9

	} else if ( scope === PositionNode.VIEW_DIRECTION ) {
	const vertexPositionNode = positionView.negate();
	outputNode = normalize( varying( vertexPositionNode ) );

In some tests that I did MaterialX seems uses viewdirection in world space for this node as default instead of view space, and apparently will have local/model space to select.

[WestLangley]

Sorry, I think you may have misunderstood my comment...

The comment is only about ensuring the sign of "view direction" is correct, and consistent with historical convention, and how three.js defines it.

In the case of local vs. world space, the two definitions should be consistent with each other.

[sunag]

Thanks for the feedback, is an important point, I'll revert this part for now and check this in other PR.