MToon outline doesn't scale when the scale of the mesh changes

[ButzYung]

When the scale of VRM mesh changes, the MToon material outline doesn't scale accordingly, resulting in for example the outline being barely visible when the mesh is scaled up by a large degree. A workaround on user app's side is to scale the outlineWidthFactor property manually.

[ButzYung]

Upon further invertigation, it seems the problem is specific to MToon material with outlineWidthFactor set to anything other than screenCoordinates.

[0b5vr]

I'm unsure if it's an intended behavior on the VRM spec level. Let me check.

[0b5vr]

@ButzYung

Unfortunately, it's an undefined behavior on the spec level 😩

https://github.com/vrm-c/vrm-specification/tree/master/specification/VRMC_materials_mtoon-1.0#outline

But, as the value name worldCoordinates implies, most of the implementations consider that it's a width in the world coordinate for now, which means it does not change the width along with the scale of the mesh.

You might need an app-side implementation to change the outline width for each material if you want to resize models dynamically.