Anisotropy is the whole argument

Ask someone to describe silk and they will talk about the way the highlight moves. Not its colour, not its softness — the fact that when you turn it, the shine travels along the cloth rather than staying put.

That behaviour has a name and a formula. An isotropic material scatters around the surface normal, so the highlight is a spot that follows the light. An anisotropic one scatters around a tangent — the direction of the weave — so the highlight is a streak that runs perpendicular to the threads and slides as the fabric turns.

Substitute one for the other and no amount of colour correction recovers the difference. This is the single line that separates satin from vinyl in a render, and it is why the fabric work on Valkyrie began with the tangent frame rather than with the palette.