So I could blend indentical substrates+layer with different roughness for their respective layers. What is the broad term for substrate/dielectric+layer ?
Another way would be to offer dual mapping slots; overall map and high frequency map which always blend at 50% as a nod to simplicity.
I expect there is a compromise here between rendering speed, and ease of use. Maybe an idea is to offer a fuzziness value to interpret any given roughness across a broader range.