I'm trying to create aged galvanized metal. Should I put oxide in a metallic map or as roughness? What opacity should i use in metallic map? If I use 100% it no longer looks like made of metal.
Oxidation should act as a roughness map first and then it will begin to act on the metallic map depending on the severity of oxidation. Large chunking of rust and speckling with no longer be metallic for the sake of energy conservation.
As for you second question, I'm not sure what you mean. Are you making a metallic object transparent? Or do you just mean whether or not something is metallic or not?
I've read that metallic map should be black or white - no in-between. But when I do it for metal, pure black (0 metalness) looks weird. Oxides should to be dielectric, but when looking at various downloaded materials they usually use light grey colour.
Galvanising itself is a form of oxidisation so a surface that is completely galvanised will be non metallic before any age related effects are felt.
in practice you might want to cheat a bit - having grey values in your metallic map isn't necessarily wrong - a pixel should take the average value of the area it covers
Yea there is nothing wrong with grays in your metalness map as poopipe points out with average value of a pixel, that is more a standard for the ease of use and is more akin to something said so that artists don't go hay wire with metalness and then you get weird lighting mishaps in levels because some artist decided to over complicate their metalness map.
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As for you second question, I'm not sure what you mean. Are you making a metallic object transparent? Or do you just mean whether or not something is metallic or not?
Here is a helpful resource as well, "PBR Bible on Polycount" : https://polycount.com/discussion/136390/pbr-physically-based-rendering-bible/p1
in practice you might want to cheat a bit - having grey values in your metallic map isn't necessarily wrong - a pixel should take the average value of the area it covers