Rusty steel — patches of bare metal corroded by deep rust, with the
oxide running matte and dielectric and the surviving metal staying
mirror-smooth and metallic. The discoloration is driven by a single
fBm mask that controls all four maps in lockstep:
Color lerps from steel grey through bright-orange flake to
deep oxide brown as the mask climbs through rustCoverage.
Metallic flips from 1.0 (bare metal) to 0.0 (oxide) at
the same threshold — rust is a dielectric and shouldn't read as
a tinted Fresnel mirror.
Roughness climbs from metalRoughness to rustRoughness,
so the rusty patches kill the IBL specular and look matte.
Normal is built from a height field that's flat on metal and
bumpy on rust, so the oxide reads as physically pitted.
Two octaves of fine fBm are mixed in on top of the coarse mask to
roughen the rust/metal boundary into the chipped, irregular edge a
uniform threshold would otherwise miss.
Rusty steel — patches of bare metal corroded by deep rust, with the oxide running matte and dielectric and the surviving metal staying mirror-smooth and metallic. The discoloration is driven by a single fBm mask that controls all four maps in lockstep:
rustCoverage.1.0(bare metal) to0.0(oxide) at the same threshold — rust is a dielectric and shouldn't read as a tinted Fresnel mirror.metalRoughnesstorustRoughness, so the rusty patches kill the IBL specular and look matte.Two octaves of fine fBm are mixed in on top of the coarse mask to roughen the rust/metal boundary into the chipped, irregular edge a uniform threshold would otherwise miss.