What black and white mean
In a conventional roughness input, low values produce a narrower reflection and high values spread it. This does not make a black roughness patch a black material. Base colour, metalness, lighting and viewing angle still contribute to the visible result. Judge the reflection on a known shape, not the greyscale file alone.
A glossiness workflow reverses that progression. Before inverting a whole library, make two small known-value patches, assign them to the destination shader and confirm which patch produces the broader highlight. This makes the conversion decision reproducible.
The one-minus conversion
For normalised values use R = 1 - G. For an eight-bit greyscale image under the same numeric convention use R8 = 255 - G8. Thus 204 becomes 51. This is an arithmetic conversion of stored values, not a guarantee that two different renderers will produce identical highlights.
Do the operation once. A source exporter may already have inverted a glossiness map. Record the original map role and the conversion in the filename or handoff note so another artist does not invert it again.
Colour space and packed channels
Treat scalar material data according to the receiving shader documentation. Applying a display-colour transform to a roughness value changes the number that drives the material. A midpoint stored in a file can therefore arrive as a different roughness even though the thumbnail looks plausible.
For glTF metallic-roughness textures, roughness uses the green channel and metallic uses blue; these values use linear interpretation. Do not connect the RGB composite as if it were a single greyscale map. Check the actual channel assignment after export.
Why a photo is only a starting point
A dark area in a photograph might be paint, shadow, dirt or a reflection. Converting brightness directly to roughness cannot distinguish those causes. Use generated maps as candidates, then separate the material regions and compare against the real finish where a reference is available.
For review, keep geometry, camera and lights fixed while comparing maps. Start with a simple neutral surface and change one input at a time. Once the map behaves predictably, return to the final art direction and judge the complete product.
