Roughness

Roughness sums up, in one value, the tiny bumps and pits of a surface that are too small to see. Low roughness means a mirror-smooth surface with sharp reflections; as it rises, light scatters in more directions and the reflection blurs, then fades to a soft sheen. It is usually entered as a number between 0 and 1 or as a greyscale map.

The hidden key to realism

One of the most common reasons a render looks computer-generated is that every surface has the same, flawless roughness. In the real world even polished marble carries fingerprints, light wear and duller paths where people walk. When a roughness map carries those variations, reflections change across the surface and the material suddenly looks real.

Typical surfaces

• Glass, mirror, polished chrome: very low roughness
• Lacquered doors, polished stone, glossy tile: low
• Oiled timber, satin paint: medium
• Exposed concrete, plaster, fabric: high

Some software asks for the same information the other way round, as glossiness. In that case the map is inverted; otherwise matte surfaces turn glossy and glossy ones turn matte.