Roughness map

A roughness map is a greyscale texture that tells the shader how rough or smooth each point of a surface is. Epic: "The Roughness input controls how rough or smooth a Material's surface is"; 0 gives a mirror-like reflection and 1 a matte, diffuse look (Unreal Engine documentation: Physically Based Materials in Unreal Engine). In the texture, black is smooth and white is rough (Adobe Substance 3D: The PBR Guide – Part 2).

#What it represents

Roughness describes microfacets, surface detail far smaller than a pixel. Adobe calls it "sub-texel geometric detail" (Adobe Substance 3D: The PBR Guide – Part 1). When the tiny facets are aligned, light reflects in one direction and the highlight is sharp; when they are scattered, the reflection spreads and blurs (Marmoset: Basic Theory of Physically-Based Rendering). Because of energy conservation, a wider highlight is also dimmer. See physically based rendering for the microfacet diagram.

#Why it matters

Adobe calls roughness "the most creative map" (Adobe Substance 3D: The PBR Guide – Part 2), and in environment art it often carries most of a surface's character. Fingerprints on glass, a worn path through a polished floor, oil on concrete, rain on a street: none of these change colour much, but all of them change roughness.

Epic chose roughness over gloss for Unreal 4 because it is "very clear in its meaning, whereas gloss always needs explaining" (Karis 2013).

#Roughness, gloss and smoothness

These are the same property with the scale flipped:

• Roughness: 0 smooth, 1 rough. Unreal, Godot, glTF, Substance.
• Glossiness: the inverse, white is smooth; used in the specular/gloss workflow (Adobe Substance 3D: The PBR Guide – Part 2).
• Smoothness: Unity's name for gloss. HDRP stores it in the mask map's alpha (High Definition Render Pipeline manual 17.0: Mask map and detail map); URP reads it from the alpha of the metallic or base map (Unity Manual: Lit shader material Inspector window…).

Converting between them is a simple invert (1 − value). Forgetting to invert is a common export mistake.

#Example

#Common mistakes

• A flat roughness value. A single number for the whole material looks like plastic. Even subtle noise helps.
• Painting roughness from the base colour. Dark paint is not automatically shinier; roughness and colour are separate properties.
• Imported as sRGB. Roughness is linear data (Adobe Substance 3D: The PBR Guide – Part 2); in sRGB every mid value shifts.
• Forgetting to invert between gloss/smoothness and roughness.
• Ignoring specular aliasing. Fine normal detail on a smooth surface sparkles at a distance. Godot can filter roughness from the normal map on import, which "can help decrease specular aliasing slightly" (Godot documentation: Importing images).
• Using Specular = 0 to get a matte look. Epic: use high roughness instead (Unreal Engine documentation: Physically Based Materials in Unreal Engine).

#In different tools

• Unreal Engine: Roughness input; usually the green channel of an ORM texture. See channel packing.
• Unity: Smoothness, not roughness. HDRP mask map alpha (High Definition Render Pipeline manual 17.0: Mask map and detail map); URP from the metallic or base map alpha (Unity Manual: Lit shader material Inspector window…).
• Godot: Roughness; 0 is "a perfect mirror", 1 "completely blurs the reflection" (Godot documentation: Standard Material 3D and ORM Material 3D). Can be read from any channel.

#Related terms

physically based rendering · texture maps explained · metallic map · base colour (albedo) map · channel packing · normal map

#Further reading and watching