PBR materials

Older game textures often had lighting painted into them: highlights on the top edge, shadows in the creases. That looked right under the one lighting setup it was painted for and wrong everywhere else. Move the asset into a night scene and it still glowed as if lit from above.

Physically based rendering separates the material from the light. The texture maps describe what the surface is: its colour, whether it is metal, how rough it is. The renderer works out what it looks like under whatever light is in the scene. Disney's 2012 shading course notes popularised the artist-friendly version of this model used in film, and real-time engines adopted it within a few years. Today it is the default in Blender's Principled BSDF, Unity, Unreal Engine, Godot and the glTF file format.

There are two common ways to store the same physical idea.

Metallic-roughness is the dominant workflow. A metallic map says which parts are metal. A roughness map says how blurry reflections are. For non-metals the reflectance is fixed at a low, realistic value, so artists cannot accidentally author physically impossible surfaces. glTF 2.0 defines its core material this way.

Specular-glossiness stores reflectance colour directly in a specular map, with glossiness as the inverse of roughness. It gives more control, and more room for mistakes. It survives in older pipelines and some game projects, and assets often need converting between the two.

Most PBR problems come from a few predictable mistakes:

• Lighting baked into base colour. Base colour should look flat and slightly dull, with no highlights or shadows. Ambient occlusion belongs in its own map.
• Wrong colour space. Base colour is a colour image (sRGB). Metallic, roughness, normal and AO maps are data and must be read as linear. A roughness map imported as sRGB makes the whole material look wrong.
• Mixed-up normal maps. OpenGL-style and DirectX-style normal maps flip the green channel. Blender, Unity and Godot use the OpenGL convention. Unreal Engine uses DirectX. A wrong convention makes bumps look like dents.
• Grey metallic values. Real surfaces are metal or not. Values between 0 and 1 are for blending at edges and rust, not for "slightly metallic" plastic.

PBR maps also depend on good UV mapping: every map is sampled through the same UV layout, so stretching or mismatched texel density shows up in all of them at once.

How well materials survive export depends on the format. glTF stores metallic-roughness materials as part of the standard, so they usually arrive intact. FBX has no single agreed PBR material, so engines interpret its materials in different ways, and textures often need reconnecting. OBJ's companion MTL format predates PBR entirely. The differences are covered in 3D file formats and glTF vs FBX.