So now to roughness. The roughness map describes how smooth or not the surface is in extreme close up, and this has a huge effect on the appearance of the reflected light. Using PBR Metal/Rough, a puddle of water and a dry leaf will have the exact same reflectance – the specular will be set to 0.5, or 4%. The reason they will look completely different is down to the roughness. Water is very smooth, so the grayscale roughness map will be be almost black, and the surface will render with sharp clear reflections while the dry leaf will be very rough, the values will be approaching white and the surface will render with reflected light so soft and blurred it may not seem to affect it at all.

While the values for base reflection are kept simple, the roughness map can be full of detail – scratches, fingerprints, smudges, pores, chips and blemishes of all kinds can sell the illusion of reality here. Any detail that is too small for the bump or normal map should be here.

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