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When light passes through a thin object, you may see shadows or silhouettes from the other side. Roughness can make the light from the other side appear to scatter more or be blurrier.

Normal/Bump

Here is where you supply a signal, either a texture or procedural pattern, to create a bump to the surface to "fake" surface details like small bumps or scratches. This means an artist doesn't have to model these tedious and often repetitious parts of a model.

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