Graphics › Basics
Surface properties: roughness, metallic, AO
Know first: Color is numbers, Normals and light, Shader: a function for every pixel
Why a wet stone is both darker and shinier than a dry one. Why the highlight on red plastic is white, while gold reflects yellow. And what the Roughness, Metallic, Specular and AO channels really mean. Together with BaseColor they are a handful of numbers that say how a material treats light. The engine works out the lighting itself from those numbers.
Why this matters
In the lesson on normals and light, brightness came from a single formula: BaseColor × light × the cosine of the angle. That formula fits matte things such as paper, dry earth and plaster. But a stone at the water’s edge glints, reflections run across metal, and a puddle shows the sky. A cosine alone cannot describe any of that.
On the main node of an Unreal material, Metallic, Specular, Roughness and Ambient Occlusion sit next to Base Color. People often set them by guesswork. They drag Roughness until it looks “nice,” set Metallic to 0.5 “for some shine,” or zero out Specular to make something matte. The result is a world of plastic, black metal in the shade, and grass that glows in the sun. Once you know what each channel means physically, you choose the numbers instead of hunting for them.
All of this rests on a model called PBR, short for physically based rendering. Its core idea is that a material describes a substance, not a picture. You describe a stone once, and it looks right in full sun, under an overcast sky and in a cave lit by a torch. Artists used to paint highlights and shadows straight into the texture, and the picture fell apart under any other light.