-materials have diffuse and specular reflection only.
-in dielectric, you have both (and for physically based workflow they should never exceed 100%). If you have a certain amount of specular reflection you will have an inversely proportional amount of diffuse reflection and vice versa.
-diffuse reflection is the light that scatters right underneath the surface and is reflected in all directions (diffuse reflection can be imagined as the base colour of the material or albedo), a material that has only the diffuse component looks flat and matte (like a very rough plaster), however, no materials in the world have only diffuse reflections.
-the specular reflection is the light reflected on the surface of the material and can be sharp or blurry
-the amount of blurriness of specular reflection is controlled by the glossiness parameter (or its inverse the roughness parameter)
-for materials like metals the diffuse component is 0% and the specular is 100%, however, for coloured metals like gold or copper, the colour is given to the specular reflections and not to the diffuse reflections
-a material that has 100% specular reflection with 100% glossiness, and 0% diffuse reflections it's a mirror
-a material that has 100% specular reflection with 0% glossiness and 0% diffuse reflections looks like a very rough metal (like a heavily sandblasted silver) however, metals in real life always have some amount of glossiness it's never 0%
-the "specularity" parameter is usually used to "weight" the % of specular reflection
-the "specular colour" parameter is used to assign the colour to the specular reflections (metals and other strange hybrid materials can have coloured reflections; non-metallic/dielectric materials usually have white specular reflections)
-the diffuse reflection might not always has a weighting factor as the specular reflections, usually it is controlled by the luminosity of its colour.
-the "diffuse colour" parameter is the same as the specular colour but assigns a colour to the diffuse reflections rather than the specular ones.
materials have diffuse and specular reflection only.
in dielectric, you have both (and for physically based workflow they should never exceed 100%). If you have a certain amount of specular reflection you will have an inversely proportional amount of diffuse reflection and vice versa.
diffuse reflection is the light that scatters right underneath the surface and is reflected in all directions (diffuse reflection can be imagined as the base colour of the material or albedo), a material that has only the diffuse component looks flat and matte (like a very rough plaster), however, no materials in the world have only diffuse reflections.
the specular reflection is the light reflected on the surface of the material and can be sharp or blurry
the amount of blurriness of specular reflection is controlled by the glossiness parameter (or its inverse the roughness parameter)
for materials like metals the diffuse component is 0% and the specular is 100%, however, for coloured metals like gold or copper, the colour is given to the specular reflections and not to the diffuse reflections
a material that has 100% specular reflection with 100% glossiness, and 0% diffuse reflections it's a mirror
a material that has 100% specular reflection with 0% glossiness and 0% diffuse reflections looks like a very rough metal (like a heavily sandblasted silver) however, metals in real life always have some amount of glossiness it's never 0%
the "specularity" parameter is usually used to "weight" the % of specular reflection
the "specular colour" parameter is used to assign the colour to the specular reflections (metals and other strange hybrid materials can have coloured reflections; non-metallic/dielectric materials usually have white specular reflections)
the diffuse reflection might not always has a weighting factor as the specular reflections, usually it is controlled by the luminosity of its colour.
the "diffuse colour" parameter is the same as the specular colour but assigns a colour to the diffuse reflections rather than the specular ones.
Additional observations:
-the metalness does not describe a different type of reflection, but it basically makes sure that when increasing the specular reflections, the diffuse reflections are lowered proportionally. (so basically, when you increase specular reflections to 100%, even if there is a colour assigned to the diffuse colour channel you won't see it because it would be weighted down to 0%)
-Do not mistake the diffuse reflection with the subsurface scattering (SSS), the SSS is an effect that scatters the light underneath the surface as the Diffuse, but it can go way farther underneath the surface compared to the diffuse, in theory, if you set the SSS distance to a very low number you could replicate the Diffuse reflection effect, however, it is much heavier to calculate, so when you don't need to go very far beneath the surface you just avoid calculating it and you rely only on the diffuse effect.
the metalness does not describe a different type of reflection, but it basically makes sure that when increasing the specular reflections, the diffuse reflections are lowered proportionally. (so basically, when you increase specular reflections to 100%, even if there is a colour assigned to the diffuse colour channel you won't see it because it would be weighted down to 0%)
Do not mistake the diffuse reflection with the subsurface scattering (SSS), the SSS is an effect that scatters the light underneath the surface as the Diffuse, but it can go way farther underneath the surface compared to the diffuse, in theory, if you set the SSS distance to a very low number you could replicate the Diffuse reflection effect, however, it is much heavier to calculate, so when you don't need to go very far beneath the surface you just avoid calculating it and you rely only on the diffuse effect.
