any reason why diffuse and specular reflection are distinct properties? i imagine a low diffuse reflection could represent a flat surface, effectively serving as specular reflection, right? an example of a material with high diffuse and high specular reflection seems to be glossy wood, but that's only true because the wood (fig. 2) is covered in a transparent gloss that light also _independently_ interacts with and added onto the underlying rough wood?
the typical model that's taught (fig. 3) illustrates how in specular reflection, neighboring surface normals are the same, while in diffuse reflection, neighboring surface normals are different.
couldn't a better model for diffuse reflection depict how light becomes trapped and lost in microscopic tunnels and caverns of, for example, fabric, and eventually exit at some "unpredictable" location? which means, even if the surface is flat, there will still be caves and ravines, indicating diffuse reflection.
also, could we extend diffuse reflection to be multidimensional? such that we can have agamograph-like materials? if the vertical roughness is minimal, while the horizontal roughness is jagged, different properties can appear at different angles. or like the material of steel, how steel has microscopic protruding lines around it.