Every time I think I understand the relationship between the two terms, I get more information that confuses me. I thought they were synonymous, but now I'm not sure.What is the difference between diffuse and albedo? Are they interchangeable terms or are they used to mean different things in practice?
Albedo vs Diffuse
rendering;shading
The short answer: They are not interchangeable, but their meaning can sometimes appear to overlap in computer graphics literature, giving the potential for confusion.Albedo is the proportion of incident light that is reflected away from a surface.Diffuse reflection is the reflection of light in many directions, rather than in just one direction like a mirror (specular reflection).In the case of ideal diffuse reflection (Lambertian reflectance), incident light is reflected in all directions independently of the angle at which it arrived. Since in computer graphics rendering literature there is sometimes a diffuse coefficient when calculating the colour of a pixel, which indicates the proportion of light reflected diffusely, there is an opportunity for confusion with the term albedo, which also means the proportion of light reflected.If you are rendering a material which only has ideal diffuse reflectance, then the albedo will be equal to the diffuse coefficient. However, in general a surface may reflect some light diffusely and other light specularly or in other direction-dependent ways, so that the diffuse coefficient is only a fraction of the albedo.Note that albedo is a term from observation of planets, moons and other large scale bodies, and is an average over the surface, and often an average over time. The albedo is thus not a useful value in itself for rendering a surface, where you need the specific, current surface property at any given location on the surface. Also note that in astronomy the term albedo can refer to different parts of the spectrum in different contexts - it will not always be refering to human visible light.Another difference, as Nathan Reed points out in a comment, is that albedo is a single average value, which gives you no colour information. For basic rendering the diffuse coefficient gives proportions for red, green and blue components separately, so albedo would only allow you to render greyscale images. For more realistic images, spectral rendering requires the reflectance of a surface as a function of the whole visible spectrum - far more than a single average value.