Physically Based Rendering/Shading
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https://www.marmoset.co/posts/basic-theory-of-physically-based-rendering/

- albedo: a color which describes the fractions of various colors of light
  that will scatter back out of a surface.
  “Diffuse color” is a phrase sometimes used synonymously.

- Reflection
- Diffusion
- Energy conservation:
    - Reflection and diffusion are mutually exclusive.
- Fresnel
    - Differing reflectivity that occurs at diffent angles
- Microsurface
    - Gloss, smoothness, roughness.

- BRDF - Bidirectional Reflectance Distribution Function.
    -   Function of View and Light direction - f(l, v)
    -   For a ray of incoming light from a given direction
        the BRDF function gives the distribution of outgoing light
        in all directions.
    -   For a given view direction, the BRDF gives the relative 
        contribution of each incoming direction.
    -   The BRDF varies by light wavelength