## ← BRDF - Interactive 3D Graphics

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Showing Revision 2 created 05/24/2016 by Udacity Robot.

1. I want to give you a taste of how materials can be represented in a more
2. general way. You'll also learn a great term to impress your friends and
3. confound your enemies. Think about a surface and how it reflects light. The two
4. variables we use are the light's incoming direction and the amount of light
5. reflecting towards the eye. So at it's simplest, a material can be represented
6. by this function,given a light and eye direction, give back an intensity. This
7. function is called the BRDF which stands for Bidirectional Reflectance
8. Distribution Function. Let's look at that phrase. First, it's a function. The
9. inputs are the light and the eye. The function depends on two directions so
10. it's bidirectional. These directions are normally given with respect to the
11. surface itself, that is, each vector is often given as two numbers, the
12. altitude angle and the azimuth. The altitude is the angle away from the normal,
13. and the azimuth is the angle of the vector when projected onto the plain. The
14. phrase reflectance distribution means how the light is spread. One simple
15. example is a perfect mirror. The reflectance distribution in this case is that
16. when the eyes direction is exactly equal to the lights reflection direction,
17. all light is reflected towards the eye. Every other eye direction gets no
18. light. Another basic distribution is diffuse reflection. For some given
19. incoming light direction, the direction to the eye doesn't matter. That's the
20. definition of diffuse reflection. Since this value is constant, diffuse is then
21. represented by the surface of a hemisphere. Specular highlights are represented
22. by lobes. This distribution represents a glossy surface, where light is
23. reflected in a general direction. The lights direction determines where most of
24. the lights energy is reflected. If the load gets wider, the specular
25. reflections spreads out. Written this way, our BRDF needs four numbers, two for
26. the light and two for the eye. But if you think about it, most materials really
27. only need three. These two altitude angles and this azimuth between them. For
28. example if you put a sheet of paper on a table top and rotate it. Both the
29. light azimuth and eye azimuth angles change with respect to the paper, but the
30. angle between the two remains the same. Most materials are fine with three
31. numbers.