## ← Reversing the Process - Interactive 3D Graphics

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Showing Revision 2 created 05/25/2016 by Udacity Robot.

1. We've seen how expensive it would be to track all the photons in a scene. To
2. avoid this level of computation. In computer graphics, we make a number of
3. simplifying assumptions. First, only those photons that reach the camera are the
4. ones needed to make the image. So, these are the only ones we would like to
5. compute. We know which objects we can see. But how do we know in advance which
6. photons will matter? Each photon is generated by a light source, but we don't
7. know where each ends up. So, we reverse the process. Instead of sending photons
8. from the light, we essentially cast a ray from the eye through each pixel and
9. see what's out there. When a surface is seen at a pixel, we then compute the
10. direct effect of each light on that surface. Add up all the light contributions
11. and you have a reasonable proximation of what the surface looks like. One more
12. approximation can be used for these lights, nothing blocks them every surface is
13. considered visible to a light unless it faces away from that light source, in
14. other words no objects casts shadows. This simplification saves us a lot of
15. computation. Rendering can get even simpler than this. For example, you could do
16. away with lights all together and just draw each object with a solid color.
17. However, I'm drawing the line there as lights add considerable realism and
18. perception of 3D to a scene. For example, look at this scene with, without
19. lights. So there are two extremes. At one end photons are carefully traced from
20. light emitters through a scene, potentially giving a highly realistic image. At
21. the other end the camera determines what gets viewed and a rough approximation
22. of what light reaches each surface is determined. This process can be
23. unrealistic but can be computed extremely fast, We'll start with a second
24. extreme and, and in later lessons show how we can remove or modify various
25. simplifications, so the more photon based computations are performed, such as