Understanding of NDC rays in NeRF

[shriajaykumar]

I have been racking my brain on this for few days. So please help me with these following queries:

1. What is the need to transform ray origins and ray directions in NDC frame ? I have already read physical intuition behind ndc_rays? #18, but still did not follow through.
   In the paper, I read that the purpose for this is to preserve parallel lines and to have linear disparity-z relationship. But, the latter , though helps in better sampling and faster calculations, creates an inaccurate 3d space reconstruction. So, why is this desired ?

2. Why is NDC rays used in NeRF but not in TinyNeRF ?

3. What does "forward facing scenes" in the paper mean ? What does this have to do with NDC ?