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how does sorting rays following similar direction help?

They're probably alluding to Disney's Hyperion renderer[1]. Sorting rays helps with cache coherency.

[1] https://disney-animation.s3.amazonaws.com/uploads/production...

Yeah, I was a bit disappointed that they started focusing on their own renderer instead of just explaining more of the basics of path tracing. Note that it isn't necessary at all to sort rays, although it can make things faster if done correctly. Also note that Disney isn't the first to consider sorting rays and tracing them at the same time, similar research was done in the 90s.

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