Where should the ray origin be put in perspective camera ray tracing?
The following gives a picture of the ray tracing in the perspective camera setting: https://www.cs.cornell.edu/courses/cs4620/2011fa/lectures/06raytracingWeb.pdf
and says that the $e$ is the ray origin, which is the same for all rays.
However, I don't understand how $e$ is supposed to be selected. Can it be the zero vector? Or perhaps it is supposed to be the camera center?