Oswald in the doorway: the blog of the Oswald Innocence Campaign, by Ralph Cinque

There is absolutely no doubt that Oswald could see Ruby coming in this image.

Human peripheral vision is almost 180 degrees when you have both eyes open, and Ruby's angle to Oswald is much less than 180. 

And, at the extremes, what you can detect most readily is: motion. Jack Ruby was moving. Oswald could see him coming. And prior to that, they had eye contact.