Illegal or legal screen (Video)

[Nevadaref]

Originally Posted by BillyMac

NFHS 4-40 Screen
ART. 3 When screening a stationary opponent from the front or side (within
the visual field), the screener may be anywhere short of contact.
ART. 4 When screening a stationary opponent from behind (outside the
visual field), the screener must allow the opponent one normal step backward
without contact.

Billy,
You had it all along! The definitions which I thought were in the Casebook are actually contained within the Rules book. The articles above tell you that the front or side is within the visual field and that from behind is not.
Although these articles are for stationary opponents the definitions provided for what constitutes the visual field apply to moving players as well.