Test if backface of facet faces camera

How can I test is a specific facet is displaying its backside to the camera or frontside.

get the facet normal and dot with the view direction.


What is a good threshold?

Because Dot<0 seems to miss the ones on the edge of the screen.

Try with Dot <= 0, as Dot = 0 means 90° between normal and view so should not be visible as a triangle has no thickness.