Finding back-facing polygons with the dot product

Here’s a video that uses ICE to demonstrate how to figure out whether a polygon faces in a certain direction, as described in Mathematics for Computer Graphics by John Vince

The Dot Product in Back-Face Detection
A standard way of identifying back-facing polygons relative to the virtual camera is to compute the angle between the polygon’s surface normal and the line of sight between the camera and the polygon. If this angle is less than 90◦ the polygon is visible; if it is equal to or greater than 90◦ the polygon is invisible.