I cant find a way to ask UE4 if a actor/component was rendered last frame, aka was/is visible for a specific camera. There also seems to-be no frustum check functions, so you can do some tests yourself.
Those features are often used to decide what hud/interaction elements to render, in combination with traces. So as example this could be a interaction display logic:
Get all actors in interaction range, that we can see and are of type xy, than do some range/type filtering.
In the examples they either use a simple overlap test, which allows interaction even if you are turned 180 degree or they add a line trace that only works with big enough actors.
Yes thx, those steps will yield similar results, but most of it is already done per frame for rendering so most engines simply expose those results in some form, so they are re-useable. Also keep in mind that the dot product of the two forward vectors only works as approximation and does not answer the question: Does the player sees object xy?
The idea is that the player should only be able to interact with objects that he, she can actually see at a given moment, so stuff does not activate, are used by accident. This is usually combined with a highlight system to further lock the “useable” object. The dot product or a cone trace may work well enough, since often only objects in the center are highlighted and used. The case where you need a frustum culling test, is when you want to highlight objects for missions/objectives the moment the player “sees” those, even if they are at the camera borders.
So thats why i request those features, which where available in Unity4 and CryEngine in some form.