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How do Draw Distance and Scene Depth actually work?

So, I have a fog effect that I’m implementing in a post-process effect, so, in order to support that I’d like to cull stuff that is fully enveloped by the fog.

I’m using 4.4.2

  • So I have some walls, I’ve set their “Desired max draw distance” to 50,000.
  • I have a Post Process fog material that uses a Scene Texture : SceneDepth node to create fog at a distance of 20,000.

Now, I’m going to assume that the distances in the editor are correct.

  • According to that, the cull distance of 50,000 actually makes objects cull at a distance of roughly 22,000. (tested by position an object with that cull distance at that distance from the camera and then walking backwards and forth a few feet and seeing it appear and disappear.)
  • post process effect reports a distance of roughly 23,000 as being 20,000 (here’s a screenshot of the material I used to test this)

These things aren’t technically problems, per se, as I can work around them, I’m just wondering if anyone has any insights as to why they’re happening, or like how you can figure out the distance that things cull at from the max draw distance (almost-half-but-not-quite?)

Other insights into exactly how culling & scene depth actually work in Unreal 4 would also be cool.