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Currently, precomputed visibility volumes do not seem to alter occluded primitives, nor do they show visibility cells when r.ShowPrecomputedVisibilityCells 1 is entered.
Repro: [reproduced internally]
1. Open Editor (Any project)
2. Select Volumes in placement mode
3. Drag a pre-computed visibility volume into level
4. Place static meshes within bounds of volume (adjust bounds as necessary to compensate) - For the test SM_Rock, SM_Shelf, and MaterialSphere were all used
5. Build level
6. In the console commandline, type both stat initviews (enter) then r.ShowPrecomputedVisibilityCells 1 (enter)
Occluded primitives remains at 0
Visibility Cells are not rendered
Statically Occluded Primitives should show >0 and visibility cells should be rendering in viewport
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-3634 in the post.