The Umbra computation for the currently loaded map is launched by the "Build Umbra" button in the editor toolbar. The computation process runs in the background and presents you with options of keeping the computed data or discarding it once the process is complete. Note that any edits to the active map while the computation is in progress will invalidate the computation results.
When you accept the computed data, the Umbra visibility data chunk will be stored in the persistent level of your world. Additionally any primitive components that have been determined to be statically occludable by Umbra will have a visibility identifier stored with them. This visibility identifier is shared with Unreal's own precomputed visibility system, and therefore Umbra can not be used simultaneously with it. Choosing to save the Umbra computation data automatically removes any precomputed visiblity data.
Umbra uses automatic incremental building. This means that after the first build results are cached and reused.
Umbra stores the exported level geometry, computation parameters, visibility data and incremental computation cache files inGameDirectory/UmbraCache. Use the Umbra viewer application to import the scene and the tome of visibility for thorough analysis and inspection!
Umbra computation parameters
Umbra computation parameters are exposed as entries in world settings, under the "Umbra" category. Refer to Umbra 3 documentation for the interpretation of these parameters.
Umbra component properties
The Umbra scene export integration will in most cases be able automatically deduce whether a component can be used as a static occluder. There are cases, however, where manual fine/tuning is needed. One such case is a static mesh with very small holes (smaller than the "smallest hole" computation parameter) through it, for example a grille or a fence. For this purpose the Umbra plugin adds an editable "use as occluder" property to all primitive components.
Umbra view volumes
To speed up Umbra computation and reduce the size of the data generated, it is advisable to roughly mark the playable volume of your map with Umbra "view volumes". You will find this Umbra-specific volume in the volumes dropdown menu. Note that view volumes are only inspected in the persistent level, adding volumes in any other levels has no effect. As a fallback, if no Umbra view volumes are present, the plugin will attempt to use "precomputed visibility volumes" instead.
Note: Umbra view volumes are currently not supported, you should use precomputed visibility volumes instead. Umbra view volumes will be added back in a future version.