The `check_visibility` system currently follows this algorithm:
1. Store all meshes that were visible last frame in the
`PreviousVisibleMeshes` set.
2. Determine which meshes are visible. For each such visible mesh,
remove it from `PreviousVisibleMeshes`.
3. Mark all meshes that remain in `PreviousVisibleMeshes` as invisible.
This algorithm would be correct if the `check_visibility` were the only
system that marked meshes visible. However, it's not: the shadow-related
systems `check_dir_light_mesh_visibility` and
`check_point_light_mesh_visibility` can as well. This results in the
following sequence of events for meshes that are in a shadow map but
*not* visible from a camera:
A. `check_visibility` runs, finds that no camera contains these meshes,
and marks them hidden, which sets the changed flag.
B. `check_dir_light_mesh_visibility` and/or
`check_point_light_mesh_visibility` run, discover that these meshes
are visible in the shadow map, and marks them as visible, again
setting the `ViewVisibility` changed flag.
C. During the extraction phase, the mesh extraction system sees that
`ViewVisibility` is changed and re-extracts the mesh.
This is inefficient and results in needless work during rendering.
This patch fixes the issue in two ways:
* The `check_dir_light_mesh_visibility` and
`check_point_light_mesh_visibility` systems now remove meshes that they
discover from `PreviousVisibleMeshes`.
* Step (3) above has been moved from `check_visibility` to a separate
system, `mark_newly_hidden_entities_invisible`. This system runs after
all visibility-determining systems, ensuring that
`PreviousVisibleMeshes` contains only those meshes that truly became
invisible on this frame.
This fix dramatically improves the performance of [the Caldera
benchmark], when combined with several other patches I've submitted.
[the Caldera benchmark]:
https://github.com/DGriffin91/bevy_caldera_scene
73970d0c12