Rebuild when the refit tree's relative SAH cost exceeds the as-built cost by
this factor (ddgi DECISIONS 2026-11-05, B1a). The cost is the expected
number of top-level nodes a ray visits, a small part of a ray's work, so 25%
more costs a few percent of trace time at most; a lower ratio would rebuild
continuously in a scene that never stops moving.
Rebuild when the refit tree's relative SAH cost exceeds the as-built cost by this factor (ddgi DECISIONS 2026-11-05, B1a). The cost is the expected number of top-level nodes a ray visits, a small part of a ray's work, so 25% more costs a few percent of trace time at most; a lower ratio would rebuild continuously in a scene that never stops moving.