必须照射到正面才会被视为可用的射线原点发射光线的百分比。
在预计算过程中，系统会计算从射线原点到集群的可见性，但也会试图拒绝位于几何体内的射线原点。如果照射到正面的光线的百分比低于 backFaceTolerance，则系统会将该射线原点标记为无效，但这不会影响光能传递计算。
For example, if backFaceTolerance is 0.0, the ray origin is rejected only if it sees nothing but backfaces. If 1.0, the ray origin is rejected if it has even one ray that hits a backface.
This setting can help control light leaking, but should be considered in the context of the mesh authoring workflow. Preventing ray origin rejection by setting a value of 0.0 will ensure Enlighten Realtime Global Illumination does not reject ray origins due to single-sided geometry. However, if the majority of the meshes are closed, so visible backfaces are rare, a positive value can reduce light leaking or darkening that intersecting geometry would otherwise cause.