Physically Based Rendering, 3rd Edition

3.9.5: Robust spawned ray origins

Computed intersection points and their error bounds give us a small 3D box that bounds a region of space. We know that the precise intersection point must be somewhere inside this box and that thus the surface must pass through the box (at least enough to present the point where the intersection is). (Recall Figure 3.43.) Having these boxes makes it possible to position the origins of rays leaving the surface so that they are always on the right side of the surface so that they don’t incorrectly reintersect it. When tracing spawned rays leaving the intersection point p, we offset their origins enough to ensure that they are past the boundary of the error box and thus won’t incorrectly re-intersect the surface. ...