It is absolutely Shadow minimum bias. Utterly unrelated, but I feel the urge to say it (lol): raytrace down, map up. That is, for lights coming down, use ray tracing, for lights pointing up, use depth mapping. Or the reverse. Creates softer fill shadows.

thou and I, my friend, can, in the most flunkey world, make, each of us, one non-flunkey, one hero, if we like: that will be two heroes to begin with. (Carlyle)