The 3d aspect indicates that the Shroud image seems to correlate very accurately with a image density gradient that relates with body to cloth distance. In other words, it acts like a spatial database that carries similar info as would a topographical map. It very clearly indicates by shades of dark to light where the body was in direct contact with the cloth and where the cloth may have been separated by a millimeter or a centimeter or fraction thereof. This is not a quality that can be produced by mere reflected light photography. This anomaly is analogous to the image becoming positive in a photo-negative.
Nobody claims to understand it.
Its just one of the many unexplained phenomena related to this remarkable artifact. The significance is that had the image been the result of any liquid substances applied to the cloth, capillary action would have caused the image to penetrate far deeper than just two microfibrils. The blood stains soak all the way through, as do the water stains that were caused when the fire of 1532 was doused. Yet the image does not penetrate the cloth and is perfectly consistent in image density over the entire surface including both ventral and dorsal body images. This precision would be virtually impossible for an artist to replicate.