The image on the Shroud has the quality of a photographic negative. The image is only on the surface of the cloth, there’s a high degree of anatomical detail combined with optical distortion, and negative reversals of the image on the Shroud are famously more clear than the Shroud itself (as one would expect when making a positive image out of a negative).
One explanation for the photographic quality of the image is the use of a camera obscura combined with an early photographic process — likely one of the closely guarded secrets of the alchemists. The line on the neck of the image and the differing distortion of body and head suggests that it was the result of two exposures, one using an actual corpse, and one using a sculpted face.
Some have also suggested that one of the only contemporaneous people who was skilled enough, smart enough and sacrilegious enough to take the job was Leonardo daVinci.
And — just for argument’s sake — if the shroud were the result of a photographic process using a corpse and a sculpted head, isn’t it interesting how the face kinda resembles daVinci’s self-portrait.
wikipedia: shroud of turin