It would turn out something like those wierd-looking world maps... you know the ones I mean, that look the same way an orange peel looks if you get it to come off in one piece.
If the shroud were 'rubbed' on a statue of some sort, and left to discolor somehow, when it was taken off and then laid out flat, the image would still be distorted.
Try taking some silly- putty,wrapping it around the face of a doll, for instance, and then taking it off and folding it out flat (image up of course)
See if the image stays anywhere near realistic, or if it distorts out.
The image on the shroud, even though it is laid out flat, still has a perfect image of a man, even though it was laid around him.
This is what is meant by the image being more of a 'photographic image' than of a transfer of some kind of medium,(paint or some kind of a dyed fluid)
The fact that the photographic negative,which was discovered in the late 19th century, shows far more detail than the positive image(the one the eye sees) is very strong evidence for the image not being the work of a 'medieval forger.'