As I’ve suggested elsewhere, I think the Shroud image was produced by a craftsman commissioned to provide some visible ‘evidence’ that the cloth displayed before the congregation at the conclusion of the Easter ‘Quem Quaeritis’ ceremony resembled one that might really have covered the body of Jesus. There was, I think, no claim at its manufacture that the Shroud was a true relic; it was no more than a liturgical accoutrement, which everybody recognised as such.
Either the craftsman himself or his patron came up with the idea of a double ‘sweat imprint’, perhaps inspired by the Latin word ‘sudarium’ (although this was never applied to such a large cloth), or simply from the fact that the cloth was used to wrap a statue of Jesus on Good Friday. It is not impossible that, as suggested by Joe Nickell and Charles Freeman, he could simply have painted the double image free-hand, but I think the very existence of the statue was a powerful incentive to use it, or something similar, as a printing block. One is tempted to guess that one or two preliminary versions were rejected. Covering a realistic statue with colourant and spreading a cloth on would have looked grotesque; and ending up with a cloth showing four feet meeting in the middle, and the heads hanging down at the ends, would have looked disrespectful. On the other hand some of the figures of Christ used in the Easter liturgy were carved in bas-relief, and folding a cloth appropriately over one of those could give the required verisimilitude and orientation. In some Good Friday liturgies, such as that used in Hereford, the wooden ‘body’ to be interred in the Easter Sepulchre was first washed, and occasionally it seems that wine was used for this as well as water. One is tempted to wonder if the imprinting medium might be vinous.
Either way, it seems likely that an appropriate bas-relief, quite possibly the actual statue used in the Easter ceremony, was used. The similarity in intensity between the ventral and dorsal images suggests to me that the image was made in two stages, front and back, and not simultaneously, lowering the image onto the cloth (the dorsal image deriving from the pressure of the image on the cloth) and then folding the cloth over the front of the body (this image deriving from the cloth being pressed down by the craftsman), but it is not easy to tell.
If the substrate were something like a woodcut*, and the print achieved in the same way as a woodcut, by applying an even pressure across the surface, then the result would look like a wood-cut print, with sharp edges and a single shade of ‘ink’. This is not what we see on the Shroud. A bas-relief, on the other hand, has the curves and dimples of a real subject, just rather flatter. A cloth spread over one would need to be pressed down by hand, and necessarily this would be firmer on the more protruberant parts and weaker on the more recessive parts. There would also be areas of non-contact, and thus no ‘ink’ at all.
The resulting image would show a relationship between darkness of colour and applied pressure, and correlatively the relationship between the applied pressure and the prominence of the bas-relief. This is what we see on the Shroud.
*(Important to point out that a woodcut has the ink filling the grooves in the wood, not on the surface, while the Shroud suggests the opposite. It is the method of applying the paper/cloth I’m looking at here, not the image itself)