Cameras obscura were almost certainly around in the late medieval period. There's some controversial evidence such devices were used as an aid to drawing by Renaissance masters, but it's inconclusive. The problem was fixing the image, and that didn't happen until the C19th. If the knowledge was available, how did it get lost? The bas relief idea is too simplistic to show the complexity of the shroud image. It would have taken unprecedented genius to reveal the complex image of a crucified adult at that time, like finding someone had developed personal inter-stellar transport while the moon shot programme was under way.
This was the cutting edge of artistic representation at the time the medieval shroud was conceived:
http://es.wikipedia.org/wiki/Archivo:Fra_Angelico_026.jpg
The problem with the camera obscura method is that it has been debunked a long time ago as a method to produce the shroud simply because it doesn't produce all of the unique features of that shroud image, but the problem goes much worse for the camera obscura method because there is very good evidence that the shroud is much older then the 13th century. people that do argue for the camera obscura method simply don't understand the image on the shroud of turin.
Here is a good article by Head Photographer of the Sturp team Barrie Schwortz who shreds through Nicholas Allen's Camera Obscura theory as an explanation of the shroud image.
https://www.shroud.com/pdfs/orvieto.pdf
But as I said the problem gets even worse. Lets say that the camera obscura method was available in the 13th century? If so you then have to scale that date back to the 12th century because in the Hungarian pray manuscript dated to 1190 ad we have an illustration of what can only be the shroud of turin complete with the 3way herringbone weave and the unique 4 poker holes found only in the shroud.
Then we have the perfect blood stain congruent match between the sudarium of Oviedo and the shroud of turin which Marc Guscin and his team of researchers showed that the shroud and the sudarium were wrapped around the same body at very close time intervals of an hour or 2 at most, and since the sudarium's history is indisputable going back to 614ad, that makes the shroud at least as old as the sudarium of Oviedo.
https://www.shroud.com/guscin.htm
There is a plethora of evidences that make the shroud much older then the 13th century which makes the camera obscura theory(which has been shown many times to not be the explanation for the image of the shroud of turin) ridiculous at best and ludicrous at worst.
Remember also that Agnostic Chemist Ray Rogers in his excellent peer reviewed chemical analysis published in the secular chemical journal Thermochimica acta(which someone already posted the link to in here) did a vanillin test and the dated the shroud off of this test to be between 1300 and 3000 years old. the reason for the wide dates is that Rogers had to account for the different conditions in which the shroud could have been stored in.
I also doubt a camera obscura could account for the xray information encoded into the regions of the hands,wrist, left femur, Jaw, gums and teeth of the man of the shroud. I doubt someone has an xray machine in the 6th century or first century.
You also have to account for a medieval forger understanding what rigor mortis was more then 700 years ago because the image on the shroud itself is of a person in rigor mortis. Rigor mortis doesn't last more then 48 hours. How long do the biblical accounts say Christ was in that tomb? No more then 2 days or a bit more or less. Friday evening to sunday morning ;)
This is exactly why I could never have formed a well informed opinion about the shroud within my first 2 years of researching it. it wasn't until my 4th year of research that I started to believe in the authenticity of the shroud as the image of the historical Jesus, and once I started to believe this, the inference to the best explanation for that image took over, and What I believe the best explanation for that image is that it was caused by the resurrection event. of course you cant the resurrection scientifically but its more then reasonable to infer to the best explanation,and in order to do this you would need to study the shroud in the totality of its evidences.
A simple googling might help a bit in having a general understanding of the shroud but to gain a deeper understanding you would need to put in the leg work of research to get that far