We know it is NOT a photograph because it exhibits none of the characteristics of a photograph and has no light artifacts. It is, rather, a representational terrain map of the body it covered in 2D, containing 3D data. No photograph contains that because of light. We also know what the image is made of, and it is not created by a photographic process. There is no photo reactive residue on the Shroud that would be there if it were a photo.
Every photo in the world has that photo-reactive residue left on it, because THAT is what the photo is made of. If there is none, then there would BE NO PHOTO. Yet the Shroud has none.
The image exists in the residue of the Soapwort used in the retting process used when the Flax that made the Shroud was cleaned before the Shroud was woven. It is less than thickness of a soap bubble. The Flax was done in a method used in the Middle East, a hank system, where it was washed and then spread on bushes as hanks before being spun and then woven into cloth. The image is merely slightly more aged than the other parts of that coating and exists only on the surface that was facing the body. The closer the cloth was to the body, the more it was "aged" and the more it was darkened.
Those images were formed AFTER the blood stains were transferred to the cloth. We know this because they do not exist under the blood stains. Trying to get the image to register to the blood stains afterwards or before, would be an impossible undertaking for any photographic process. By the way, the blood stains are real human blood.
No medieval artist would need to use real human blood for an "artistic project" because it would have been unnecessary. It also bears ZERO similarity to any other artistic style of the period. Such a postulated artist would have to be a super-genius, with a pan-authority on things we are only discovering about the 1st Century through exceedingly deep scholarship, as well as human physiological reactions to severe traumatic death we have only learned over a Century of medicine. He would also have had to have had a thorough grounding in Palynology (the study of pollens, specifically of the pollens of the Jerusalem area, including a plant that went extinct in ~800AD that was used for packing around bodies by Jews), the geology of the Jerusalem area (so he could embed dust from the exact right kind of Travertine aragonite found only outside of the East Gate of Jerusalem) on the feet, knees, backside, and shoulders, of the image on the Shroud), and he would have to have made a mistake in the placing of the nail exit wound in the back of the wrist, contrary to all medieval art, showing the wound in the palm, but exactly where it would be if placed at the base if the palm, following natural spaces in the wrist bones, something experienced Crucifixion execution soldiers would know. . . which would support the weight of a man. Anywhere else in the palm would not support that weight of a man writhing in pain for long and would eventually tear through. This hypothetical genius somehow KNEW this, when no one else of the supposed period of fakery did. All of this points more to the Shroud being authentic than it being a fake.
Is that accurate as far as you know?