A reader writes:

The article is interesting but I’m a bit confused about the progressive opacity overlays. Is there any technical documentation on the size, scale, skew and tilt of the compared images? They can’t all be so perfect, can they? I’m guessing that the mosaic is important but that there may have been other sources as well. I’m thinking the Turin shroud may be the first source in a complex tree of sources, mostly lost to history.