For some years, the retroReveal website hosted by the University of Utah facilitated:
"the collaborative discovery of hidden content in documents, manuscripts, music, and artefacts".
Using image processing algorithms, images are separated into component colour channels to which various transformations are applied to retroactively reveal hidden elements in images that would otherwise be barely visible to the naked eye.
retroReveal (also known as retro Reveal) was popular with researchers in several fields.
Unfortunately, retroReveal went offline in May 2022 and seems unlikely to return.
However, all is not lost.
ImageSleuth is a free,
fully functional replacement for retroReveal which also includes several enhancements.
ImageSleuth is cross-platform; it may be downloaded for Windows, Mac or Linux. Once installed, it does not require an internet connection to function. Nor is it necessary to rely on the continued availability of a third-party website.