Found it!!
I did a little experiment after taking a photo of a footprint in the sand that looked embossed. It's the position of the shadows, as I said @ozziehoffy
Astronomers have dealt with this problem for years. Mainly while looking at craters. If the lighting is just right, a crater will appear to be a dome rather than a bowl. Often, they'll rotate the image 180 degrees, and it corrects the problem. Obviously you can't do that with your photo, but it's a neat experiment on how the brain percieves 3 dimensions.
@ozziehoffy@onie - not only can you re-orientate the image to change it from looking sunken to looking raised, but if you stare at one of these type of images and then change the dof of your vision, you can change the perception inside your head. Bit like those old books that had the dotty pages and if you stared at the right focus, you could see the "image" appear from the dots. One of the things people do when visiting the Volcanos in PNG is to take a photo of their foot print in the ash, and yeah, I've done it and I've also stared at it for a while working out whether it be sunken or raised.
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As I go off searching for the side by side shot I did.....it's on my computer somewhere!
I did a little experiment after taking a photo of a footprint in the sand that looked embossed. It's the position of the shadows, as I said
@ozziehoffy