Optical illusion due to direction of light – are these shapes indented or sticking out?

Optical illusion due to light direction - are these shapes indented or sticking out?

Optical illusions
How we interprete form in two dimensional images
An illusion due to direction of light
How light direction affects how we read shapes and form

Are these shapes indented or sticking out?

This is a photograph of the word ‘post’ on the side of a British post box (the red pillar box).
Viewed upside down the word looks as though it is indented. The right way up it appears to stand out (which it does).
Why is this?
It’s because in normal everyday circumstances light tends to come from above (such as from the sun in the sky), so that’s where we assume light is coming from unless there is obvious evidence to the contrary. In the upside down version the light is interpreted to come from above, so the shadows and highlights are interpreted in this light
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