Spacial ambiguity is an optical illusion in which the brain is not able to wholly decipher depth perception throughout the entire image.
The foreground and the background can be swapped by focusing on different parts of the image. Sometimes hiding parts of the image from view makes the spacial ambiguity evident. Gradual falloff of light can also make a flat surface look like a rounded column. In some images the ambiguity is unsolvable
This effect seems to be more prevalent when there are multiple parallel lines and varying colour tonality.