The overlay on the above H-a image was purposely defocused (using a mild Gaussian Blur) and the dynamic range
was compressed (resulting in fewer discrete levels) to simulate what at lower resolution appears to be an almost
crater-like depression in the solar chromosphere.

Moving  the cursor onto the image reveals the actual topographical features as detected with higher resolution and
wider dynamic range. It actually does contain a "floor" area of plage with activity rising all around and over it, but not
at the exaggerated depth as perceived with lowered resolution and dynamic range. There,  the darker areas lacking
structure and detail, blend and create the illusion of deep shadows... an effect further enhanced by encircling
filaments that seem to define an outer edge or rim... but this is merely an astro-Trompe l'oeil (fool the eye)!

Granted, dimensional variations do occur, as the chromosphere abounds in raging activity thrust from below... but
optical limitations and nuances can create illusions of something that is not there (not unlike canals or faces on

The full disk image from which these croppings were taken may be seen on my
21-July-2004 H-a page.
Depth Illusion
Effects of optical limitations: things may not always be what they seem!
Move mouse on/off image to observe depth illusion