how to determine the geographical area that is shown?


I wonder if it is possible to determine the geographical area that is
currently being shown in a cesium viewer, even if by just an approximation?


See Screen Coordinates.

did you somehow catch the event of the user moving the camera
around, zooming, etc. and call after each such movement? or were you
working with a static camera view position?

I save the properties of the camera and I compare it with the current camera all time on tick() function. If the camera changes (the user moved the camera or zoom) I call the getExtentView function.

thanks, such polling is something I thought about myself

Lots of people want this functionality, including me, but it’s actually pretty difficult to pull off accurately. I wrote an issue up a while ago ( hopefully Dan or someone better at Math than me can look into it in the near viewer.