You cannot take a picture of a person where the person is the subject of the photo. This is not the case when you take a picture of a statue and there's people in the background, for example.
You would have to justify it anyway and personal rights in Germany always trump the rights of others to create art, for example. You'll find many cases that are about surveillance cameras as well and even there's it's often in favor of the person not wanting to be on camera.
https://dejure.org/dienste/vernetzung/rechtsprechung?Text=VI...
http://lrbw.juris.de/cgi-bin/laender_rechtsprechung/document...