Is it illegal to photograph others' backs?

Taking pictures of other people's backs is not an infringement of the right to portrait. Because the key element that constitutes the infringement of portrait right is that the object of infringement needs to have distinct directivity, and it is impossible to effectively identify this person simply by relying on the back, then without directivity, it is impossible to determine the victim. Without the victim, there is no violation of the right to portrait.