CCTV footage of the alleged attack slap by Dappy on a man outside Chicago's nightclub in Chelmsford, Essex, was played to the court.
The black-and-white footage, which lasts about 10 seconds, showed the former N-Dubz singer "surrounded by security" when he turned and "slapped" George Chittock, Prosecutor Jaqueline Carey said.
Dappy, who's real name is Dino Costas Contostavlos, was acting out of "annoyance" in a "violent and unjustifiable manner" after claiming he was verbally abused during the night, she told the court.
Jon Harrison, defending, said early in the evening Mr Chittock told Contostavlos: "Dappy, you're a mug, I'm going to knock you out."
Mr Chittock replied: "Incorrect."
More top news
Defying cross-continental outrage and an international ban, Japan's whaling fleet has set off for a decades-old whale hunt.
Star Wars actress Carrie Fisher has hit out at the "messed up" film industry where "the only thing that matters is weight and appearance".
US marine Joseph Scott Pemberton has been found guilty of killing a transgender woman in a Philippines hotel.