Hirani is currently busy filming the biopic on Dutt, which features Ranbir in the role of the "Khalnayak" actor.
Recently Ranbir's new look from the film went viral online.
"The pictures shouldn't have come out. We were shooting in Carter road (in Bandra, a suburb in Mumbai) so it came out. But I am very happy. The film is being made with a lot of happiness and when a film is made with happiness, it turns out good. It's a great story," Hirani told reporters here.
Hirani was speaking at the look of filmmaker Tanuja Chandra's debut book "Bijnis Woman".