Lee, Will Smith and other crew members work on the filming set.[Photo provided to China Daily]

Lee said at a news conference before the film's Shanghai premiere on Monday that he may know Smith's face "better than his birth mother".

"People perceive human faces in strange ways," Lee says.

"The lighting, the circumstances … lots of elements are involved. Sometimes, even the imagery from his early films wasn't convincing enough."

Shooting at a high frame rate also means more information is shown every second. That, in turn, means higher demand for creative ideas, strategies and performances.

"We live in a digital age, and young people today have sharp eyes," he says.