The loneliness of big city life, what it means to be human, and how movies can teach you all you need to know are just some of the themes in a Japanese film about a sex doll that comes to life, says its director.

"She's a doll but she becomes human because she can feel inside her the breath of someone else, the soul of someone else," Hirokazu Kore-eda said in an interview after the Cannes festival screening of "Air Doll."

South Korean actress Bae Doona plays the eponymous doll whose creator secretly fits a human heart into the rubber body which is then bought by a solitary middle-aged waiter in Tokyo.

Kore-eda took the idea from a short manga comic book and broadened it out to have the doll ditch her owner and take to the streets where she meets and falls in love with a video store clerk.

The result -- which got its world premiere Thursday at the non-competitive Un Certain Regard section of the Cannes festival -- is a bittersweet exploration of both the pain and joy of being alive.

The world is new to her and she begins to learn its ways through the varied cast of characters she encounters, explained Kore-eda, who has shown his work before at Cannes and whose previous film was titled "Still Walking".

She quickly realises however that everyone she meets in the Japanese megalopolis has a similar kind of hollowness to her own. But she overcomes that emptiness by reaching out to others, said Kore-eda.

"She has a rich life because of the way she lives and the way she's in contact with other people, and of course she needs someone else to put air inside her," he said.

"That might make people think about the need to connect with other people in order to fill the emptiness in their own lives," he added.

Bae Doona said in the film's production notes that the character she portrays has a soul "as pure, as beautiful and as spotless as a new-born baby's."

But Kore-eda added that becoming human also meant being subjected to human imperfections.

"What I wanted to show is a doll that grows from a baby, then becomes a child and then a woman. And at the end she kills her boyfriend," he said. "So she is more realistic, and that comes with love."

The doll works for a while with her boyfriend in the video store and is introduced to a long list of contemporary and classic movies.

"All the films mentioned in the film are films that I like. When I was in my teens and my twenties I learned a lot from watching films, and I wanted to show this doll who also grows up and learns a lot through films," said the director.

Kore-eda said he had been aware that some critics might draw a parallel between a Korean playing a woman used as a sex object by a Japanese man and events in the Second World War when tens of thousands of Korean women served as sex slaves for Japanese soldiers.

"I knew there was a risk that some people might see it that way but to act this role I couldn't think of any Japanese actress," he said.