It's not enough any more just to make a good film. For any self-respecting director, it's become about 'filming the unfilmable' - from Steven Spielberg's equine epic 'War Horse' to Tom Hooper's singalong-a-verite 'Les Miserables', via some time-hopping acrobatics in the Wachowskis' 'Cloud Atlas'. Ang Lee is the latest with his dream-like 'Life of Pi'.

And for the most part, he's pulled it off wonderfully. The many, many fans of Yann Martel's Booker prize-winning novel will wonder how anyone can capture the wandering, dreamlike odyssey of Pi, following the shipwreck that separates him from his zoo-keeping Indian family and other animals, on their way to a new life in Canada. For the main part of the opus, this wide-eyed teenager is stranded on a very big, very blue sea, accompanied only by a hyena, a zebra, an orangutan and a very non-sleeping tiger named Richard Parker.

The answer is a big water tank, lots of CGI and a miracle of casting. First, the CGI. I have it from the young actor's mouth that he has never actually met a tiger (red carpet publicists take note), but you'd never guess from the interaction between young Pi and the ferocious cat that threatens his existence even while it feels like they are two of very few souls left peopling the earth. The zebra is slightly less convincing, but the narrative sweeps along quickly enough not to worry too much.

Similarly, the wide seascapes come alive with 3D and Ang Lee's visual touch. The director has said that he is "humble in the face of water", and a mystical surrender to the elements is evident throughout, from the enormous storm that sends the zoo's animals flying into the deep, to Pi's experiences on a beautiful but deadly organic coral island. An aerial shot, as though from space, finding Pi alone on the sea while a whale swims underneath but undiscovered, is one of the most beautiful and symbolic images of the entire film.

And Suraj Sharma, an unknown teenager who only went to the auditions to accompany his brother, was born to play this role - from rubber-boned young urchin to weary-eyed loner overcome with experience and solitude, he is both intensely physical and movingly spiritual. Ang Lee has waxed lyrical about how Sharma became the beating heart of the film's production, and he must be delighted in his decision to place this epic on such skinny but pure shoulders.

Without giving away plot spoilers for those who haven't read the book, the whole thing only unravels - narrative-wise - in the final quarter of an hour or so, when the epic we have sat through for the past 120 minutes is 'explained'. Compared with the drawn-out sagas of the sea, Pi's concluding chat with a boggle-eyed Rafe Spall is told so quickly and elliptically that I wanted to hit 'Rewind' and hear that bit again, to make sure I got it all sorted. Except I wasn't at home with a box set. I was in a cinema wearing a pair of 3D speccies, so I just had to be as confused as Mr Spall and surrender instead to the spectacle of all that had gone before.

Life of Pi is in UK cinemas from Thursday 20 December. See some stunning stills from Ang Lee's epic below...

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