As three of Titian's greatest masterpieces are shown together for the first time at the National Gallery, imagine... goes behind the scenes of a unique collaboration with the Royal Ballet.Alan Yentob meets the all-star creative team who are transforming mythological paintings of the goddess Diana into contemporary dance - among them Turner Prize winning artists Chris Ofili and Mark Wallinger, choreographers Wayne McGregor and Christopher Wheeldon, and composer Nico Muhly.Alan also explores why Titian's paintings - and his source material, Ovid's Metamorphoses - have continued to inspire so many artists to this day; and along the way, he encounters Seamus Heaney... and a dancing robot.