"Zidane: A 21st-Century Portrait" is based on a gloriously simple and audacious idea. To train the camera on the great footballer over the course of a single match: 90 minutes, in real time. On the ball, and mostly off the ball: just Zidane. Douglas Gordon occasionally inserts TV coverage clips for context but otherwise the camera remains on Zidane and his face, as gaunt and impassive as an Easter Island statue, massively dignified in the deafening cauldron of noise. He runs; he frowns; he pants; he spits. He is always watchful. Occasionally, he bursts into action.