Philip Glass Live on Film, The Barbican, London

In his fascinating new book on Philip Glass, my colleague Robert Maycock observes that "you probably do not listen to the music in [a] film from the front of your mind". Yet, as Glass reminded us in a public interview after Shorts, the first night of this week-long festival, images alone can be very ambiguous. Only when supplied with music does their mood become clarified (but even then, music can be demandingly, deliciously ambivalent too).

As usual, the images seem to have come before the music in the process of making the four new films offered here (plus two older ones by Godfrey Reggio). But the most unusual aspect of their creation was that the composer selected the film-makers. And with one exception, apparently (Peter Greenaway's The Man in the Bath), Glass achieved a flexible collaboration in which he doubtless got his own way much more than film-music composers normally do: something aptly reinforced here by the immediacy of hearing the music performed live.

But if you think this means that the results consist mainly of Glass's signature chugging arpeggios, you'd be wrong. Well, some of the time, anyway. The most successful musical departures came in the dissonantly hieratic score for Atom Egoyan's Diaspora, which turned its footage of fleeing, multicoloured sheep into a resonantly disturbing experience. A pity that in spite of evidently close collaboration, the composer wasn't stretched by Michal Rovner's groups of moving figures on inclined planes, like notes on a musical stave.

If you think Glass's music is so transparent that it'll fit any old image, then think again too. For Shirin Neshat's Passage, focusing powerfully on a desert burial ritual, Glass filtered his usual fare through a Persian perspective provided by transformations of Neshat's own chanting.

The following night, the performance of La belle et la bête, Glass's 1994 reworking of Jean Cocteau's 1946 film, demonstrated his ability not only to follow Cocteau's emotional as well as structural leads, but also to put his own spin on a remarkable film classic. In substituting his setting for the original soundtrack, Glass manages to make anything sounding familiar from his earlier scores seem remarkably fresh and dramatically apposite. Any awkwardnesses of word setting or harmonic motion deriving from the limitations this ingenious approach imposes – here including lapses in synchronisation between the live singers and the actors on screen – seem minor. Some problems of projection and balance between voices and instruments marred an otherwise good performance.