'Purple Rain': a poor movie, but fans flock to see rock star Prince.

Beneath its flashy visual style ''Purple Rain'' is a standard drama about a young man with a bad family life. The director, Albert Magnoli, pumps up the story with lighting and moving-camera tricks, as if he were staging ''Mourning Becomes Electra'' for MTV. But there's little tension to the plot, and its attempt to blend melodrama and comedy just doesn't work.

So why has ''Purple Rain'' become one of the year's hits, finishing sixth in the summer box-office sweepstakes? Because its star is Prince, a leading member of the current rock-and-roll royalty. As one critic suggested recently, he's like the flip side of Michael Jackson - projecting a darker and more turbulent image, just as the Rolling Stones embodied the nervous underside of the Beatles' generous optimism. He's an inventive and energetic performer, too, as the zillion concert scenes in ''Purple Rain'' attest.

Other elements of ''Purple Rain'' also play directly to the huge young audience that Hollywood zealously courts these days. There's nudity, some surprisingly graphic sex, music that's almost nonstop, and a shiny motorcycle. The warmed-over plot might contribute to the picture's success as well: The hero's conflicts with his family reflect common anxieties among young people.

But what about the film's nasty attitude toward women? Is this a reflection of current folkways - in which case there's terrible trouble brewing - or is it just a cynical way of dragging emotional violence into the story? Whatever the answer, it's hard to stomach a movie that keeps nudging you to gasp or giggle at aggression toward female characters; and the screenplay's feeble psychology (junior is only mirroring his troubled dad) is far too weak to serve as an excuse.

It would be fun to see Prince in a more substantial and less questionable enterprise, and given the box-office results of his movie debut, we'll probably have the chance before long. In the meantime, the rock star called Sting will also be inhabiting the large screen this year, in the upcoming ''Dune.'' So take heart, older viewers! If there's a trend toward musicians with one name - Prince , Sting - can a movie comeback for Liberace be far off? 'Cal'

There has been little intelligent filmmaking about current problems in Northern Ireland. ''Cal,'' a new movie by Irish director Pat O'Connor, can't fill the gap by itself, but it makes a heartfelt try.

The title character is a 19-year-old Roman Catholic boy who lives with his father in a depressed Ulster housing project. One of his more pugnacious friends has become involved with the illegal Irish Republican Army and occasionally recruits Cal to drive a getaway car.

On one such errand a Protestant policeman was killed, and this crime weighs on Cal's conscience. When he later meets the victim's wife, he falls in love with her. Having no idea of his IRA link, she innocently returns his affection. The results are tragic for Cal, reflecting the web of violence he heedlessly entered by failing to reject the terrorist behavior around him.

By trying to be a love story as well as a social and political drama, ''Cal'' spreads itself too thin. Since the romance cuts into the major issues and vice versa, neither casual nor serious moviegoers are likely to be fully pleased with the picture. The plot also leans heavily on coincidence.

In other respects, though, the movie's sense of balance serves it well. The screenplay (by Bernard MacLaverty) painstakingly shows vices and virtues on both sides in the Irish troubles, depicting rank anti-Catholic prejudice and Catholic economic woes as well as thuggish attitudes and violence associated with the IRA. The turmoil in Ulster is seen as a complex and self-perpetuating cycle with no easy scapegoats or solutions.

The performances are generally strong, though it's hard to see why Helen Mirren (as Cal's lover) won the Cannes Film Festival award for best actress earlier this year.

The producer was David Puttnam, whose impressive list of credits includes ''Local Hero'' and the forthcoming ''The Killing Fields.'' The rating is R, reflecting some graphic scenes of violence and sex.

Since the evenhandedness of ''Cal'' impressed me - it's a thoughtful drama, not a polemical one - I met with director O'Connor when he visited New York recently and asked how he came by such a philosophical view of Irish problems. He told me he has spent much time away from Ireland - at school in the United States and Canada, among other places -and so has developed some perspective on the situation.

Echoing the movie, he feels that all participants in the Ulster struggle have real grievances to make and real culpability to bear. Going beyond the movie, he feels progress toward a solution will occur when all sides become more concerned with dialogue, communication, and mutual understanding.

Can a motion picture help channel things in this direction? ''If it opens some minds and makes for a positive atmosphere,'' he answered, ''I think it can play a part.''

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