The prejudices and ideology implicit in Clint Eastwood's movies are ceaselessly fascinating - and certain to be a rich resource to social historians of various persuasions for years to come. Consider "Escape From Alcatraz," which opens today.

The movie begins with a portentous tease. Eastwood plays a convict brought to Alcatraz on a stormy night, stripped down and escorted to a cell. As he trudges along in the buff, between two prison guards, the suspense becomes killingly funny. Will the shadowy lighting and intermittent lightning reveal more of the star than we've ever seen before? The answer is just obvious as the tease.

One of the inmates, as obese psycho played by Bruce M. Fischer, evidently gets the wrong idea about this introductory promenade and tries to proposition Eastwood in the shower. He gets a sock in the gut and a cake of soap in the mouth for his pains, but he's kept around to pose a gratuitous menace whever the filmmakers consider it helpful.

Having cleared the air with the homosexual element, Eastwood proceeds to negotiate an accord with the blacks. After some amusing verbal sparring, the most esteemed black convict, Paul Benjamin, becomes so cordial with Eastwood that he steps in to disarm the vengeful Fischer at one crucial point.

The calculated diplomacy of the film, in which the hero takes a hard line with homosexuals while achieving detente with the blacks, suggests a potentially explosive idea for Eastwood. He could get the drop on projects such as "Cruising" by reviving Dirty Harry in a story about a murder wave in San Francisco's gay environs. It would be completely in character.

That character is both the strength and the problem with "Escape From Alcatraz." It falls between two stools: a fictionalized reconstruction of an authentic prison escape and an obligatory crowd-pleasing vehicle for Eastwood. And it's more ingenious and diverting when the filmmakers sneak onto the second stool.

The role of a convict on Alcatraz would appear to limit Eastwood's characteristic traits: steely indomitability and open contempt for perverts and figures of vested authority. The setting doesn't lend itself to throwing one's weight around or putting inferiors in their place.

The limitations are finessed with some cleverness, however, and there are a number of memorably definitive Eastwood remarks. For example, when a fellow inmate asks, "What kinda childhood didja have?" Eastwood replies: "Short." Another bulls-eye for the hard guy - but modest compared with one facetiously hostile punch line with a timing and inflection guaranteed to bring down the house.

Eastwood is spared the task of indicating that kind of smoldering defiance in his first audience with the warden, because the warden is played by Mr. Supercillious himself, Patrick McGoohan. Conceived as a pompous despot, the character is rendered so instantly hateful by McGoohan's peerless snootiness that no response is called for.

Curiously, the filmmakers didn't trust this loaded piece of casting and the prison environment itself. Screen-writer Richard Tuggle ends up straining to "justify" a three-man breakout, led by the Eastwood character, by showing the warden breaking the hearts of poor old inmates: He reduces Roberts Blossom to despair by taking away his painting privileges and provokes a coronary in Frank Ronzio by crushing his chrysanthemum. Who needs these absurd depradations? McGoohan doesn't have to do anything to be disagreeable. The mere sight of him is enough to set a thousand escape plans hatching.

Director Don Siegel and his colleagues tend to bog down when they turn from Eastwood's familiar image to mere escape melodrama. The script is based on the true story of the Rock's last breakout, on June 11, 1962, which suffers from built-in dramatic liabilities. The fate of those escapees has never been determined. Perhaps they got away, perhaps they drowned in San Francisco Bay. Since the film-makers do not speculate about what happened to them, the movie is destined to fade out without a payoff.

The 1962 episode exploded the legend of Alcatraz as the country's most forbidding penitentiary. The escapees had patiently exploited the fact that the joint was beginning to fall apart. Corroded over the years by the salt air, the cement walls proved vulnerable to men sufficiently motivated to chip holes through them. The cost of making the necessary repairs seemed so excessive that the prison was phased out and shut down less than a year later.

Although Siegel and cinematographer Bruce Surtees attempt to depict the details of the escape work diligently and suspensefully, they never take hold in the obsessive way of similar minutiae in Robert Bresson's "A Man Escaped" and Jacques Becker's "Le Trou."

In fact, the busy work connected with the escape may not have enough clarity to keep audiences from becoming restless or drowsy. A lot of the detail work seemed not quite visible, either because the compositions were slightly oblique or the lighting was too murky.

Watching the French films, one does not feel confused about what the hands or objects were accomplishing, and they certainly weren't operating under full illumination. It must have been more dramatic illumination. The escapes in those movies, and in John Sturges' adventure epic "The Great Escape," certainly appeared more photogenic than of "Escape From Alcatraz."

"Escape From Alcatraz" never seems to find a comfortable niche in the alcove of prison-movie tradition. It's neither an exceptional escape story nor an effective, semi-tendentious shocker such as "Brute Force," "Caged" or Siegel's own "Riot in Cell Block 11." It's not a biographical story like "Birdman of Alcatraz" or an urgent, realistic update on prison culture, such as "Short Eyes" or "On the Yard."

It's a half-baked stopover in the big house, relying on Eastwood, rather thana particular prison "theme," for forcus and continuity. For better and worse, Eastwood's peculiarly intimidating personality - solitary, sarcastic, fearless - has become its own predominant, suggestive theme. "Escape From Alcatraz" is poorly orchestrated, but the Eastwood melody still comes through, laconic and clear. CAPTION: Picture 1, Clint Eastwood in "Escape From Alcatraz"; Picture 2, Clint Eastwood in "Escape From Alcatraz"