What makes Shohei Imamura’s films so robust and pleasurable, even when the camera seems to impose an almost clinical gaze, is their irrepressibly earthy sensibility. There’s an often grimy, grindhouse candor to animate his social critiques, which most American audiences know only by such refined, latter-day productions as the elegiac “The Ballad of Narayama” (1983) and the similarly death-shadowed “Black Rain” (1988).

It’s a quality that was accurately implied by the programmers at BAMcinématek when they decided on a theme for their landmark Imamura retrospective, which begins tonight and runs through March. Pimps, prostitutes, and pigs were some of the director’s best friends, not to mention serial killers, bar girls, hapless pornographers, and petty hoodlums.

Imamura, who died last year at 80, had, during a 45-year career, become Japan’s greatest living filmmaker not only because he enjoyed a splendid and productive late phase — with festival prizes and art-house fanfare for his final films, “The Eel” (1997), “Dr. Agaki” (1998), and “Warm Water Under a Red Bridge” (2001) — but also for the way he unflinchingly mapped the gamier precincts of the postwar landscape.

Those porkers are no mere metaphor. Though BAM’s series abounds with all the rare and scarcely distributed Imamura titles any fan could ever hope to see, chief among its 20 selections is the rib-splitting “Pigs and Battleships” (1961). A broadly comic saga of occupied Japan, its lowlife antics transpire in the port town of Yokosuka, whose black market thrives amid the influx of American servicemen. A series of unfortunate events turns a wannabe gangster’s pork-vending scam into so much hogwash, as hundreds of pigs stampede, trampling the exploitative intents of the local crime syndicate and the Yankee arrivals alike.

The film, in all its sordid vigor, represents Imamura at his most freewheeling. The most memorable scene involves a yakuza version of the Three Stooges who, having whacked a rival and tossed him in the pigpen, later slaughter one of the swine for supper and discover … well, let’s say they need a few extra toothpicks for this barbecue.

The evident irony in Imamura’s biography is that he came to such a bawdy aesthetic. His father was a physician, and his formative introduction to the film industry was as an assistant to Yasujiro Ozu at Shochiku studios, where the master crafted his delicate studies of Japanese domesticity with a Zen-like stillness and immeasurably placid tone.

When, after an initial run of features in the late 1950s, Imamura busted out of the gate with “Pigs,” he did so with a fervor that seemed revolutionary, suggesting a whole new way for Japan to look at itself. Of course, as all the bios mention, the collegiate Imamura also worked for a spell as a black marketer, which surely lent his vision its indelible twist.

By the time of “The Pornographers” (1966), the most widely seen of the director’s earlier works, Imamura was happily adept at incisive low humor, but he also continued to polish his skills as a social observer. His sympathetic depiction of a hamstrung smut peddler as cultural anthropologist had satirical and self-reflective intent.

A filmmaker is a filmmaker, after all, and few of Imamura’s images are as revealing as actor Shoichi Ozawa’s eyeball scrunched-up to a cluster of Super-8 cameras that he’s lashed to a board in a primitive bid for mass production. Lensing bargain-basement kink in the forgotten corners of Osaka, the titular pornographer Ogata finds it’s never easy being sleazy, though his scopophilia is more than matched by the perversely inventive Imamura, who shoots one sex scene from the point of view of a carp in an adjacent fishbowl.

Films like “The Pornographers” and the often astonishing portrait of a serial killer, “Vengeance Is Mine” (1979) — which is being screened in a new print — are only the appetizers in BAM’s remarkably comprehensive retrospective. Among others not to miss are such absolute rarities as the documentary “History of Postwar Japan as Told by a Bar Hostess” (1970; the epic “Deep Desire of the Gods” (1968), an ethnographic drama set on the remote Ryukyu Islands, whose natural landscape proved ideal for Imamura’s first color shoot; and “Intentions of Murder” (1964), a signature effort in its sympathies for strong women in a weak-minded world, which deploys avant-garde style to render a story of sexual abuse and payback.

All in all, it’s as singular and sustained a body of work as any filmmaker can aspire to, made by a prophet of his nation’s subcultural funk: something that would propel many a lesser voyeuristic film or photography career for decades after the 1960s. Before Japan was shocking and extreme, Shohei Imamura was doing the dirty work.

Through March 29 (30 Lafayette Ave., between St. Felix Street and Ashland Place, 718-636-4100).