“This is not fun yet,” a man says halfway through “Caniba,” shortly after piercing his own skin with multiple knife blades. The audience for Verena Paravel and Lucien Castaing-Taylor’s severe, baldly shock-strewn documentary may feel something similar, while suspecting that the “yet” is probably unnecessary. A 90-minute wallow in the intimate company of Japanese cannibal Issei Sagawa — poring over the salacious crimes of his youth, the grim death wish of his future and the love-hate relationship he has with his brother and carer Jun — this latest unidentified factual object from Harvard’s convention-busting Sensory Ethnography Lab project makes a proud point of its unpleasantness. Audiences are invited to consider the nature of their revulsion, and whether it’s rooted simply in fear of the other, or recognition of the abject in themselves.

How illuminating or challenging “Caniba” proves for viewers will depend on their amenability to Paravel and Castaing-Taylor’s amoral stance and literally up-in-your-face technique. Those who aren’t provoked by its ambiguous psychological inquiry, however, may wish for a bigger human picture from this relentlessly close-up exercise. The sheer, brazen extremity of its subject matter, coupled with its unblinking perspective, guarantees “Caniba” a long, loud festival run, already off to a contentious, walkout-heavy start in Toronto and Venice, where it took a special prize from the Horizons jury. It’s a film made for post-screening debates on the liberal-minded documentary circuit, though what, if any, commercial potential distributors might see in it is another question. Where Castaing-Taylor and Paravel’s celebrated 2012 fishing-trawler immersion “Leviathan” attained niche cinematic event status on the strength of its bravura technical innovation, this is a dingier work in all respects.

That murkiness begins with the image that dominates the film: a tight, unforgiving, often grimily unfocused close-up on the ravaged face of Sagawa, now in his mid-sixties, as he reflects (or evasively doesn’t, as the case may be) directly to camera on a life of unacceptable desires. In 1981, when he was a mature student at the Sorbonne in Paris, Sagawa was arrested for the murder of his classmate Renée Hartevelt — two days after he had shot her in the head during a joint study session, raped her corpse and ate a significant portion of her remains.

Declared legally insane and unfit for trial, he returned to Japan, where he has lived off his lurid crime ever since, whether by writing books and manga comics recreating the murder, performing in particularly specialized pornography, dabbling in food criticism, or submitting to the scrutiny of multiple documentary makers. Of the latter, Paravel and Castaing-Taylor position themselves as, if not the most sympathetic, the most open-minded to their subject’s history. Per the film’s press notes, the pair wish to distance themselves from other “predictable moralistic pieties” about Sagawa, instead seeking parallels between cannibalism and the larger human condition. Not insignificantly, those key horrific facts about Sagawa’s past are scrunched together in a dense, fast block of text at the film’s outset; that his potted biography is hard to read in detail seems deliberate, as the directors instead prioritize our present-tense appraisal of the man himself.

It’s not a pretty picture, either way. Severely incapacitated by diabetes and the after-effects of a recent stroke, Sagawa spends his days haunted by his anthropophagous acts, which is not to say he’s remorseful. Still consumed by the desire for human flesh, he yearns to die at the hands of another cannibal, and thus to be released from a life of ongoing local notoriety that, if hardly equivalent to imprisonment, comes with its own restrictions. He is wholly dependent on the care of his brother Jun, ostensibly a more functional man who still professes incomprehension over Sagawa’s affliction — but gradually reveals unorthodox sexual and sadomasochistic fetishes of his own, in what seemingly emerges as a peculiar bid to upstage his more (in)famous sibling. Could it be that he’s at once aghast at his brother’s cannibalism and perversely envious of it?

As the film’s claustrophobic focus shifts between the brothers — both narratively and visually, as their cropped faces jostle for the foreground of the frame — their improprieties become oddly indistinguishable. In so blurring their identities and urges, “Caniba” further tacitly invites viewers to consider their own most nominally freakish impulses, and the distance between those and Sagawa’s. It’s a mileage-may-vary tactic that stops just short of empathy (an arguably unnecessary but safeguarding disclaimer asserts that the film does not condone its subject’s actions), but is nonetheless startling in the way it deflects attention from the principal cause of Sagawa’s celebrity. The film is rather more intriguing as a warped fraternal study than it is as an individual portrait — by this point, Sagawa himself seems quite exhausted by his own past — though it’s perhaps a subject not most expansively or satisfyingly served by the film’s physical tunnel vision. With so little of their lives visible or discernible beyond their faces’ craggy confines, more questions arise — about their upbringing, their external relationships, their place in the Japanese system — than are answered.

While Paravel and Castaing-Taylor take a lateral, liberal view of their subject, “Caniba’s” gaze isn’t necessarily a humane one: The nosy, clammy closeness of the camera can have an alienating effect on the incomplete faces it studies, often from an austerely low angle that makes Sagawa appear, however near, far from approachable. That the filmmakers themselves seem undecided in their view lends proceedings queasy tension, only somewhat relieved when they go in for stray, slightly smug jabs of ghoulish black humor. When the film knowingly flirts with outright exploitation by dipping into Sagawa’s bizarre pornography, or segues disorientingly into a perky karaoke routine set to British new-wave band The Stranglers’ Sagawa-inspired 1981 track “La Folie,” its tongue is wedged most firmly in cheek — which is, given the topic at hand, perhaps the safest place for it.