I’m guessing the ads for “Infinitely Polar Bear” won’t say “From the writer of ‘Diary of a Wimpy Kid 3: Dog Days’!” But Maya Forbes did write that film, along with “Monsters vs. Aliens” and a bunch of other Hollywood films that one wouldn’t exactly call personal.

But Forbes dug deep into her family history for her debut as a writer-director, and “Polar Bear” is a beautiful story, warm and generous of spirit. Family dramas often ask the audience to pick sides, or conjure up a villain. Here’s a story about a family who loves each other just . . . coping . . . with some difficult circumstances, and it’s wonderful filmmaking.

Cam Stuart (Kenosha’s own Mark Ruffalo, in his best role since “You Can Count on Me”) was a scion of Boston blue blood who got kicked out of Harvard and Exeter for his free-thinking, high-living ways. And this is in the ’60s. But those ways attracted Maggie (Zoe Saldana), who married him , and they had two biracial daughters, Amelia (Imogene Wolodarsky, Forbes’ daughter) and Faith (Ashley Aufderheide).

Turns out that Cam wasn’t just eccentric, he was bipolar, and hospitalized after a massive nervous breakdown. Most of “Polar Bear” takes place in 1978, when Cam has just been released to a halfway house and is slowly rebuilding himself. If he keeps to his lithium, he can function. Maggie still loves him but is wary, and she is deeply worried that the family is running out of money.

She decides she needs to go to business school in New York to keep the family afloat, and has a proposition — Cam will take care of the girls while she’s away. This brought me up a little short, because we’re used to seeing people with mental illness treated as children at best. But Forbes’ father really was bipolar (“Polar Bear” was his joking nickname for his condition) and her mother did go away to business school. She knows that dealing with mental illness is a matter of managing, day by day, and that knowledge and compassion imbues every frame of the film.

Ruffalo is almost always good (hey, he made the best Hulk, the ultimate in bipolar characters) but Cam Stuart gives him so much to dig into. He’s a doting father and husband, but he’s also got this hunger to connect to the world around him, whether he’s digging into eccentric projects on the living room table or being overly friendly to his neighbors. And then there’s the fact that he comes from old-money Boston, waving his cigarette between two fingers and speaking as if he’s in a Noel Coward play. There’s so many layers, so many seeming contrasts that all get pulled together to make this funny, heartbreaking, completely fascinating person.

Many movies would make the practical Maggie as sort of a wet blanket on Cam’s parade, but Forbes is careful to show that she’s just as loving a parent as her husband, under tremendous pressures to provide for her family, and Saldana captures all that in a lovely and deeply-felt performance. The daughters seem like real kids, resilient and doting, alternately worried for and embarrassed by their larger-than-life dad.

It just seems like a rare portrait of a real family, just coping together with the many challenges thrown their way. Forbes has done such a beautiful job with “Infinitely Polar Bear” — the only downside may be that she’ll find it hard to go back to “Wimpy Kid 4” after this level of filmmaking.