Infinitely Polar Bear Mark Ruffalo, Zoe Saldana, Imogene Wolodarsky. Directed by Maya Forbes. Rated R. Opens Friday.

Writer-director Maya Forbes based Infinitely Polar Bear on her own experiences growing up with a manic-depressive father, and the movie exudes familial love and forgiveness. That spirit of generosity also helps the movie avoid typical mental-illness melodrama, even as Mark Ruffalo goes big in his performance as bipolar bohemian Cam Stuart. Cam’s problems are serious, but he’s also a genuinely devoted father to Amelia (Imogene Wolodarsky) and Faith (Ashley Aufderheide) and husband to Maggie (Zoe Saldana), even if she’s not sure she can stay married to him.

She does trust him enough to take care of their kids while she goes away to business school, though, and Cam’s efforts to be a good father are hilarious and heartbreaking. The kids, too, mix precociousness with sadness as they deal with the mental illness of a man they love and revere. Set in 1978, Polar Bear is loosely plotted and has the nostalgic feel of home movies (including some footage that mimics the look of home movies of the era). It doesn’t glamorize mental illness, but it also doesn’t pretend that mentally ill people can’t be caring, worthwhile human beings. For Forbes and for her characters, that’s just another part of growing up.