"All happy families resemble one another," Tolstoy wrote in the opening sentence of Anna Karenina. "Each unhappy family is unhappy in its own way."
The distinguishing feature of unhappiness for the Block family, which is lovingly dissected in the documentary 51 Birch Street, was its invisibility.
Doug Block, whose documentary film about his parents' marriage is named after the address of the family home in suburban Long Island, near New York, wonders ruefully in voiceover whether things weren't better that way: "When it comes to your parents," he says, "maybe ignorance is bliss."
Block, who makes a living doing wedding videos, has gathered footage of his family - and in particular of his parents - throughout his adult life. More than 10 years ago, he filmed long interviews with his mum, Mina, and dad, Mike, purely as a family document; despite the fact that he had some track record as a documentary maker, he had no intention of making a movie for public consumption. Even when his mother died and his father decided, after 54 years of marriage, to marry Kitty, a woman who had been his secretary 35 years before and go to live in Florida, he didn't see a movie in it.
"Then I went out to the house a couple of weeks before the movers came, thinking it might be my final time," he says, speaking from his New York apartment. "I went to gather some stuff and took my camera and I decided I would interview my father.
"He was not a guy who ever talked about himself and all of a sudden he couldn't shut up. It was the first time I ever heard him talk about my mother. And it was because she wasn't around any more. So I kind of decided on the spur of the moment to come back and film my father some more. I still didn't think there was a film there but when I came back and we were in the car and I asked him whether he missed Mom and he said no that it wasn't a loving association - it was just a functioning one. That was the moment when I thought there might be a film there."
More crucially, helping clear out his childhood home, he discovered three boxes full of 35 years of diaries in which his mother had kept a daily record of her emotional processes. What emerged from them was the portrait of a marriage far more complicated and troubled than he had ever imagined.
Block's film weaves together footage gathered over many years into a textured and nuanced family drama that has, at times, the shape of a thriller. There's plenty of artifice - even artistry - on show; it's an extremely accomplished piece of film-making, cleverly edited and scored. But Block says the process was far from easy.
"Let's just say there were a number of sleepless nights," he says. "When we were editing the section with the diaries, I felt an enormous burden of trying to capture who my mother was from little bits and pieces of words and phrases. She was a very complicated woman and I loved her dearly and she wasn't always easy, but to try and capture her in all of her complexity I was constantly worried that I was missing something."
Block is pretty meticulous about not letting his own feelings intrude; indeed, even in interview he struggles and stammers as he tries to articulate what he admits was a "complicated response".
"Partly I felt glad that Dad had someone in his life. He was 83 and the last thing I wanted him to be doing was living alone in that big house and Kitty obviously cared for him a lot. On the other hand I wasn't at first all that bothered. I think I was a bit stunned - I thought 'Okay good for him'. But sometimes I don't even know how I'm feeling until I start going there and I start thinking about it from my mother's point of view and I think, 'Hey, wait a second. How long was this going on?'."
Block is particularly gratified that the film means different things to different audiences. He showed it at the Miami Film Festival where the audience age averaged 60 and "the women were just so charged up, they thought my mother was some sort of feminist hero of the order of Betty Friedan and Gloria Steinem."
"I really thought this was going to be a film primarily for baby boomers - people in middle age dealing with aged parents, but different people are coming at it and getting different things out of it: younger people are seeing it as a film about marriage and older men relate to the father-son story."
But for the maker, it is finally a film about his father, who is unquestionably the film's strongest character.
"I really wanted to see what it was like for him," he says. "I think one of the reasons the film is so powerful is that you actually see me and him develop a closeness and a real relationship over the course of the film. That's something you see an awful lot in fiction films and in theatre and in novels but you rarely see that happening in documentaries - that things change over time."
Festival documentary picks
The Festival's documentary line-up this year - more than 50 titles including half a dozen music films - paints the picture of a planet in environmental, economic and political crisis. But the gloomy titles are only part of the picture.
Iraq in Fragments and The Blood of My Brother: Both take us deep into the hearts and minds of Iraqis to show a society suffering the collateral damage of the United States-led invasion - and are far from united about whether things are better or worse than under Saddam.
Black Gold and China Blue: Docos that will change the way you look at, respectively, your latte and your new blue jeans. Powerful indictments of the fiction that globalisation is good news for the Third World.
Memory for Max, Claire, Ida and company and A Lion in the House: This year's triumphs of the pure fly-on-the-wall genre, heart-rending but ultimately uplifting, look at, respectively, life in a home for elderly afflicted by Alzheimer's disease and children battling cancer in a Cincinnati hospital. They sound depressing; they are fantastic.
His Big White Self: The new film from Nick Broomfield who did the two documentaries on serial killer Aileen Wuornos (whom Charlize Theron played in Monster) is a deceptively low-key but chilling portrait of the unrepentantly racist South African neo-Nazi Eugene Terre'Blanche.
An Inconvenient Truth: Al Gore's lecture about global warming is riveting. If you ever felt a shred of scepticism about the crisis we face, see this film.By Peter Calder Email Peter