LOS ANGELES • Something seismic was happening during the Academy Awards ceremony on Sunday.

The Hollywood establishment, excoriated for its longtime exclusion of women and minorities, recognised African-American production design and costume virtuosos for the first time. Asian-American film-makers were honoured. A movie about a gay rock star collected four trophies.

"I want to thank the academy for recognising a film centred around an indigenous woman," Alfonso Cuaron said as he accepted the award for best director for Roma, about a domestic worker in Mexico City.

But then came Green Book. In a choice that prompted immediate blowback - from, among others, director Spike Lee, who threw up his hands in frustration and started to walk out of the theatre - the Academy of Motion Picture Arts and Sciences gave the best picture Oscar to a segregation-era buddy film.

While admired by some as a feel-good depiction of people uniting against the odds, the movie was criticised by others as a simplistic take on race relations, both woefully retrograde and borderline bigoted.

It was the ultimate Lucy-pulling-away-the-football moment for those who had hoped the film academy was going to reveal itself as a definitively progressive organisation. That the 2017 selection of Moonlight as best picture was not a fluke. That the efforts to diversify its membership - albeit still 69 per cent male and 84 per cent white - had been transformational.

Adding to the anger over Green Book were the other choices available.

Ryan Coogler's Black Panther was a cultural and commercial phenomenon, shattering a myth about the overseas viability of movies with Afrocentric storylines. Roma, a nuanced examination of class that was made by an almost entirely Latino cast and crew, had been showered with honours at the pre-Oscars award shows.

And Lee's BlacKkKlansman, about an African-American police officer who infiltrates the Ku Klux Klan with the help of a Jewish surrogate, was a chance for the academy to recognise one of cinema's singular, groundbreaking film-makers - one who had been repeatedly overlooked in the past.

"A lot of people may have allowed their expectations of the academy to become too great," said Dr Todd Boyd, a cinema and media studies professor at the University of Southern California who focuses on popular culture and race. "We can see some signs of changes, but there has not been a full transformation."

Green Book, based on a true story, focuses on a working-class Italian-American man (Viggo Mortensen, nominated for best actor) who gets a job as a chauffeur and bodyguard for a gay African-American pianist (Mahershala Ali, who won the Oscar for best supporting actor). As they drive through the South in 1962, the mismatched men begin to realise they have common ground. At one point, they bond over fried chicken.

"The whole story is about love," Peter Farrelly, the film's director and one of its writers, said in his best-picture acceptance speech. "It's about loving each other despite our differences and finding the truth about who we are. We're the same people."

That message resonated with many people, including black cinema standouts like writer-director John Singleton (Boyz N The Hood, 1991) and Octavia Spencer, an Oscar winner in 2012 for The Help, who served as a Green Book executive producer.

Green Book received an A-plus grade from ticket buyers in CinemaScore exit polls and has become a modest hit, collecting US$144 million (S$194 million) worldwide for Universal Pictures and Participant Media.

Despite pre-Oscars backing for Green Book, vocal supporters were hard to find after it won. A half-dozen academy members declined to be interviewed on Monday about the film's victory, including several who supported Green Book.

Several lamented that the Oscars were no longer about movies but rather advocacy.

Others in Hollywood spent the day scratching their heads and wondering if Green Book had won simply because it was a studio-backed film. Roma was released by Netflix - largely skipping theatres - and some voters seemed unready to crown the streaming service.

It was also possible that Green Book benefited from the academy's complicated "preferential" voting system for best picture, in which nominees are ranked first through eighth, and the second-and third-place positions can carry as much weight as first place.

Cries of foul, however, were plentiful.

Dr Boyd said that, in part, he saw the selection of Green Book as pushback by older, more conservative voters. "There are inherently people in the academy who think the organisation's diversity efforts are going too far," he said.