There are almost 65,000 frames in the feature film Loving Vincent, each one of them painted by hand in oils.
Animation is always an arduous process, but "ours was the slowest form of animation ever invented", says Hugh Welchman, co-director and co-writer of the film. "Everything you see in the film is a photograph of a painting - we weren't using different layers [as happens in most animation], it was very purist."
Welchman, an Englishman who won the Oscar for best animated short in 2008 for Peter & the Wolf, spent six years working on this tale about the last days of Vincent van Gogh. His co-writer and co-director Dorota Kobiela, who is Polish, spent even longer on it.
"She spent seven years on it," says Welchman of his wife. "She'd already got the grant to do this as a short film, but in Poland you have to wait three or six months until the money comes into your account. So in the interim she applied for a job at my company, and I disrupted her plans.
"We fell in love, I got her to work on my film [Peter & the Wolf] and while we were doing that we talked about Loving Vincent and visited lots of galleries. And as we were doing the research I realised how huge his following is around the world, and so we decided to make it as a feature film."
There are two distinct visual styles in the movie: van Gogh's trademark heavy brushstrokes, thick textures and bold colours; and a black-and-white realist style informed by 19th-century photography. In both, some of the characters seem vaguely, and sometimes instantly, recognisable - their scenes were played by actors including Chris O'Dowd, Aidan Turner and his Poldark co-star Eleanor Tomlinson, and Saoirse Ronan.
Their performances were captured on film, and that footage was used as a reference point for the painters to render them in either van Gogh or photo style.
"The painter would have a monitor at eye level, then they'd have a canvas underneath, and they'd just paint what they could see, but they'd have to reimagine it into van Gogh style," explains Welchman. "Once they'd done the first frame, they were using that for reference. They'd reimagined it, so they had to stick with that, and they had to not only reference their own frames, but what others had done in the same scene or with the same character."
Such a labour-intensive process necessarily demands a lot of labourers. Luckily, in Poland Welchman and Kobiela knew there was a suitably skilled (and relatively cheap) workforce, because of the country's strong tradition in art training. From 1000 applicants, they picked 65 painters. But putting together the finance for the project was almost as arduous as making the thing, so they were only able to hire about 20 at the outset.
By the time the funding came together, they were massively behind schedule and they felt they'd exhausted the talent pool in Poland - given the pay, about $1000 a month, it's likely many had moved on to more lucrative offers - so they put the message out internationally. They couldn't believe the response.
"We had this teaser on our website, and a fan of one of the actors put it on Facebook and within 24 hours we had 2 million views and within three months we had 200 million views. As a result of that we closed the finance, and we got 4000 new applications from painters around the world."
They hired another 60 painters - from Australia, Japan, North America, South America, India - and it gave the production the boost it needed. "A lot of our painters had been working on it for a year and a half and were pretty exhausted, and then all these painters descended from all over the world and were terribly excited," says Welchman. "They were only there for six months, so they managed to maintain their enthusiasm to the end."
Andrew Grimmer was the sole Australian who worked on the project, contributing about 12 seconds of footage (at 12 frames a second, that's about 140 paintings), mostly of an exchange between the central character Armand Roulin (Douglas Booth), who investigates Vincent's death, and Dr Mazery (Bill Thomas), who has a controversial theory about it.
"I made three or four frames a day, six days a week for over five months," says Grimmer, a skilled portraitist from the Mornington Peninsula in Victoria. "It doesn't equate to much of the movie, but I'm happy with the quality I achieved while having to paint at such speed."
Grimmer heard about the project when a friend sent him a link, with the words "this could change your life". He applied, heard nothing for three months, then one day opened an email. "You've been chosen to test in Poland, we will Skype tomorrow."
Three days later, the production got in touch and offered him a spot. He'd need to be in Gdansk, joining a team of 65 painters in a studio, in two weeks. "I had no passport, no money, and my partner and two-year-old child were in Australia," he says. "But I agreed."
It was hard work, Grimmer says, but "the comradeship was beautiful and I made life-long friends. There were 15 or more of us living together in a hostel and we grew very close." He would do it again in a heartbeat.
Despite the challenges, Welchman would too. In fact, he and Kobiela are already plotting a new movie in a similar style.
"We're going to do a painted horror filming, based on the late paintings of Goya," he says. "You've got to set yourself a new challenge in life."
Loving Vincent is in cinemas now.