“The Music That Inspired the Movie ‘Michael Jackson’s This Is It’ ” (Sony)

The first of what’s sure to be many posthumous releases from the late, great Michael Jackson is a two-disc chronicle of his final days, rehearsing for his comeback. A leadoff single, “This Is It,” arrives Oct. 12 with backing vocals from Jackson’s brothers.

Co-producer John McClain told Billboard magazine the song “only defines, once again, what the world already knows – that Michael is one of God’s greatest gifts.” In addition to the title tracks and previously unreleased recordings, the CD includes the original masters of his hits in the order in which they appear in the film.

MOVIES | Director hopes ‘This Is It’ captures Jackson’s vision

As establishing legacies goes, this is indeed it. The world’s final, fleeting moments of Michael Jackson, who died suddenly in Los Angeles on June 25, will come courtesy of “Michael Jackson: This Is It,” a film composed of footage the late musician had shot for his 50-date comeback gig in London: as reference material for his choreography, movie segments intended to be shown during each concert and footage intended as bonus content for an inevitable DVD.

Famed choreographer and filmmaker Kenny Ortega, who was directing the concert show for his friend, says it’s a movie that almost didn’t happen.

“I didn’t want to do it,” the director of the “High School Musical” franchise and the upcoming “Footloose” remake says. “Michael’s death weighed on me hugely and still does. I was so shaken up, overwhelmed with grief and sadness, I couldn’t even think of editing the footage we had into a final film.”

It’s been a crushing year for Ortega, who also is mourning the deaths of Patrick Swayze and John Hughes, his colleagues on several films. Jackson holds a special place, though.

“We enjoyed a lot of the same things,” Ortega says. “While we were putting this concert together, we would giggle like kids, whispering ideas back and forth, and sometimes we would just sit together singing Broadway songs from ‘Oliver’ or ‘The Roar of the Greasepaint — The Smell of the Crowd.’ “

Ultimately, Ortega says he realized he had a certain responsibility to both Jackson himself and Jackson’s fans.

“I was so shaken up and overwhelmed with grief, but there was this film footage and a film was going to be made and I understood why,” he says. “In the days immediately after his death, there was such an outpouring from his fans. Many of them wanted to know what Michael had planned for them in that last tour. At the end, it became kind of a responsibility. It was part of the journey. Michael had all of these significant reasons why he wanted to go back out into the world again and do this tour. As long as we could bring those things into the film, then we would be sharing things that were important to Michael.”

And Jackson planned to give those lucky enough to score a ticket to the sold-out London concerts quite a show. Jackson’s “Thriller” video would be in 3-D with Jackson and dancers (as ghouls) popping out of the screen. Jackson’s “Earth Song” would be performed against the backdrop of stunning environmental shots, also shown in 3-D.

“It wasn’t enough to just have ghouls jump out of the ‘Thriller’ film to dance on stage. We had to have them jump off the stage and have them dancing down the aisles,” Ortega says. “It was about surprise. Just when you think Michael couldn’t possibly top something, he would say, ‘We’ve got to come back and continually surprise them.’ “

Jackson planned for the concerts to mark his retirement from world touring (but not necessarily recording or performing).

“The size of the production was enormous and ever-growing,” Ortega says. “There wasn’t a day we came to work that Michael didn’t have another idea that he was bringing into it.”

You could try to rein Jackson in, but it wouldn’t have any impact.

“He was the maestro, the architect, the bandleader. Michael was everything,” Ortega says. “You couldn’t stop this guy thinking. You couldn’t turn him off. Once he got on a roll, you just wrote it all down and somehow made it happen.”

No idea was too grand or too crazy.

“One night, I got a call at 3 o’clock in the morning from Michael and he told me, ‘We need Queen Victoria Falls,'” Ortega says with a chuckle. “We already had the world’s largest 3-D, high-def screen, so of course Michael wanted footage of one of the seven greatest natural wonders of the world on stage with him. He wanted the stage to reflect the world in all its beauty and all its drama.”

Jackson was working hard, but was he working too hard? Ortega doesn’t think preparations for the concerts contributed to the circumstances surrounding Jackson’s untimely death.

“The show was giving Michael purpose. It was nourishing Michael. It was the most important thing in his life outside of his children,” he says. “He was so looking forward to going out there and being with his fans. And it wasn’t going to stop after the 50 shows in London. He was really, really hoping that he would have had the personal strength and the success of the show to be able to take this out there for a major world tour.”

There were no outwardly telltale signs that anything was wrong.

“I knew he wasn’t sleeping a lot and I was concerned about that. He told me not to worry about it, he had it under control,” Ortega says. “I was concerned he was losing weight and that he wasn’t eating enough, and so I watched over him and always made sure he had plenty of health food and that he was hydrated. This was not a man who was burned out, though. The film shows you that Michael was at the top of his game.”

Jackson was all about working hard. “Roll up your sleeves, put on your dance shoes and when you’re done, all that should be left is a bunch of sweat on the dance floor,” Ortega says.

And so, a singer’s final curtain call for touring becomes his final curtain call.

“Michael’s hope at the end of the concert was that people wouldn’t be able to go to sleep at night,” Ortega says. “He told me, ‘I want them to be filled with so much from this experience that they’ll want to talk about it for the whole night.’

“He wanted to entertain them and give them the show of all shows,” he adds.

And with “This Is It,” he just might still do that.

It opens Wednesday, with some late-night screenings T