The rare 16mm concert footage sat in a barn, undeveloped for “30some odd years.”

As it was of a performance by Jimi Hendrix, with the infamous legal complications that tied up his music, estate and legacy after his death involved, we don’t have to wonder too hard why that was.

But it was shot by cinematographer soon-to-be-director (“The Buddy Holly Story”) Steve Rash, who kept it. And it’s Hendrix live, the “Voodoo Child,” for Pete’s sake. It couldn’t stay hidden forever.

“Jimi Hendrix: Electric Church” premiered on Showtime. But no matter how big your TV, you have to think the biggest screen is where this wondrous relic of the pop festivals of the 1960s belongs. Now his 48 minute late-night set from the July 4, 1970 edition of the Atlanta Pop Festival is making its way into select cinemas. (The current booking list is here.).

John McDermott’s film opens with a relatively thorough half hour of context — with people like Billy Cox (the Jimi Hendrix Experience’s last bassist), Paul McCartney and Steve Winwood singing the praises of the original “Guitar God.”

Paul McCartney says it was the Jimi Hendrix moment — “He was finally coming in the front door,” a star whose time in Britain gave him the swagger and confidence to accept his due — great fame. “We worshipped him.”

Rick Robinson of The Black Crows marvels at his “purity of intention.” Derek Trucks opines that “When something’s real, it’s going to last.”

And Kirk Hammett from Metallica adds that “He took a fairly pedestrian instrument, a (Fender) guitar primarily used by surf bands and country musicians, and turned it into a lethal weapon.”

Context? The Atlanta Pop Festival was staged on a race track and neighboring pecan orchard, with epic traffic jams, a lot of nudity, sex, pot and litter, hundreds of thousands of “hippies”  in the middle of segregated South Georgia (in Byron, Ga., 100 miles from Atlanta) pretty much without incident. It was the “last great pop festival” of the era, coming before The Rolling Stones/Hell’s Angels debacle at Altamont, California.

Jimi’s soundman, Abe Jacob, recalls that everything this band needed on tour was packed into a 19 foot truck — sound system, instruments and merchandise to sell to the fans, plus a sound man, two roadies and a driver. That turned Abe into a philosopher of rock.

“The amount of talent you have is inversely proportional to the number of trucks it takes to put your show on the road.”

And this “Electric Church” that Jimi was ministering to? Dick Cavett asked him about this “ambition.”

“It’s a belief I have…That the electricity comes through us to the crowd,” Hendrix told Cavett on his TV show. The Jimi Hendrix sound “doesn’t hit through the eardrums…we plan for our sound to go through the soul of the person” listening, “waking something inside of them…There are so many sleeping people.”

The audience of 500,000 was the largest live audience Hendrix ever played for. He overdosed and died two months later.

The set, launched at 1230 or so at night — epic in hindsight, but was just another blistering Jimi jam — begins with Jimi apologizing.

“Really hope it isn’t too loud for you.” At one point, he starts into “All Along the Watchtower” and backs away from the mike. He’s in the wrong key, but so unflappable he doesn’t let on. He chunks through the guitar intro again and repeats “As I was sayin’, ‘There must be some kinda way outta here…”

“Purple Haze,” “Hey Joe,” “Foxy Lady,” “Voodoo Child” “Stone Free” finishing with the solo guitar version of “O Say Can you See?”

As the lone sheriff of the town at the time remembers the teeming mass of humanity that flooded the village, “It was July the fourth, and they were gonna hear ‘The Star Spangled Banner,’ by golly.”

And so they did. And so, now, can you.

MPAA Rating: unrated, nudity

Cast: Jimi Hendrix, Bob Merlis, Billy Cox, Susan Tedeschi, Paul McCartney, Derek Trucks, Kirk Hammett

Credits: Directed by John McDermott. An Abramorama release.

Running time: 1:28