SAN FRANCISCO — Over the past two years, inside the high-tech sanctuary of Industrial Light and Magic, the man who built a virtual virgin jungle for the last “Indiana Jones” movie and conjured 150-foot-tall aliens for “War of the Worlds” has been confronting his most difficult task yet: creating a digital version of the beloved Teenage Mutant Ninja Turtles that could realistically interact on screen with Megan Fox.

On this assignment, Pablo Helman needed more than just turtle power.

“For me, in the 19 years that I’ve been at ILM, this is one of the most challenging projects I’ve worked on,” the visual effects supervisor said in a recent interview at his office. “Technologically, it’s very difficult to capture someone’s performance, put it on a character and make it believable. In this case, we had to design a way to combine performances that were taken at many different times.”

“Teenage Mutant Ninja Turtles,” the live-action reimagining of the 30-year-old comic book franchise in theaters Friday, features a completely computerized version of the four sewer-dwelling superheroes, a take more akin to Gollum from “The Lord of the Rings” films or Caesar from the recent “Planet of the Apes” movies than the rubbery renditions from the 1990s live-action “Turtles” films.

The revitalized reptiles were fashioned at ILM by blending computer-generated imagery with several motion-capture performances by four actors. It’s a radical departure from the original ’90s film trilogy, when Jim Henson’s Creature Shop crafted puppety suits for actors playing the half-shell heroes.

For the reboot, the performers physically portraying each Ninja Turtle donned skintight grey getups and shell-shaped backpacks, while helmets equipped with cameras captured their facial expressions. The actors’ bodies were replaced on screen by their counterparts — massive talking turtles who know kung fu — and their facial expressions were grafted onto the Ninja Turtles’ green noggins.

Despite the effort to construct Ninja Turtles for the digital age, die-hard fans didn’t initially deem the makeover of Leonardo, Michelangelo, Raphael and Donatello totally tubular. Instead, many were shell-shocked to see in early teasers and trailers that the filmmakers added nostrils and lips to the turtles’ faces, a different anatomy than the one from the previous comics, cartoons, toys and films.

“This whole gritty, doofy, straight-out-of-’Avatar’ look is not working for the iconic cartoon turtles,” Jason Schreier wrote on the blog Kotaku last May. “Teenage Mutant Ninja Turtles has never exactly been cool — Leonardo and crew were always dorky and cheesy in a loveable sort of way — but they have never had ridiculous zombie nostrils and gaping mouths like this before. It sure looks dumb.”

Helman defends the humanlike faces because it allows the computer-generated characters, who he said are onscreen for about two-thirds of the movie, to be more expressive.

“You’re never going to please everybody because what you’re fighting is that magical moment when, in this case, someone first discovered the Ninja Turtles,” said the Academy Award-nominated visual effects guru. “It’s not possible to convince someone that these are the Ninja Turtles they fell in love with 30 years ago. The idea is that you have to take the original intent and make it your own.”

“Ninja Turtles” director Jonathan Liebesman noted that producer Michael Bay, the man responsible for bringing “Transformers” to life, originally laid out three commandments for the overhaul of the Ninja Turtles: they should be charming, intimidating and individually recognizable — not just to kids but also their mothers. Liebesman believes the filmmakers accomplished their mission.

“I feel like once people see the movie, they will understand why we made these decisions,” said the “Wrath of the Titans” and “Battle: Los Angeles” director. “We’re trying to make them more lifelike and realistic. I don’t think it sacrifices anything fans love, once they go and see the movie. I think hating on design is just a part of fandom, which is fine. There’s a lot of value to what fans have to say.”

It’s not the first backlash that Bay and the team at his Platinum Dunes production company, which is producing “Ninja Turtles,” have experienced. When the filmmaker originally unveiled his computer-generated interpretation of the Transformers, hardcore fans were enraged that Bay added flames to Optimus Prime’s paint job.  The film franchise went on to make more than $3.5 billion.

“You can’t win, so we’ve just tried to present the best version of what we’re doing,” said “Ninja Turtles” producer Brad Fuller, who previously worked on “The Purge” films. “The movie speaks for itself. I wonder if all the discourse about the film is a good thing or a bad thing. I don’t know if there’s actually a way that you can determine whether it’s good or bad.”