Pop quiz: What is the hardest part of adapting 600 pages of notes written by James Cameron onto the big screen? For the VFX artists of Alita: Battle Angel, now on Blu-ray, it was bringing the complicated design of Alita herself.

“The single most challenging thing we had was the stylized nature of Alita,” visual effects supervisor Nick Epstein of Weta Digital tells Inverse.

The renowned studio has created special effects of Avatar (2009), Aquaman (2019), and trilogies like Lord of the Rings and Planet of the Apes, but Alita turned out to be its greatest challenge yet.

“We didn’t have any reference for how Alita talked or moved,” Epstein says. “That’s what we had to figure out.”

Alita: Battle Angel, Robert Rodriguez’s adaptation of the Japanese manga Gunnm by Yukito Kishiro, is the story of “Alita,” a cyborg warrior who discovers her true nature in the dystopian future of Iron City. The film stars Rosa Salazar, who wore a complex motion capture rig while shooting her scenes and gave the VFX team a real, physical foundation to work from.

But given the unusual blend of live-action and stylized aesthetics of anime — Alita sports oversized cherubic eyes and realistic skin, hair, and facial features — initial reactions to Alita were polarizing, with fans and critics citing the uncanny valley as a turnoff.

“It’s not something your brain naturally accepts,” Epstein says. “We had a beautiful performance from Rosa, but the challenge was making that real performance a believable part as any of the other human inhabitants.”

Alita was the source for many problems faced by Weta’s VFX team. Thankfully, a simple solution cured “80 to 90 percent” of all their headaches.

In 'Alita: Battle Angel,' Rosa Salazar, seen here opposite Ed Skrein, stars as the awakened cyborg warrior "Alita" in a dystopian future city. 20th Century Fox

“Alita is a manga-fied version of Rosa. That was our biggest challenge,” says Epstein, who said problems arose when the VFX artists tried to directly adapt Rosa’s performance into digital for Alita. “Once we transferred Rosa’s performance to Alita, the issue we had was something in the transfer was distorting our perception, in the mouth and eyes in particular.”

So, what was the fix? As the team learned, the simplest solution was the best one: They simply copied and pasted Rosa’s mouth onto Alita.

“We took Rosa’s digital mouth and put it wholesale on Alita,” Epstein says. “There was something non-proportional in the mouth scaling from Rosa to Alita, and that was really hurting the mouth shapes. We were lucky we hadn’t distorted Alita’s features so much that we couldn’t do this.”

Not all of their problems went away, nor did this solution make the work any easier. “We still had work to do in the corners of the mouth, that was tricky,” recalls Epstein, “But that simple decision to put Rosa’s mouth on Alita was a good way of solving the issues.”

In the effects-heavy sci-fi 'Alita: Battle Angel,' Rosa Salazar's animated lead stands alongside the authentically human characters of Iron City. 20th Century Fox

Another major assist on Alita: Battle Angel came from lessons learned from a film Weta Digital handled under unfortunate circumstances: 2015’s Furious 7, after lead actor Paul Walker’s death. To give Walker an emotional, respectful send-off, the filmmakers turned to Weta Digital, who inserted Paul Walker’s face over his real-life brothers who finished the late actor’s unfinished scenes.

Innovating on Furious 7, with something called “posed-face textures,” would have a direct impact on Alita years later.

“When an actor hits a certain ‘pose,’ we would capture the textures as well as the face shapes. When your digital characters hits that pose, these textures are activated and that helps in terms of realism,” Epstein explains. “You’re not using a static set of textures, you’re triggering textures that are captured at different points in the scanning stage of their performance. That was a technique we developed for Paul Walker and that’s what we carried forward for Alita.”

With all this work behind them, Epstein tells Inverse he’s “grateful” for the creative direction of Alita: Battle Angel.

“I am grateful the filmmakers wanted to make a live-action manga movie. Thats why I was attracted to the project,” he says. “I think it was a bold choice, and one that worked very well.”

Epstein believes the work on Alita could just as well influence how the big effects spectacles of tomorrow are made. “Four or five years ago, bringing something to the screen like Alita would have been much harder. But technology’s come far enough that we can [make this film]. That comes down to our experience at Weta. We’ve come quite a long way.”

Alita: Battle Angel is available now on Blu-ray, DVD, and Digital HD.