No detail is too subtle, no film too arthouse, to escape digital tinkering - not even Henry Cavill’s facial hair, or the Call Me By Your Name star’s nether regions. But are film-makers in danger of losing our trust?

From Transformers to Aliens, we all know that computer-generated imagery features heavily in big-budget Hollywood blockbusters – it’s right there in front of us on the big screen. But most CGI is imperceptible. Take the revelation this week that Armie Hammer’s genitals had to be digitally edited out of the acclaimed gay romance Call Me By Your Name. The actor’s oh-so-short shorts had failed to contain him and some work was required in post-production to tidy things up. This is an arthouse, realist drama that is using visual effects to alter what audiences see – and it’s increasingly common in indie movies. Just ask people at one of London’s busiest post-production houses.

“Simple but high quality digital visual effects are now at price point where low budget movies can afford them. We change the time, on clocks, on wristwatches. We grade day for night. We fix cloud continuity in the sky. We make summer shoots look autumnal and we add snow and rain when called for,” says Ben Robards, a creative partner at Absolute Post. “If your 80s movie isn’t convincingly period, we can liberally sprinkle Rubik’s Cubes and Walkmans around; if your Jane Austen adaptation has electricity pylons and hybrid cars in shot, we can remove them.”

So, much of CGI is about what you are not seeing, rather than what you are. Hammer has had many predecessors – not all of them human: legend has it that one poor animator on the movie Babe spent months painstakingly removing every frame of the title character’s anus. More recently, editors on Justice League had to remove a moustache grown by Superman himself, Henry Cavill, for his role in Mission: Impossible 6.

Facebook Twitter Pinterest Bum deal … Babe Photograph: Allstar/UNIVERSAL

All these practical considerations are perfectly understandable, but the possibilities are becoming extreme. Audiences were divided when the late Peter Cushing was digitally reanimated for Rogue One: A Star Wars Story, asking if it was ethical to put words into the mouths of deceased actors.

Writer-director Rupert Jones, who recently directed the British thriller Kaleidoscope, is relaxed about such techniques, citing a scene in Blade Runner 2049 that incorporated iconic film-star footage into the sci-fi epic’s futuristic setting. “The replication of Elvis Presley and Marilyn Monroe in that extraordinary fight sequence was clearly adding to the world and the story. Oliver Reed being ‘reborn’ in Gladiator meant that the film could be completed. I suppose what I would object to is the idea of resuscitating old stars because we’d rather regurgitate what worked before than try to imagine new things. That feels like the ‘show’ being finally swallowed by the ‘business’”.

More concerning is the possibility that real moving images can be manipulated for nefarious influence. Now that experts are warning that fake news purveyors might soon digitally doctor video and audio, should similar concerns apply to moviemaking; and in particular to documentaries?

“We are getting close to the point where the moving image can be altered, supplemented and synthesised to such a convincing degree that even experts seek additional cues to find veracity,” says Robards. “But the intention to alter visual history and warp public perception is nothing new – the ancient Romans redacted names from monuments, Stalin airbrushed opponents from photographs. I take comfort from the fact that, historically, we have usually found an ethical redress against the misuse of power no matter what technology is deployed.”

Maybe this ability is dangerous only in the wrong hands. And of, course, developments in technology may have made us more alert to potential deception. “Common wisdom suggests that society is less deferential than it was half a century ago when TV was a relatively new innovation for many households,” says Prof Patrick Leman from the Institute of Psychiatry, Psychology and Neuroscience at King’s College London. “What may be changing is what people are likely to be believing of. Take the recent photo of Osama bin Laden after his death. Many in the west will trust the accuracy of the image, but some conspiracy theorists elsewhere will regard it as fake. Same as the moon landing photos and numerous other examples such as the Zapruder film of Kennedy’s assassination. I think people may be more trusting of images from a source they believe is reliable (and corresponds to their general worldview), but less trusting of those that do not come from such a source.”

Facebook Twitter Pinterest Oliver Reed … resurrected for some scenes in Gladiator. Photograph: Allstar/DREAMWORKS

Perhaps it’s my general worldview, but, as a film critic, I trust that the vast majority of film-makers use visual effects for the best reasons. Dazzling sci-fi aliens and animated worlds aside, studios need to use visual effects for more practical reasons, including legal, factual and certification issues. And in the case of arthouse fare, I have found that directors are passionate about communicating with their audience and making sure their story is told with clarity and without distractions.

Surely trust is all part of the process, as it is when reading a novel by an author? “As a film-maker, one is trying to engage the audience to such a degree that it forgets itself and ‘disappears’ into the story,” says Jones. “One has all these creative resources – the lighting, the design, the hair, the acting, the grade etc – with which to persuade the audience of the story’s verisimilitude. Visual effects are no different. If it doesn’t draw attention to itself, then it’s working – and so trust doesn’t really enter into it.”

“There is an unspoken contract with the audience,” adds Robards. “A documentary demands a very different level of authenticity to fiction, for example. There does seem to be a tacit understanding of what is acceptable and what is not. If Blue Planet II was shown to be ‘faking it’, people would feel far more aggrieved than if we had enhanced an eruption of an unidentifiable volcano for an historical recreation.”

And if anyone is feeling aggrieved about the absence of Hammer’s genitals, perhaps they should be seeking out another kind of film altogether.