The history of sci-fi movies is very lightly punctuated with ground-breaking moments that advanced and redefined what could be done visually with the genre: Fritz Lang’s seminal Metropolis brought a scale and technical wizardry to movies that had only been hinted at in Cecil B. De Mille’s biblical epics; MGM’s Forbidden Planet was possibly the first sound-era science-fiction production to not only treat of an intelligent theme – a science-fiction version of Shakespeare’s The Tempest – but to throw enormous production resources and ingenuity at the problem; twelve years later Stanley Kubrick almost single-handedly ushered science-fiction movies out of steerage and into first-class with 2001: A Space Odyssey; In 1977 George Lucas and John Dykstra brought dazzling innovation into the field of optical effects with the Dykstraflex motion-control system from which Industrial Light and Magic would found its optical-effects empire.

‘Maria’; the Krell labs; going ‘into the infinite’; watching the first imperial star-destroyer thundering overhead to the sound of a thousand dropped jaws…

And then came the T-Rex.

For me, the emergence of the Rex in Spielberg’s Jurassic Park is the most exciting moment I can ever recall in the cinema, and that includes Star Wars. Waving a very fond farewell and a million thanks to immortal stop-motion animators such as Ray Harryhausen and Phil Tippett (whose stop-motion dinosaurs were replaced with computer generated ones after dazzling proof-of-concept CGI work from ILM during the shooting of Jurassic Park), it seemed that cinema – any cinema, not just sci-fi - would never be the same again.

The less-convincing brachiosaur earlier in the movie had only hinted at the revelation that the Rex turned out to be, and the shot of the creature walking away to check out Ellie’s torch-shenanigans, filmed from Sam Neill’s POV inside the car, is a benchmark of verisimilitude that neither the JP sequels nor their numerous imitators could repeat.

Cut to fourteen years later, and I walk out of the press screening of I Am Legend, shaking my head at the pointless Hanna-Barbera CGI vampires that helped to blow a visually stunning movie which had been doing pretty well up until their appearance. And this happens movie after movie now, like a bunch of shiny Christmas crackers with the same dull plastic trinket in them. So what happened to the promise of T-Rex? What happened to the magic?

I believe that the problem lies in the natural but mistaken assumption that improvements in technology were going to yield even better results than the T-Rex. Hell yeah, we thought – soon they’ll be able to show dinosaurs in full daylight without rain. Unfortunately, the problem with CGI is one of narrative credibility, not technological prowess, and it will not be solved by even the coolest rendering algorithms, texturing techniques, physics emulation or motion capture.

The excitement of both Michael Crichton’s original Jurassic Park novel and the film Spielberg made from it lay in the science that explained why dinosaurs could credibly be menacing humans in the 20th Century. It takes more than technical ability to suspend disbelief; maybe the background science of the narrative in JP was specious, but it contributed enormously to the reality of the T-Rex.

Secondly, as many have pointed out, we have no idea how dinosaurs really moved or what they really looked like; imposing upon Rex the lumbering gait and leathery skin of large and familiar mammals like elephants and hippos was all it took to sell the movement and appearance.

But if there is anything we are truly familiar with, it’s people. The human face is the very first thing we learn to distinguish as separate from ourselves when infants, and the slightest change of countenance can signal many things to us. No wonder then that despite the best efforts of CGI wizards in films like The Polar Express, Beowulf, The Hulk, the Mummy films and others, CGI humans perennially fail to convince.

But these are side issues that I have looked at before. To home in on the title of this week’s column, why use CGI for things it is will never be suited for? I Am Legend was a superb opportunity to wake up Stan Winston and his talented cronies, or to call in Greg Nicotero to create hordes of hideous on-set vampires. Instead we get these cartoonish freaks who are humanoid in every respect except a Mummy-like distension of the jaw that blows the very little credibility they had in the first place. The skin is clearly rendered rather than filmed, and the eyes are lifeless and flat. The creatures in Legend are actually less frightening than the CGI characters in Polar Express, because at least in Legend they’re supposed to look dead.

I can think of a couple of reasons why I Am Legend went down the CGI road against all logic:

1) Economies of scale and the burgeoning number of digital effects houses have driven down the legendarily-prohibitive costs of CGI to a point where prostheses and real-world creature effects would have extended principal photography at far greater expense. If this is the case, the major studios might be using CGI’s potentially-fallacious ‘big budget’ reputation to boost the cachet of their big Summer and Xmas releases whilst reducing outlay and risk (their favourite pastime).

2) CGI monsters have featured in many successful Hollywood blockbusters, such as the Lord Of The Rings trilogy, so people will obviously ‘buy’ them despite the complaints of pedants like myself. The problem with this rationale is context: middle-earth is a place of total fantasy populated by fictitious monsters who belong in an unreal environment and could not have been practically created with creature effects. Legend instead depicts a brutalised version of the real world featuring completely humanoid monsters upon whose verisimilitude the film hangs.

Incidentally, I don’t agree with the various criticisms of the CGI deer that Will Smith hunts at the beginning of I Am Legend – the animals simply couldn’t be more convincing from a photorealistic point of view. What gives away their unreality is the common-sense understanding that Manhattan would have had to shut for a week - rather than a morning - to get those shots using real deer. Once again, the problem is not technical in nature – it is the viewer’s common-sense that blows the illusion, that removes us from the thrilling notion of ‘filmed theatre’ and deposits us back in the realm of cartoons, where we are expected to offer the film-maker a far greater leash to depart from reality – and where the expenditure of effort is diminished along with the rewards .

Commentaries both on DVDs and in forums and blogs indicate an insurgent backlash against Hollywood’s lazy recourse to CGI. The ‘it was actually there’ factor has a big part to play in achieving suspension of disbelief, and, to boot, a lot of movie actors would probably welcome the opportunity to shoot at something they could actually see once more.

Martin’s review of I Am Legend is here .

Martin writes his (mostly) sci-fi column every Friday at Den Of Geek.