By Darren Waters
BBC News Online entertainment staff
Visual effects are an essential element of almost every film made today. BBC News Online was given a behind the scenes look at the work of effects firm Cinesite, which has just completed King Arthur.
On the left is the original shot and on the right the visual effects are in place
There is an eerie quiet around the offices of visual effects firm Cinesite, based in Soho, London's film and TV heartland.
Spread over two floors, the offices resemble an apocalyptic horror movie with empty cubicles, computers on but hibernating, lights dimmed and chairs abandoned.
A handful of staff are working on preliminary effects for Hitch-hikers Guide to the Galaxy but most are taking a well-earned breather after completing work on Disney's King Arthur and Warner Bros' Alien vs Predator.
The two films are high-profile releases this summer and both feature an enormous number of visual effects.
But while the effects in Alien vs Predator are eye-popping, in King Arthur their effect is altogether much more subtle.
Most of the film's visual effects are designed to create a mood or an atmosphere and would pass largely unnoticed by the audience.
"If someone walks out of a film and did not notice the visual effects then I know I've done my job," says Matt Johnson, visual effects supervisor on King Arthur, who spent 18 months working on the film, including relocating to Ireland where it was shot.
Mr Johnson worked side by side with director Antoine Fuqua and producer Jerry Bruckheimer to bring the story to the screen.
"This kind of film is not about the visual effects, it's about the action that is going on.
"The best visual effects are really story driven - if you can do effects and the audience is not taken out of the story then we have done a good job."
One of the highlights of the film is a battle between opposing forces on a frozen lake in a snowy valley.
The Hollywood Reporter described it as "one of the great cinematic fight scenes of all time".
But while the fighting is real - or at least Hollywood real - much of the sequence is not.
The actual valley in Ireland in which the scene was shot was too green and leafy to pass as an icy locale in Roman Britain.
Instead Cinesite created the snowy valley complete with frozen lake inside a computer.
The effect is dramatic but it is the small touches that make the difference - occasional snowfalls down the valley side, swaying trees and bushes and a rolling mist across the whole scene.
"I'm obsessed by putting in little realistic details," says Mr Johnson.
As the battle develops in the scene the camera shifts perspective beneath the lake.
"Everything in this shot is computer generated - from the guys walking across the top of the ice through to the cracks.
"There's even little bits of broken twig in the water. They are not part of the scene but help ground it in some sense of reality."
Cinesite did visual effects for Alien vs Predator
The "money shot" of the sequence, an extended camera swoop from beneath the ice, breaking through the surface and into the heart of the battle, is known at the "Bruckheimer shot".
Bruckheimer has made a name for himself as the architect of some of the biggest blockbusters in cinema history, including Pearl Harbor and Pirates of the Caribbean.
"It's great for a visual effects person to work on a Jerry Bruckheimer movie because it gives you a chance to turn the knob up to 11."
There are more than 500 different visual effects in the movie - a mix of digital, physical effects and composite shots - mixing sequences together to form a whole.
The effects range from the spectacular - a virtual army of thousands of Saxons on the march - to the subtle - cracks in the ice.
The company has worked on all three Harry Potter movies and is working on the fourth.
Work on Charlie and the Chocolate Factory and Hitch-hiker's Guide to the Galaxy will keep the firm busy over the next 12 months.
Cinesite created this scene using live action, a model of Hogwarts and computer graphics
Early work on the two films is currently underway but kept tightly under wraps.
Mr Johnson says the company prefers to use a range of techniques.
"It's finding the best tool for the job at the time.
Sometimes it's very easy to get carried away and make everything a digital effect
"A lot of my job as a visual effects supervisor is saying 'don't do a visual effect'.
"Just because you have invented a bit of software that can make things look like wood doesn't mean it isn't better to simply get a plank of wood."
King Arthur is released in the UK on 30 July. Alien vs Predator is released in the US on 13 August and the UK on 22 October.