This moment comes before the slow motion explosion, which was a practical fire with some CG debris and other embellishments.

But the most daunting sequence is the climactic fight on top of London Bridge under construction. "There was a lot of photographic reference for what the bridge looked like, which we used, and a lot of reference for what the buildings around it looked like: the type of boats and all that stuff," Jarrett continues. "The tricky thing was that it was in broad daylight on a bright day and you're up 250 feet in the air looking down on London. And every single bit of it has to be manufactured. There's none of real London that's there now, with the exception of maybe half a dozen buildings. But the most effective approach was to have it be full-CG, with CG water, obviously for the river, and simulations for the debris floating around the river and past boats.

"The buildings about half a mile in all directions from the bridge are full CG with shaders and textures, not just matte paintings projected on. There is a mixture of live-action elements of smoke and atmospherics along with the CG smoke and steam from the boats. Little details were important like trying to keep the sky moving all the time and drifting and having enough smoke and atmospherics, but not to overcook it. We were trying to make sure that every single smoke element looked different and was blown at a different direction or had a different color. We wanted to make sure there was no sense of repetition in the background.

"You know, London is a city with a lot of character -- it's been around for a long time but also different areas within the city have got a very different feeling. So, for example, just to the east of that bridge is Docklands where the historic port was. Up to that point, boats could come up with their tall masts but they couldn't get beyond Tower Bridge, so if you look east it's all the kind of down and dirty London, lots more chimneys, more industrial; if you look west, it's the complete opposite: it's the fancy buildings by the people that own the docks; boats couldn't get up there with tall masts because there's a bridge in the way; only smaller boats and lower boats; there's much less atmospherics. So we were always thinking about what would make it a viable place and I do think it has a nice character, but the hardest sequence in the movie with about 320 shots. And when the DVD comes out, you'll actually see that the buildings are laid out as they were really laid out at the time."

Bill Desowitz is senior editor of AWN & VFXWorld.