"And there were some other tricks to give us a sense of lighting within the clouds, first revealing the Warner Bros. logo and then the Harry Potter logo coming through. It's a very architectural set up and then as you begin to push through this, the lightning flashes are timed with the soundtrack, which sounds like a distant war, which relates to an underlying theme of all-out war.

Eventually this pulls us through to modern-day London, which we actually shot on location at the building of the Great London Assembly. It's a very ultra-modernist building very much at odds with the Wizarding world of Harry Potter, which always has that wistful, Dickensian look to it. David Yates has liked using modernist architecture for depicting the Muggle world.

An atrium of clouds builds up with a very strong architectural sense. The huge Dark Mark falls over London. That was a real achievement using DN Squirt. We wrote a few new tricks and tools for it that allowed us to work with the underlying geometry of the Dark Mark, particularly a huge scull with giant fangs, and basically fill it with clouds. But then as we're animating it, we're getting all of the edges tearing away and bleeding away. It's a balance between natural cloud forms and the stylistic shape of this skull. We wanted it to look unnatural."

"We got a hold of street plans. And we created a very detailed 3D model of this section of London and then populated it with CG people and vehicles. We developed a new system using Houdini to procedurally model all of the trees. From my point of view, the route that the camera flies in is pretty much the same one that I walk from Charing Cross Station into Double Negative every morning. One of the things about Diagon Alley is that everything had to be carefully hand-made. We took the components of the set usually to extend it based on our imaginations.

"After the Death Eaters grab Ollivander, we cut to the Millennium Bridge. This is a fantastic horizontal suspension bridge outside of the Tate Modern Gallery on the banks of the Thames. We're back in the modern world of steel and glass. The ominous sky is a matte painting derived from a plate that was shot off the roof of Double Negative, with lightning flashes added to it. The three plumes of the Death Eaters descend on the Millennium Bridge and twist around it flying back and forth. They vanish but they cause the bridge to turn and twist and ripple.

We switch to a fully CG bridge and replace the river with a CG river using the dnOcean toolset so that we can create waves and swirling tracks left by the Death Eaters. We shot some close-ups of people on greenscreen. We then created the London environment as a 3D reprojected painting behind that. The bridge begins to break up and we treated it effectively like a character animation and rigged it using our character toolset because the deformations were so extreme. But then the character animation of the bridge drives a secondary dynamic system, which is built using our dnDynamite toolset, and this causes things like the decking planks to pop away and eventually as it really breaks up, we see the suspension cables snaking and whipping away, also using dnDynamite. Eventually, the bridge snaps in two and they originally had animations of people falling into the river but that was deemed to be [too intense]. We were able to animate proper water surfaces using Squirt and some enhancements to Houdini on the top of it."

"In the end, the most difficult and interesting work for us was the water and fire simulation," Alexander recalls. "We had three shots external to the cave where Harry and Dumbledore are looking back toward the entrance and that was a fully CG ocean. They actually shot a helicopter plate of it, but the water wasn't wild enough for David Yates. So they asked us to give them a much stormier ocean. And once we go into the cave, it's dominated by a giant lake and the island that Harry and Dumbledore are going to is in the middle of that lake. All of the water is computer generated, even the water that Harry scoops out of the bowl that holds the Horcrux."

According to Tim Burke, the overall visual effects supervisor, "David was pretty keen on giving them an unnerving presence but didn't want in any way to create a zombie type of character. He wanted a human character you could empathize with, that even evoked sadness. At the end of the day, these are dead souls. We came up with a design that went beyond what was possible with an emaciated human: very skinny with all the bones and ribcage showing and spending all the time underwater with the effects of gray skin. We developed this concept with ILM. They developed different variations of heads in ZBrush so we could populate this world with hundreds of these characters. And we started looking at how they moved and having done some studies of movement and getting to the point where we didn't have any reference material, we turned it over to ILM to work on their animation skills and they came up with pretty normal human type movement."