Sparking Fire in Harry Potter and the Half-Blood Prince

The July issue of Post Magazine, a trade resource for editors of video and film, features a cover story regarding the creation of a number of elements used in the cave scene in the Harry Potter and the Half-Blood Prince film. One element, fire, was a particular difficult special effect to create, the article explains. Noting that the sequence encompasses “complex compositions of live actors, greenscreen, CG characters,
environment, simulated natural phenomena and simulated fantasy phenomena,” this article goes in to much detail about the work Industrial Light and
Magic (ILM) put into the creation of the fiery climax of the film, relating
the many steps in the process. Tim Alexander, a VFX supervisor on the film for ILM, relates the fact that the sequence “chewed up a great deal of our time and energy. Fire’s pretty difficult and we didn’t have a solid solution for it.” He goes on to say, “You have to have such a high density of particles to create the detail
that you need in fire that it very quickly becomes unwieldy and slow… We still did a particle sim, but it’s a very low particle-count, like
fuel flying around in the air, just to tell us where it’s going to
burn.” This process, a creation of ILM dubbed the Verte engine, the article goes on to describe it as:

Horvath’s engine calculated how the particles would burn “but he does
it in two-dimensional slices so he can do very high-resolution
simulations very quickly,” Alexander says. The fire effect’s 3D volume
is “sliced” like a loaf of bread, focusing on the X and Y dimensions,
but not the Z axis. “You can get really nice detail around the edges of
the fire and you can get speed out of it because you’re not worrying
about depth.”

Since the fire itself is moving, the particles move from slice to slice
in 3D. “If you look at a fire, it’s actually pretty difficult to tell
how deep it is. By piling up a bunch of slices together, you still get
dimensionality to it but you’re giving your time and energy to the
screen space, where you need it, like the edges of the flame where you
want to see those little licks.

The story continues, relating the many people and processes involved in creating such a intensive special effect. Additionally, the article speaks to the creation of the Inferi, which were created to “not look like grotesque zombies ” rather, they appear very skinny and sickly and make Gollum seem like a he was on steroids.” A decision by director David Yates, the article goes on to speak of the process of creating these creatures, and the live-action and CGI production of the underwater scene.

The full cover story article in Post Magazine, covering the many parts of the creation of special effects in Half-Blood Prince, can be read right here.

Many thanks to Wayne for owling!