Harry Potter and the Philosopher's Stone was released 20 years ago. The dueling sequences between Dumbeldore, Credence, and Grindelwald were breathtaking in a way that the creators of the earlier Harry Potter movies could never have imagined.
Of course, these special effects do not come without hours of arduous work, even today. They are done with a combination of filming techniques and computer editing in several layers before everything comes together. A prime example of this is the Quidditch matches in the series. The work to pull off a single Quidditch match could take moviemakers months, and the Harry Potter behind-the-scenes features and the book Harry Potter: Film Wizardry (by Brian Sibley) shares everything die-hard fans need to know about the making of everyone's favorite wizarding sport.
Everything Starts With A Storyboard Sketch
Before filming starts, the Quidditch matches must be meticulously planned. This is done first with hand-drawn storyboard sketches, which detail exactly what each character will be doing on the field.
Once this is completed to the creators' liking, the sketches are transferred to a computer, where a more detailed and precise scene is created using CGI. This is called "previsualization" or "pre-viz." This process may take several weeks, but once everyone is happy with how the match looks, they will move on to actual filming.
Each Player Was Filmed One By One
The filming process for Quidditch is long and complicated, mainly since each player must be filmed individually. So even if it looks like the characters are interacting with one another, their movements were actually filmed solo in front of a green screen.
An entire week of filming could result in only a few-second clip to be used in the film. This means that each movement and sequence must be done with great care, ensuring that everything will seamlessly come together in the end.
The Broom Rigs Required Frequent Updating
Of course, the actors could not be filmed on actual flying brooms, so engineers had to get creative with how to make it appear that way in the film. In Sorcerer's Stone, rigs were made that could move the child actors up, down, left, and right at a relatively low speed.
However, in later films, the rig design had to get more complicated. For starters, the actors were now considerably heavier than they had been at 12 years old. The new rigs could not only bare heavier actors but could rotate in almost any way imaginable.
The Movement Of The Cameras Was Critical
Quidditch is meant to be a very fast ed and dangerous game. Of course, the child actors could not safely be thrown through the air on their broom rigs at the speeds needed for the movies. Even with stunt doubles, the risks were just too high.
To create the illusion of speed, motion cameras would move in the opposite direction of the broom rigs. The combined forward velocity of the brooms, with the backward momentum of the cameras, make it look like the quidditch players are flying at breakneck speeds.
Backdrops Helped The Illusion Of Speed
Just like a plane in the sky looks like it is traveling much slower than it really is, actors edited into a scene with a clear blue sky could look as if they were leisurely soaring around. Since film creators wanted the magical sport to look daring and exciting, this presented a problem. To solve this, producers put the players against a more complex backdrop whenever possible.
The risers surrounding the Quidditch pitch were helpful for this, but in scenes like Draco and Harry's race for the snitch in Chamber of Secrets, audiences needed to see just how fast the students and the rogue Bludger were going. Therefore, the scene was put underground in a trench, adding more to the speed and excitement.
The Brooms Were Terribly Uncomfortable
The broom rigs that the actors would sit on for hours were, thankfully, more than just a single handle. They were mounted with a bicycle seat, which was meant to help the actors to sit more comfortably (and would either be hidden by clothing or edited out in post).
However, after an extended period, the actors would become extremely uncomfortable on the broom rigs, with the first film's stunt coordinator, Greg Powell, stating that after a while, he could see the actors' "eyes cross." Over the years, features were added to the brooms, like metal stirrups, to help alleviate some of the uncomfortable pressure, but it became a pain that the actors had to grow used to.
Lighting Was The Key To Believability
Anyone who has tried to film against a green screen and make the end product look believable knows that it is much more complicated than it looks. Several elements must be considered for the actor to look like they were initially filmed against the backdrop they were edited into.
One of the most important of these elements is lighting. If an actor is shot under dark lighting and then placed in a bright and sunny scene, the clip will never look believable. If the sun is setting in the background and the light is shining on the actor from the front, it would break the illusion of the magic. Good CGI is all in the details.
The Size Of The Quidditch World Cup Was A Challenge
One of the biggest changes from the Harry Potter books to movies was the omission of the majority of the Quidditch World Cup match in Goblet of Fire. This change left many fans disappointed, but just the simple introduction of the two teams took several months of arduous editing to complete, leaving a more complex match near impossible.
This was due simply to the massive scale of the event. The pitch was several times larger than the one at Hogwarts, and the stands contained thousands of Quidditch fans that each needed to be edited in. The CGI supervisor of Goblet of Fire, Doug Smythe, explained how each strut, beam, and chair had to be put in one step at a time, followed by all the digital Quidditch fans.
Lessons Were Learned With Each Film
The director of the first two Harry Potter films, Chris Columbus, explained that the special effects for Quidditch were done near the end of filming in Philosopher's Stone and that that only left them with a limited time to make the match look the way they wanted it to. So, when it came time to start filming for Chamber of Secrets, the flying sequences were filmed first, allowing five or six months dedicated explicitly to Quidditch.
This lesson carried on to other films and other directors. Quidditch and other CGI-intensive scenes were filmed as early in the movie-making process as possible, allowing the creators to make the much more magical Quidditch scenes in Half-Blood Prince and other later Harry Potter movies.
The Actors Used Special Swings And Trampolines
As the films continued, the techniques used to make actors look like they were zooming through the skies got more creative. While fans could often be used to make someone look like their hair and skin are blowing back from their speed, there are always visual benefits to filming someone actually flying at a high velocity.
For this reason, actors might be placed on a trampoline with a broom between their legs. This might look funny to an observer, but the effects were undeniable after editing. Another technique was to put stunt people on large contraptions known as Russian swings, often used by trapeze artists, to throw them through the air onto cushioned pits.
