Theme park rides have been delighting Disneyland and Universal Studios guests for decades. While some of their attractions are decades-old themselves, people flock to them again and again because they’re entertaining, create a sense of wonder and (for some adults in the crowd) evoke happy childhood memories.
Part of what makes these rides so entertaining is that many of them include effects that, to the average viewer, defy the laws of reality. Disneyland’s The Haunted Mansion, for example, includes a dining room scene where a gaggle of transparent ‘ghosts’ dance through tables and hang from chandeliers. Another example a short walk from The Haunted Mansion is Roger Rabbit’s Car Toon Spin. At the end of this ride, Roger uses a movable hole (you know, like the ones you see in cartoons) to make a tunnel appear on what looked like a solid wall. Universal Studios has impressive illusions as well; if you head to their Orlando theme park, you can step into a re-creation of London’s King’s Cross Station and watch your muggle friends melt into a brick archway that takes you to Station 9 ¾ (just like Harry Potter!).
Most theme park guests, if they think about it at all, likely imagine that these effects are from cutting-edge technologies (Holograms! Lasers!). These illusions, however, come from techniques developed more than 150 years ago, a time when the horse and buggy was the most advanced mode of transportation.
How have these antique techniques stood the test of time? The answer is obvious to those well-familiar with the history of magic: Magnets.
Not magnets…that’s the wrong M-Word. Today’s M-Word is Mirrors.
So, what about mirrors? Well, mirrors (and clear polished glass) cause reflections, and sometimes those reflections—when they are angled properly and appropriately lighted—get projected in ways that make people think they’re seeing a ghost or something in front of them inexplicably appear or disappear.
Let’s start with the dancing ghost effect in The Haunted Mansion, which is a favorite example of one of the oldest illusion reflection techniques. Despite what some may think, the spooky spirits aren’t holograms: no lasers were involved in the development of this illusion.
Have you ever stood outside on a cold, dark night and looked longingly at something in a shop window or (hopefully not) into someone’s home? If so, when you looked into that well-lit space, you might have seen a transparent image of yourself in that room. This phenomenon stems from the same principle that creates the specters of The Haunted Mansion, where the ghosts are reflections through silvered glass of animatronics found above and below the ride’s moving car.
This effect is commonly known as Pepper’s Ghost (much to the chagrin of its co-inventor, Henry Dircks), and was first presented in 1862 by John Henry Pepper on the Royal Polytechnic stage in London. Pepper intended to present his illusion as a scientific curiosity, but immediately realized the effect had the potential to be more than just a scientific presentation. “He was ready at the end of the first performance to walk out on stage and start to explain to everybody how it worked, because that’s what he did as a science exhibitor,” Jim Steinmeyer, an illusion creator and historian who has documented the science and history of Pepper’s Ghost, explains. “But when he heard the response to it…he never explained it on stage.”
This technique moved from the theater to sideshows and eventually to theme parks. Today, several attractions use Pepper’s Ghost; for example, this concept in conjunction with other mirror-based effects is what makes people magically walk ‘through’ the wall to Station 9 3/4. Other rides also use this technique in tandem with digital video technology. In the queue for Universal Studios’ Harry Potter and The Forbidden Journey, for example, guests watch Harry, Ron and Hermione chat on a balcony and then disappear under Harry’s Invisibility Cloak. Here, the image guests see of the magical trio is the reflection of digital video through a sheet of glass, the same basic technique that Pepper used in the 1860s.
Pepper’s Ghost was one of the first mirror-based illusion techniques, but not the last. “It inspired a number of important illusions,” Steinmeyer says. “It started people thinking about those principles of reflection and the use of mirrors on stage.”
By 1865, Pepper’s colleague Thomas William Tobin developed a technique called The Sphinx, which used a tri-mirror setup to make a portion of a person or object invisible to viewers (viewers in the right sight line, at least). The Sphinx was quite popular when it premiered, and designers use similar concepts in theme parks to this day. For example, The Amazing Adventures of Spider-Man ride at Universal Studios uses this technique to hide the mechanics that make a crate rise into the air, giving the impression that it is floating. Similar methods are also what cause the tunnel to suddenly ‘appear’ to riders on Roger Rabbit’s Car Toon Spin.
Other theme park rides use double parabolic mirrors to create hologram-like images. For this technique, two concave parabolic mirrors are sandwiched together, with the top one having a small hole in its center. The mirrors then reflect a 3D image above this opening of any object placed inside the parabolic sandwich. This phenomenon is used in optical illusion toys and is also used in rides like Snow White’s Scary Adventure at Disneyland, where the Witch offers guests a poisoned apple that passes through their hands if they try to touch it. The apple guests see is not the actual apple, of course, but a 3D image created by parabolic mirrors.
It took almost a century for Pepper’s Ghost and other mirror-based illusions to make their way from the theater to theme park attractions. Once designers incorporated them into the ride experience, however, it became clear that these techniques had found a home in amusement parks.
Theme park rides are perfect for this type of illusion because everything a person experiences on a theme park attraction is controlled, from when they see something, to how long they see it, to what angle they see it at. This precise level of control gives ride designers and engineers an ideal environment for creating illusions. “You can guarantee what the sight lines are…that gives the ride a new impetus, and that’s where you see these fantastic, wonderful, perfect views of these effects,” Steinmeyer explains. “[Designers] use the motion of the ride itself to create the progression of the illusion or the special effect or the story.”
And that illusion is made even more fantastic by the fact it’s seamlessly integrated into the world of the ride—of course spooky ghosts are haunting a haunted house, and of course a cartoon rabbit can carry a removable hole he can stick on anything. For Station 9 3/4, the illusion helps people relive a favorite scene from the Harry Potter books and movies; they’re in that world, not just reading or watching it.
And remember, these effects aren’t some new-fangled discovery. As Steinmeyer explains, “Successful tricks like this are really simple, elegant, and slightly crude in terms of how they’re done.” And so, the next time you’re at Disneyland and your know-it-all friend comments on how cool The Haunted Mansion’s holograms are, just smile smugly and nod your head.
Graduate student Xuan Luo has developed a pretty damn amazing Pepper’s Cone effect using stuff you probably have lying around your house right now: some clear plastic (a cup will do), an iPad, and a nickel.
If you try to reflect a normal image into a cone or pyramid, as the above video shows, it’ll warp and not work from all viewpoints. Working with Professors Jason Lawrence and Steven Seitz, Luo has put together an algorithm that pre-warps the image being projected, so that it looks perfect in the 3d display regardless of the point from which it’s viewed. From the description:
This display allows viewing a three-dimensional object from any direction over a 360-degree path of travel without the use of special glasses. Inspired by the classic Pepper’s Ghost illusion, our approach uses a curved transparent surface to reflect the image displayed on a 2D display. By properly pre-distorting the displayed image our system can produce a perspective-correct image to the viewer that appears to be suspended inside the reflector. We use the gyroscope integrated into modern tablets to adjust the rendered image based on the relative orientation of the viewer. Our particular reflector geometry was determined by analyzing the optical performance and stereo-compatibility of a space of rotationally-symmetric conic surfaces.
The effect is remarkable, even supporting animation and audio. The code for the algorithm has yet to be released, but will be open sourced when it is. Check out the project page for more detailed information, or to sign up to be informed when the code is available. If you’re more tech-minded, you can read the full details in the project’s PDF.