Slow Dance | Source: https://www.nickolaspeter.com/slow-dance
The feathers in the frame appear to be in "slow motion" motion. But in fact, it moves fairly fast—after adding some lighting and vibration design, it seems to be swaying in an incredibly lazy rhythm due to the limits of human visual perception.
Do you see? It doesn't matter if you don't understand. Jeff Lieberman, the installation's designer, said: "Confusion arises when what is in front of you conflicts with your understanding of reality. As host of Discover Channel's show Time Warp, Lieberman uses high-speed cameras to deconstruct the physics involved in motion processes such as bullet launches, ballets, and explosions. Lieberman refers to his latest installation as "Mandala Dance." On the crowdfunding platform Kickstarter, he describes it as "a picture frame that slows down time."
In fact, the mechanism of "Rambling Dance" is surprisingly simple, but it is incredibly subtle. Lieberman said: "Once you choose to understand how it works directly, you will lose the fun of exploring and revealing it forever." But if you're really desperate to understand, Lieberman advises you to take more time to figure it out.
In order to uncover the secrets of "long dance", you may need to add a concept first - visual retention.
"Go to the marquee, the lamp to go, the lights to go out and the horse to stop." | Source: Qianjiang Evening News
In fact, the principle of "visual retention" was applied to folk art as early as ancient China. The Southern Song Dynasty carefully recorded in the "Past Events of Wulin" that "Ruosha plays with shadow lights, horses ride characters, and rotates like a fly", which refers to this special lantern.
Marquee structure and principle| source: popular science China
The principle of the marquee is to insert a wire as a vertical shaft in a paper lantern, and the impeller above the shaft, and the paper-cut villain is fixed with a wire around the shaft. When the candle at the bottom of the lantern is lit, the heated air rises to form a convection, and the air flow causes the impeller to rotate and drives the shaft as well as the villain to rotate. The figure is cast on the outside of the lantern, forming an image of people and horses chasing and changing scenery.
The principle of "magic slides" | Source: Ars Magna Lucis et Umbrae, Atanasius Kiecher
"Magic Slide" shows the | "Illusion" Image Source: M Plus
In the far West in the 17th century, Europeans also invented an early slide projector and began to slide show "illusions" and "miracles", which at that time was called Magic Lanterns – through the combination of lenses to quickly project hand-drawn images to a certain location, forming a visual persistence. So what is the principle behind visual persistence?
Vision is actually a whole process of imaging from the lens of the eye, cone cells sensing light, and converting light signals into nerve currents that are transmitted back to the brain. The optic nerve takes about 1/16 second to respond, and the light response time for different frequencies will vary. The length of the optic nerve's response creates a lag in the visual image in the brain, known as "visual persistence" or "posterior image".
Fenach Mirror | Source: Dick Balzers Website Phenakistascopes
Fenach Mirror | Source: thisiscolossal.com
This phenomenon was discovered by the Belgian physicist Joseph Plateau in 1835 in an experiment to observe the sun, based on which he invented the "Fenach mirror" to prove the physiological characteristics of this human body, and he himself was blinded by this observation (looking directly at the sun for 25 seconds), but the shadow of the sun was forever imprinted in his eyes.
Fenach Mirror | Source: The Richard Balzer Collection
In 1825, dr. John Parris of the United Kingdom invented the "phantom disk" has always been considered the origin of film imaging, and theoretically the phenomenon of visual persistence can only allow us to see the static picture of the bird overlapping with the cage, so what makes us "see" the non-existent action of "the bird flying into the cage"?
"Phantom Disk" | Source: "Guillotine Valley"
Visual persistence will make the image last 0.1-0.4 seconds in the person's mind, but when the GIF is played at intervals larger than this time range, we will still feel that the image forms a continuous action. For this situation, which does not produce motion, but we perceive movement, the German experimental psychologist Wertheimer called it phiphenomenon. Motion-like phenomena provide a psychological explanation for human visual perception of motion and are the beginning of Gestalt psychology. The Gestalt school argues that the working principles of the human brain belong to holism, and that "the whole is different from the sum of its parts." Gestalt is a transliteration of the German word Gestalt, meaning "pattern, shape, form", etc., meaning "dynamic wholes".
Gestalt Principle | Source: Wiki
A misconception that "twenty-four frames per second of a film is determined by the phenomenon of human visual persistence." If the frame rate standard is derived only from "visual persistence", then based on the response speed of the optic nerve of 1/16 second, the image should be continuous as long as the frame rate is higher than 16 frames per second. If only 16 frames are higher, then in theory any number above 16 frames is feasible, why is it set at 24 frames?
Fast silent film | source: Chaplin, City Lights
In the silent film era, whether it was a hand-cranked or electrically driven projector, the projectionist set the playback speed. Due to various factors, most movies before 1920 were played at 14 to 18 frames per second. Over time, the average speed gradually increased, and in the period of sound films, everyone used 24 frames, and this became the norm in the sound film industry. During this period, rates above or below 24 frames were occasionally seen in color film experiments and on the devices of film enthusiasts.
The rate of silent films varies, with the mainstream being 16 frames. That's why a lot of older films look "fast" today — because we're using a 24-frame device to play a 16-frame source. Then came the Vitaphone system, where a 16-inch record plays exactly 24 frames per second at 33 1/3 rpm for an 11-minute movie. Later, the various studios pressured the director to shoot a 24-frame film, so that the Vitaphone system could be used to produce sound films.
The number 24 is also technically superior — compatible with the sample rate of audio (44.1khz and 48khz @ 24hz = 1838 1 2 and 2000 samples per frame); it is an easy-to-handle complex number (2^3*3). This may explain why 24 frames have been widely accepted and retained as an industry norm to this day.
That is, "The history of 24fps is all about sound" – the history of 24 frames is actually about the history of sound.
Finally, let's uncover the secret of the "long dance" at the beginning of the article – the answer is the careful choreography of light and movement. The LED lights hidden at the edges of the wooden frame blink at frequencies ranging from 76 to 84 times per second, fast enough to exceed your visual perception (the upper limit of the human eye's perception of light frequencies, the "critical flicker frequency" is about 60 Hz). At the same time, the electromagnetic motor at the bottom of the frame vibrates its clamped objects at a frequency of 80 hertz – feathers, leaves, flowers, etc.
The flickering light illuminates the shaking object, and its frequency is exactly 0 to 4 Hz between the frequency and the vibration frequency of the object. If the frequencies are consistent, the object does not appear to move at all due to the "stroboscopic effect". To take a small example of a straightforward " strobe droplet " : When you use a normal camera to observe the drops of water, it is no different from what you see in the real world, but if you adjust the camera shutter frequency to match the frequency of the drops falling, you will record the wonderful scene of the droplets floating in the air.
"I'm going to make the raindrops stand still!" | Source: "The Amazing Demon Bandit 2"
Water droplets can not only stand still, but even flow backwards| source: gfycat.com
Instead, when light briefly illuminates an object at different locations in its trajectory, each illumination event is like a strobe photograph — but the shutter is pressed so frequently that your brain treats it as a continuous film, rather than as a separate image, that the object moves. Visual persistence makes it occur, and the phenomenon of motion makes it continuous. When the frequency difference between light and vibration drops to 0, the object will appear to be stationary; turn off the flickering light, and the object will shake rapidly. It is precisely because of this small frequency difference of only 0 to 4 hertz that the object looks slow and changes slowly, which is different from the wind and grass in the real world, and appears beautiful and gentle.
Like many amazing magic tricks, knowing how "Rambling" works doesn't diminish its beauty. Looking at objects through Lieberman's frame is like peeking into an unborn, slow world. Suddenly, you could see the bending of the tail ends of the feathers; the plants seemed to have another kind of life force, because you could clearly grasp their movements every moment. Lieberman hopes his creations will help people slow down, even for a moment. "As adults, we gradually lose interest in 'mystery', and 'rambling' can really put people into a state of curiosity."
Visual persistence is a limitation of the human body's mechanism, but it is precisely this "defect" that leaves space for technology, giving human curiosity and imagination the opportunity to fill in the gaps between frames and create wonderful visual art.
The reproduced content represents the views of the author only
Does not represent the position of the Institute of Physics, Chinese Academy of Sciences
Source: Center for the Study of Science and Arts
Edit: Paarthurnax