Framerate modulation—you may have seen this phrase being tossed around in certain sakuga circles. What does it mean?

Here’s a scene from One-Punch Man. Watch how the Sea King writhes in the flames with fluid animation at first, then he starts to move a bit more jerkily. You might notice that the fire is moving in the background while the character remains in the same pose. Also watch closely as the monster is kicked in the head: at the moment of impact, his face his held on the screen for a bit longer than the other drawings. The reduced framerate makes his pained expression register better, and the impact feel more intense. That’s framerate modulation.

As you may know, film is made up of a series of still images that give the illusion of movement. The framerate or FPS (frames per second) is the frequency at which the consecutive images are displayed, or the length of time an individual image is held on the screen. Both live-action film and animation are almost always shot at 24 FPS. Early sound films needed a standardized framerate for the audio to play properly, and the 20 to 26 FPS range was established as a sufficient speed in the 1920’s. By the 1930’s, 24 frames per second had become the standard for both live-action films and animation. However, animated drawings can get away with something that live-action can’t: it’s possible to shoot the same drawing twice or thrice or more so that the same image appears on multiple frames, and it won’t look too jerky or unnatural.

Animation at the same rate as standard live-action is called animation “on the ones.” With 24 drawings per second, every frame has a new drawing. Some animators, like Richard Williams, swear by ones. In The Animator’s Survival Kit, he contends that ones look better in nearly every case. He asserts that the smoothest possible motion makes for what he calls “compulsive viewing.” In his belief, the beauty of smooth animation itself is what audiences find engrossing. Williams does acknowledge that the ever-changing-ness of ones can look “mushy,” but he makes an important distinction: ones don’t look as good with “dumb, mechanical inbetweens.” They have to be planned well. That is to say, the quality and placement of the drawings is more important than the rate at which the drawings appear. Then again, the same could said for animation at any other framerate.

Consider this clip from March Comes in like a Lion:

The animation is on smooth ones, but the movement is stilted and unnatural. The bag seems to have no weight or momentum, and the inbetweens are evenly spaced, as if they were auto-generated in Flash. The motion looks soulless and artificial. This is what Richard Williams means by “dumb, mechanical inbetweens.” You can tell the animator had trouble keeping the complex character designs looking solid; details like the facial features and tie threaten to float and shift off their proper body positions. This goes to show that animating on ones does not automatically mean “higher quality.” Having too many details to keep track of tends to result in an ugly mess, therefore cartoons with full animation usually keep the character designs simple.

Another problem with animating on the ones is the time and expense required. An average 21-minute TV cartoon would take 30,240 drawings to be fully animated on the ones. An hour-and-a-half feature film would need 129,600 drawings, and that’s only if you count one layer! Disney’s 1937 masterpiece Snow White and the Seven Dwarves was partially animated on ones. It wound up in the ballpark of 250,000 drawings, at a cost of nearly $1,500,000 or over $25,000,000 today. For comparison Akira, which also partly used ones, cost about $9,400,000 or $19,000,000 today.

As an alternative to the costly and labor intensive ones, you can animate on “twos.” Each drawing will be photographed twice, resulting in a new drawing every other frame, or 12 frames of new animation per second, half as many as on ones. You can also animate on threes or fours—8 or 6 new drawings per second, respectively. This will save more time and money, though the movement may look more jerky.

Western cartoons often increase framerates in scenes that call for fast motion—you can depict more action in less time with 24 drawings per second rather than 12 or less. Also, if the camera is panning across a background, you must have a new drawing for each frame (ones), or else the movement of the background and the animation will not match up (anime often doesn’t bother to do this). On the other hand, 12 drawings per second generally looks smooth enough for normal scenes, so most western animators default to twos. There is a case to be made for animating on twos, threes, and even fours. Movement at lower framerates may look less smooth and realistic, but realism doesn’t have to be the be-all-end-all goal. Having fewer drawings per second can create an impressionistic, eye-catching optical effect. Japanese animators in particular have made extensive and innovative use of low framerates, as well as the framerate modulation technique.

So what does framerate modulation mean, exactly? It can mean a number of different things, but the most general definition is “changes in the rate of drawings per second.” In other words, the animation switches from ones to threes, or from fours to twos, or any variation thereof. Most sakuga fans use the phrase in the context of a single, uninterrupted cut. Actually, “framerate modulation” is a bit of a misnomer: it specifically refers to the rate of the animation drawings, whereas the framerate of the actual film usually stays the same.

What makes framerate modulation more than just a flashy animation gimmick? For one, you can vary the framerate of different animated actions to convey differences in scale and weight. Disney did this in films like Giantland and Fantasia to give the drawings a sense of enormous size. You can also use the technique to establish a visual hierarchy of motion. In this clip from Mamoru Hosoda’s Digimon Adventure, the fireball and explosion effects move at different framerates than the characters, which draws your eyes to the most important movement.

Playing the clip in slow-motion, we can see the different framerates in action. The fireball moves on ones, while the giant bird moves on twos—until it reacts to the pain of the fireball destroying its wing, at which point it also moves on ones. Meanwhile, the electric-looking effects start out on ones, then switch to threes (you can see the fireball moving away for three frames), which pop out on the screen and makes us feel the power of the impact.

Before the 1960’s, Japanese animation more or less complied to the western norm. The films were typically animated at a uniform 12 or 24 drawings per second. However, once television came around, the animators had to reduce the number of drawings in order to feasibly produce a new episode every week. Animation on threes became the new standard. Western* televised animation also became more limited, but rather than resorting to threes, they more often used other time-saving tricks: reused drawings, cutouts, and static held shots between actions. Anime also relied on these methods, but being on threes gave it greater flexibility in terms of new poses and prolonged movement.

*By “western,” I mostly mean Hollywood-produced TV cartoons. Of course there were exceptions; e.g. New York TV animation did its own thing for a while.

Osamu Tezuka’s 1963 show Astro Boy is often cited as setting the look of TV anime, but he had already pioneered limited animation a year earlier with films like Male and Tales of a Street Corner. Both of these films show heavy influence from the UPA studio, which had spearheaded the modernist look of American animation in the late 40’s and 50’s. Its influence manifests through bold lines, sharp angles, simplified and graphic shapes and colors, and a stylized approach to motion. In Tales of a Street Corner, most of the flesh-and-blood characters are fully animated on ones and twos, while the posters-come-to-life move on threes. The moth, animated by Gisaburō Sugii, anticipates the future development of framerate modulation with its nervous, semi-limited flittering. In one sequence, the moth goes through a sort of insect mind-trip and switches between ones, twos, and threes in the same cut, each shift reflecting the critter’s mental state.

Other Japanese artists would also start to experiment with framerate modulation. Yasuo Otsuka’s animation in Horus, Prince of the Sun (1968) employs the technique in much the same way Disney did, in order to depict the massive sizes of the rock golem and ice monster. Partially animated on threes, their slow, deliberate movements suggest their titanic power and weight. According to Ben Ettinger, Otsuka’s approach to framerate modulation was so influential that it became standard practice in the industry. However, some animators took the concept even further.

Keiichiro Kimura, perhaps more than any animator before him, made framerate modulation a defining part of his work. He is best known as the man who inspired Yoshinori Kanada’s distinctive style, which is evident in his loose, vigorous linework and audacious poses. His drawings alone are striking, but the way he moves them is also incredible. Within the same cut, he will emphasize a single piece of action—sometimes just a few drawings—with a faster or slower framerate. The constant variation in his timing makes his animation exciting and unpredictable; it is “compulsive viewing” as Richard Williams would say. Check out this clip from the 1969 Tiger Mask opening: it starts out on twos, then switches to ones on the cape swirl, drawing our attention as the protagonist transforms into the titular wrestling hero.

By the early 70’s, animators like Sadao Tsukioka and Hayao Miyazaki were also using modulation to bring a catchy flair and energy to their scenes. This scene from Samurai Giants, animated by Miyazaki, has most of its action on threes while the speed of the pitcher’s throw is conveyed with twos.

Eventually, Yoshinori Kanada inherited Kimura’s offbeat timing and inspired a new generation of animators to do the same. His early work on Invincible Superman Zambot 3 clearly shows Kimura’s influence in its sketchy lines and wild poses. The irregular framerate makes the motions feel more lifelike and organic than they would be if evenly timed.

Lower framerates can, in fact, work well for fast motion, as this Gurren Lagaan sequence by Shingo Natsume demonstrates. The movement here is more implied than explicitly shown, yet each drawing is spaced and posed in a logical way, and so the action is still easy to follow. Much like what Scott McCloud calls “closure” between comic panels, your brain can fill in the gaps between the drawings. Note how the framerate slows down as the robot hurtles towards the sky, accentuating its vast distance, then it speeds up again as the machine comes crashing to the ground.

It might be a stretch to call framerate modulation the most unique aspect of Japanese animation, but it’s a great example of making an asset out of a limitation, and the spread of the technique signifies a industry that is willing to experiment and shrug off conventional wisdom. It is emblematic of the free creative spirit that makes the best anime so fun and engaging to watch.