What does it mean for animation to be done "on ones" or "on twos"?

Question

Animation is sometimes described as being done "on ones" or "on twos" or "on threes". What does that mean?

Answer 1

Basically:

Fundamentally, all anime today is produced at a rate of 24 frames per second. This is the same framerate used for most (all?) film today (e.g. in Hollywood movies). For a filmmaker with a camera, this just means setting your camera to expose 24 frames of film each second. But for an animator, this means drawing 24 images for each second of animation. This can be time-consuming.

To reduce the amount of drawing that needs to be done, many animations reuse images for multiple frames - rather than drawing 24 images for each second, they might draw just 12 or 8 images for each second, and then repeat each image for two or three consecutive frames. That is, one second of animation would look like the "twos" or "threes" row on the following schematic, in which each letter represents a different image and each column represents one frame:

frame#  01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
        
ones    A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  
twos    A--A  C--C  E--E  G--G  I--I  K--K  M--M  O--O  Q--Q  S--S  U--U  W--W  
threes  A--A--A  D--D--D  G--G--G  J--J--J  M--M--M  P--P--P  S--S--S  V--V--V  

As the schematic suggests, using 12 distinct images per second is called "shooting on twos" or "animating on twos", and likewise, using 8 distinct images per second is called "shooting on threes". Drawing a distinct image for each frame is "shooting on ones", and is akin to what happens in film.

But note that:

Mind you, it is not necessarily the case that all the pieces that go together to form a particular segment of animation will be shot at the same rate. For example, if you have some people walking in the foreground with a pan over a static backdrop in the background, the foreground might be animated on threes (since the walking animation doesn't need to be all that fluid), while the background might be animated on ones (since it takes very little extra effort to shoot more frames on a pan).

Most animation in anime is done on ones, twos, or threes - anything slower will look decidedly jerky. Nonetheless, you can speak of shooting on fours (6 images per second) or fives (24 images per 5 seconds) or higher numbers as well. Rates that are effectively non-integral are also possible, e.g. "shooting on two-point-fives", as below, is not uncommon:

frame#  01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
        
2.5s    A--A  C--C--C  F--F  H--H--H  K--K  M--M--M  P--P  R--R--R  U--U  W--W--

But I don't know if there is an actual name for doing that. If there is one, though, I'd bet that "shooting on two-point-fives" isn't it.