I’ve already posted this in a comment and explained it in Show 3, but to keep things organized and findable, here’s where you can find my explanation of “What are ones and twos?”
ONES and TWOS
Here’s my explanation of ‘ones and twos.’ I also talk about it at the end of Show 3.
Film has 24 frames per second. Each frame has a different drawing, or image, and when they are viewed in succession through a projector, the images connect and create the illusion of motion thanks to what is called persistence of vision. That’s all simple enough.
If each drawing is numbered according to which frame it falls on, it would look like this:
1 2 3 4 5 6 7 8 9 10 11 . . .
In traditional (hand-drawn) animation, every frame can hold a different image but animators realized long ago that they could get by with only doing a drawing every other frame, saving extra work. The drawings would be “held” on two consecutive frames so that in one second, instead of doing 24 drawings (one every frame) they could do 12 drawings (one every other frame).
The numbered drawings would look like this:
1 – 3 – 5 – 7 – 9 – 11 . . .
and drawing 1 is repeated in frame 2, drawing 3 repeats in frame 4, etc.
Persistence of vision comes into play again and the action still looks fluid. A drawing held for two frames is called “on twos.” Logically, when each consecutive frame has a new drawing, it is called “on ones.”
It’s up to the animator to decide when to animate on ones and twos. Generally, the determining factor is how it will look in the end. If it is animated on twos (12 drawings per second) and there seem to be gaps in the action, the animator can go in and add new drawings to make it look more fluid.
If this series of drawing doesn’t look smooth enough (between 3 and 7):
1 – 3 – 5 – 7 – 9 – 11 . . .
we can add a drawing where it is needed like this:
1 – 3 4 5 6 7 – 9 – 11 . . .
Something that didn’t make sense to me when I first heard it is that faster action should be animated on ones. I thought, “Well won’t more drawings slow it down?” Not really, since we’re not adding time. The frames stay the same, we’re just putting more images in to connect the action. You may think of it as a strobe light. If it flashes slowly (on twos) and you wave your arms around, it may be hard to follow the motion. If we speed up the flashing (on ones) the action connects visually because there are more images shown in the same amount of time.
This is REALLY hard to get across in words alone, and I know I can tend to be verbose, but I hope this helps. If you pause traditional animation and advance frame by frame, you will be able to see which sections are on ones and which are on twos. Of course if you’re looking at some Saturday morning cartoons, they get away with animating on 12s, 24s, or worse.
We commented about The Thief and the Cobbler in Show 2 because Richard Williams prefers to animate nearly everything on ones.
CG is nearly always done on ones. In Fantasia 2000 some of the Tin Soldier sequence was animated on twos, but that is a rare instance.