What is onion skinning?
Onion skinning in animation is an editing technique used to see several frames of an animation simultaneously. This means the animator can tell whether the lines of each frame are correctly lined up — which is crucial to create an animation with smooth motion.
Historically, onion skinning in animation was made possible by the semi-transparent paper or ‘cels’ used by animators at Walt Disney Studios from the 1920s.
Why is it called onion skinning?
The name ‘onion skinning’ derives from comparisons between the editing technique and the humble root vegetable. Much like an onion is made up of multiple translucent layers of skin, animation frames are drawn on semi-transparent sheets known as cels.
This led animators at Disney in the 1920s to dub them ‘onion skins’ and the practice of layering these on top of one another, much like the layers of an onion itself, became known as ‘onion skinning’.
Cel animation is something else you can try at home. Learn more in our guide to cel animation.
Why is onion skinning used in animation?
Traditional animations consist of multiple individual sketches, with slight differences, presented in quick succession. For the person watching, the sketches appear as one continuous moving image. This is an optical illusion known as the ‘Persistence of Vision.’
To achieve this effect, each individual image must be replaced by the next one while your brain and eye still retain the previous image. Because the eye ‘sees’ this original image for a fraction of a second as the new image appears, it creates a feeling of motion or flow between the two images or frames1. When this process is repeated across multiple animation frames, it creates a moving image.
It’s this process and the concept of the Persistence of Vision, that makes animation through onion skinning such a useful tool to creatives.
If the angles and shape of the drawing on one frame differ too much from the following frame, the illusion of fluid movement between the two is lost. Onion skinning therefore enabled animators to be highly accurate when positioning the outlines of their characters and scenes. By placing the next frame on top of the previous one, they could see through the semi-transparent paper to know exactly where the lines should be drawn, to suggest natural movement.
With such a view, the artists can easily see and correct inaccuracies. Onion skin animation is therefore central to creating smooth, flowing animations.
How traditional onion skin animation works.