How does stop-motion animation work?

Nothing on a stop-motion set ever moves on its own. An animator bends the puppet a tiny bit, steps out of the way, and the camera takes one photo, called a frame. Then they do it again. And again.

Play the photos back quickly and your brain joins them into smooth movement (the same trick as all film, see FilmClear). Cinema runs at 24 frames a second. Many stop-motion films are animated on twos: each pose is held for two frames, so the animator makes 12 poses for each second.

That means 1 minute of film needs 1,440 frames, or 720 poses on twos. No wonder it is slow. At Aardman, the studio behind Wallace & Gromit, one animator makes about 4 seconds of film a week. On Laika's Coraline (2009), 29 animators together made about 90 to 100 seconds a week.

The size of each nudge sets the speed. Big moves between frames make a fast, jumpy walk. Tiny moves make a slow, smooth one.

A stop-motion puppet has to hold any pose you give it, for as long as the camera needs, and then move again. Inside most puppets is an armature: a small metal skeleton with ball-and-socket joints. Each joint is squeezed by a screw so it is stiff enough to hold a pose, but loose enough to move by hand.

Over the armature goes the skin. Clay (plasticine) is easy to reshape, which is why clay films look so hand-made. Silicone is poured into a mould and is tough and skin-like. Foam latex is light and squashy but wears out, so spares are made.

A puppet only stands on its own while its centre of mass is above its feet. In a walk or a lean it often isn't, so animators use tie-downs: a screw from under the baseboard into a threaded plate in the foot, or a magnet under the floor and a steel plate in the shoe.

For a jump, a rig (a posable arm clamped to the puppet) holds it in the air. The rig shows in the photos, so it is painted out later, a job for the visual effects team (see VFXClear).

To make a puppet talk, animators listen to the voice recording and write down which sound is heard on which frame. This is called lip sync. Most speech can be shown with about eight mouth shapes: M-B-P (lips shut), A-I, E, O, U, F-V, L, and rest.

There are two ways to change the mouth. Sculpting: push the clay into a new shape every frame. It can make any shape, but it is slow, and each mouth is slightly different, so it shimmers. Replacement animation: make each mouth in advance as a separate piece, and swap it on, often held by tiny magnets. It is fast and neat on set.

The studio LAIKA 3D-prints its replacement faces. It printed about 20,000 faces for Coraline (2009), and over 100,000 for Missing Link (2019). Upper and lower halves mix, so a face can make millions of expressions.

Stop-motion animators shoot through frame-grab software on a computer (Dragonframe is a popular one). It shows the camera's live view with the last few frames laid over it, faded, like thin layers of onion skin. This onion skin lets you see exactly how far you moved the puppet since the last frame.

That distance is the spacing, and it is where the life comes from. A falling ball speeds up, so its spacing gets wider near the floor and closer at the top. Move it the same amount every frame and it looks mechanical. (Spacing and ease are two of the 12 principles of animation, see Anim2DClear.)

A photo is taken minutes apart from the next one, so anything that changes in between shows up as a jump. Daylight drifting through a window, a camera that gets bumped, or a camera that re-sets its exposure each shot all make the picture flicker. The fix: black out the windows, use steady lamps (see LightingClear), set the camera to manual, and lock off the tripod.

Stop-motion is not only puppets. Anything you can move a little and photograph can be animated.

Cut-out animation uses flat paper shapes with pinned joints. On a multiplane stage, layers sit on sheets of glass at different heights under a camera pointing down. Near layers are moved further each frame than far ones, which gives a feeling of depth. Lotte Reiniger made a whole feature from paper silhouettes, The Adventures of Prince Achmed (1926).

Pixilation animates real people. Photograph someone at the top of every jump, a little further along each time, and they seem to float. Norman McLaren's Neighbours (1952) made it famous. Object animation moves everyday things: in 1915 Dadasaheb Phalke, the father of Indian cinema, made a short film of dancing matchsticks. Sand animation pushes sand around on a glass sheet lit from below.

And puppets go back a long way: in the 1910s Ladislas Starewicz made films with insect puppets.

You can make stop-motion with a phone. Put it on a small tripod, build a tiny set on a desk with a sheet of card behind it, and use a free stop-motion app: it takes the photos, shows onion skin and plays your film back.

Plan first. A 5-second clip at 12 frames a second is 60 frames. Split it into shots: a wide shot to show the place, a close-up for a feeling, and a detail. Move things in small steps: a few millimetres a frame for a slow walk. Keep a pose for a few frames when the character thinks.

Avoid the five classic mistakes. A phone that gets knocked makes the picture jump. Tapping the screen shakes it too. Auto exposure makes the brightness pump. Daylight from a window makes the film flicker. And a puppet that is not stuck down slides and falls. Fix all five and your film looks steady and smooth.

Want more? The camera is explained in CameraClear, light in LightingClear, and sound effects in SFXClear.