Smear frames: distorting a frame to show speed in Blender

Pause a cartoon on a fast gesture: an arm stretches like rubber, a head leaves three copies behind it, a leg turns into a streak of colour. At normal speed, this frame goes unnoticed, but it is what gives the movement its speed. That is a smear frame. This guide explains the principle and four ways to build one in Blender.

What is a smear frame?

In animation, a smear, or smear frame, is a transition frame that distorts the subject to represent the path it travels between two poses. When an object moves too fast, two sharp poses on two consecutive frames produce a jerky effect, known as strobing: the eye sees two distant positions instead of a movement. The distortion fills the gap.

The technique comes from hand-drawn 2D animation, where the animator distorts the drawing by hand. In 3D, it has to be rebuilt, because the engine computes sharp frames: the Blender manual points out that, by default, animations are rendered as a sequence of perfectly still images.

The three forms of smear

• Stretching: the subject elongates along its path, between its position on the previous frame and its position on the next one. This is the simplest form to build in 3D.
• Multiples: several copies of the limb or object, spread along the path, over one or two frames.
• The trail: lines or a coloured shape behind the subject, without the subject itself being distorted.

Smear frame or motion blur

Blender's motion blur (Motion Blur, in the render properties of both Cycles and EEVEE) simulates a camera shutter: the object is rendered blurred in the direction of its movement, over a duration set by Shutter and expressed in frames. With Shutter at 1.0, the blur covers the duration of one frame. It is a physical, realistic effect.

A smear is a graphic choice. It keeps a clean outline and bold colours, which suits a stylised render, where photographic blur would look out of place. The guide cel shading in Blender describes this kind of render. The two can coexist in the same shot: a smear on the character, light blur on the background scrolling past.

The principle in Blender: one frame, with Constant interpolation

Every method follows the same logic: the distortion is zero on the frame before the gesture, reaches its maximum on the smear frame, and returns to zero on the next frame. For it to exist only on that frame, with no gradual build-up, set the keyframes involved to Constant interpolation (Key > Interpolation Mode, shortcut T, in the Graph Editor). The value then stays fixed until the next keyframe, which gives a clean change.

Work on the smear last, once the timing of the gesture is approved: its shape depends on the exact position of the subject on the neighbouring frames.

Method 1: a shape key

Shape keys deform an object towards a new shape, controlled by a value between 0 and 1. This is the most direct method for an object or a single part.

1. In the Object Data tab, Shape Keys panel, add a Basis key, then a second key, named for example "smear".
2. Select the smear key, switch to Edit Mode and stretch the geometry along the path, towards the position the subject will occupy on the next frame.
3. Set keyframes on the value of this shape key: 0 before the gesture, 1 on the smear frame, 0 after, with Constant interpolation.

Limitation: the shape key corresponds to a fixed distortion. If the gesture changes direction from one shot to another, you need one shape key per direction.

Method 2: Lattice or Simple Deform

To stretch a whole part without editing the mesh, two deform modifiers are enough.

• Lattice: the modifier deforms the object according to the shape of a Lattice object, a cage of points. Stretch the cage along the path, then animate the cage itself or the modifier's Strength. A Vertex Group limits the effect to part of the mesh, a hand or an arm for example.
• Simple Deform: in Stretch mode, it stretches the object along an axis; in Bend mode, it bends it; in Taper mode, it narrows it on one side. Animate the Deform Factor (Stretch, Taper) or the Deform Angle (Bend): 0 outside the smear, the value you want on the smear frame. The calculation is done in the object's local coordinates: check the orientation of its axes before choosing the deform axis.

Method 3: multiples

For the copies effect, duplicate the limb or object (Shift+D) on the smear frame and spread the copies along the path, between the start position and the end position. Make them disappear on the other frames by animating their render visibility, or their scale down to zero, again with Constant interpolation. Two or three copies are usually enough; more muddies the silhouette.

Method 4: a Grease Pencil stroke

The trail is drawn. Add a Grease Pencil object and, on the smear frame, draw a shape that extends the subject backwards, in the colour of its surface or a lighter shade. The Grease Pencil object lives in the 3D scene: the stroke keeps its place if the camera moves. A blank keyframe on the next frame (Insert Blank Keyframe, Shift+I) makes it disappear. The guide Grease Pencil in Blender introduces the tool.

This is the method closest to traditional hand-drawn animation. It allows shapes that geometry handles poorly: speed lines, splashes of colour, motion arcs.

Common mistakes

• The smear appears gradually or lasts several frames: the keyframes were left on Bézier interpolation. Switch them to Constant.
• The smear goes the wrong way: it must connect the subject's position on the previous frame to its position on the next frame, following the arc of the movement.
• The smear jumps out during playback: it lasts too long or it is too strong. Judge it at full speed, not frame by frame.
• The smear turns into a blurry smudge: motion blur is being added on top. Each object has its own blur settings in its object properties: turn it off on the object concerned.
• Simple Deform stretches along the wrong axis: the distortion follows the object's local axes, not the world axes.

Rendering a sequence with smears

A smear can only be judged in motion, on the final render: a 3D Viewport preview shows neither the final lighting nor the stylised material. Render the passage concerned first, a few dozen frames, before starting the full sequence. For engine and graphics card settings, see the guide on how to render in Blender.