Blender Grease Pencil Interpolate Sequence Explained

Blender’s Grease Pencil provides powerful 2D animation tools, among which the Interpolate Sequence operator is essential for traditional animators looking to speed up their workflow. This guide explains what the Interpolate Sequence operator is, how it automates the creation of in-between frames (inbetweening), the technical requirements for strokes, and how to configure its settings to achieve smooth frame-by-frame animation.

What is the Interpolate Sequence Operator?

The Interpolate Sequence operator is a Grease Pencil feature in Blender designed to automatically generate breakdown frames (inbetweens) across a range of empty frames between two existing keyframes. Instead of manually drawing each transition frame between an extreme pose and a target pose, the operator computes the geometric difference between the drawings and populates the timeline with intermediate drawings.

How the Operator Works

Grease Pencil interpolation operates based on stroke data, point counts, and stroke order rather than standard mesh interpolation:

  1. Stroke Matching: Blender pairs strokes on the starting keyframe with strokes on the ending keyframe based on their drawing order (index) within the layer.
  2. Point Calculation: If the start and end strokes have matching point counts and directions, Blender smoothly transforms the points from their initial positions to their final coordinates. If point counts differ, Blender temporarily subdivides or normalizes the points to morph the shape.

Step-by-Step Workflow

To use the Interpolate Sequence operator:

  1. Draw your starting pose on a keyframe in the timeline.
  2. Move the playhead forward to your desired target frame and draw the ending pose on the same Grease Pencil layer.
  3. Switch from Draw Mode to Edit Mode.
  4. Select the strokes you wish to inbetween (press A to select all strokes).
  5. Open the top menu and navigate to Grease Pencil > Interpolate > Sequence, or use the keyboard shortcut Ctrl + Shift + E.
  6. Blender instantly generates drawing keyframes across all empty frames between the selected extremes.

Key Settings and Customization

Once the operator runs, an Adjust Last Operation panel appears in the lower-left corner of the viewport (or press F9), providing several parameters:

  • Interpolation Type: Choose between Linear, Custom Curve, or standard Easing algorithms (such as Back, Bounce, Elastic, or Quintic).
  • Easing: Define where the acceleration occurs (Ease In, Ease Out, or Ease In/Out) to control the timing and weight of the animation.
  • Step: Define frame skipping to create inbetweens on "twos" (every 2 frames) or "threes" rather than every single frame, matching traditional animation cadences.

Best Practices for Clean Results

To prevent unwanted twisting or collapsing of strokes during interpolation:

  • Draw strokes in the same order and direction on both keyframes.
  • Keep the point counts similar across both drawings using the Stroke > Simplify or Subdivide operators in Edit Mode before running the interpolation.
  • For complex characters, interpolate individual parts (e.g., limbs, hair, facial features) separately rather than running the operator on the entire drawing at once.