What Is the Stretch To Constraint in Blender?

The Stretch To constraint in Blender is a dynamic rigging tool that scales an object or bone along a specific axis to point continuously at a target object. It is widely used in character animation to achieve classic "squash and stretch" effects, create flexible cartoon rigs, and simulate realistic muscle movements or mechanical components like springs and hydraulics. This article provides a clear, straight-to-the-point explanation of how the constraint functions, its core settings, and practical applications.

How the Stretch To Constraint Works

When applied, the Stretch To constraint calculates the distance between the owner (the object or bone with the constraint) and a target object. As the distance changes, Blender automatically scales the owner along its primary axis so that it always reaches the target.

By default, the constraint preserves the volume of the owner. As the object stretches longer, it shrinks thinner along its other axes. Conversely, when the target moves closer and the object compresses, it expands wider to maintain a believable, consistent volume.

Key Settings and Controls

  • Target: The object or bone that the constrained object stretches toward.
  • Rest Length: Sets the baseline distance between the owner and the target. Moving the target to this distance results in a scale factor of 1. Clicking the Reset or Recalculate button updates this value to the target's current position.
  • Volume Variation (Squash and Stretch): Controls how the non-stretching axes react during movement:
    • Volume Maintains: Defines how the volume scales (e.g., XZ, X, or Z).
    • Volume: Adjusts the intensity of the volume preservation. Setting this to 0 disables squash and stretch, leaving only the primary axis stretching.
  • Rotation: Controls how the constrained object rotates to track the target:
    • XZ / ZX: Determines the orientation of the secondary axes as the object tracks the target.

Common Use Cases in Blender

  • Character Rigging: Essential for cartoon limbs, tails, bendy bones, and facial features that require dynamic stretching.
  • Anatomy and Muscles: Used to simulate dynamic muscle flexing, tendons, and stretching skin as underlying bones move.
  • Mechanical Animations: Ideal for objects that need to span a variable distance between two points, such as pistons, rubber hoses, or springs.