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B-Spline Mask Node

By Kriss Norde

The B-Spline Mask uses B-Spline curves (a single point per control) instead of Bézier (point + tangents). The result: smooth curves with far fewer points, ideal for rotoscoping organic shapes (faces, bodies, curved objects). Faster to draw and animate than a Polygon Mask.

Overview

The B-Spline Mask generates a mask through B-Spline interpolation. This mathematical interpolation automatically produces smooth curves between the control points - no tangents to adjust as with Bézier.

Its main advantage: to trace a face, 6-8 B-Spline points are enough where a Polygon would need 15-20 points with adjusted tangents. Animation is faster because there are fewer points to keyframe.

Where to find this node

  • Effects Library: Tools > Mask > B-Spline
  • Quick add: right-click in the Node Editor → Add Tool > Mask > B-Spline

B-Spline vs Polygon Mask

B-Spline
A single point per control, no tangents. Smooth curves automatically. Fewer points needed for an organic shape. Faster to animate.
Polygon (Bézier)
A center point + 2 adjustable tangents. More precise for corners and details. More points needed for smooth curves. Slower to animate.

Rule of thumb: smooth organic shapes (face, arm, fabric) → B-Spline. Angular shapes (buildings, geometric objects) → Polygon.

Inputs

  • Effect Mask (blue) - optional. A mask to combine through the Paint Mode.

Setup

Rotoscoping a face:
MediaIn → Color Corrector ← B-Spline Mask (skin roto, 6-8 points)
       → MediaOut

Animated body roto:
MediaIn → localized effect ← B-Spline Mask (body silhouette, animated)
       → MediaOut

Combined with a Polygon for a hybrid shape:
B-Spline (organic outline) → Polygon Mask (Paint Mode Add) to add
an angular detail → final Effect Mask

Controls tab

Center X / Y
Overall position of the mask.
Size
Overall scale. Does not affect the point animation (independent scale).
X / Y / Z Rotation
Rotation around the 3 axes (Z is the standard one for 2D rotation).
Tension
Controls how smooth the curves are. Holding W while dragging changes the tension of a specific point. More details in the dedicated section.
Level
Mask opacity (0-1).
Soft Edge
Feathers the edges. Selectable filter (Box, Bartlett, Multi-box, Gaussian).
Solid
Filled or outline only.
Border Width
Outline thickness when not Solid.
Invert
Inverts the mask.
Paint Mode
How it combines with the incoming mask: Merge, Add, Subtract, Multiply, Replace, etc.

Tension: controlling the smoothing

Tension controls how closely the B-Spline curves follow the control points:

Low tension (smooth)
The curves pass "far" from the points - a very rounded shape. Sometimes the points themselves are not on the visible curve.
Moderate tension (default)
A natural compromise - the curves pass close to the points without being too constrained.
High tension (firm)
The curves stick to the points - a more "polygonal" look. At the extreme, it is equivalent to a Polygon Mask without tangents.

Tip: holding W while dragging changes the Tension of a specific point. This lets you have smooth areas and sharper areas in the same mask.

Use cases

  • Rotoscoping a face - 6-8 B-Spline points to trace a face or head outline efficiently. Fewer points = simpler frame-by-frame animation.
  • Rotoscoping a human body - 12-15 points for a full silhouette, where a Polygon would need 25+ points with tangent adjustments.
  • Moving fabric - flag, curtain, floating cloth: B-Spline curves animate naturally without odd corners.
  • Animals / organic shapes - animal rotos, foliage, water - anything without straight edges.
  • Localized skin grading - a quick mask on a character's skin for a localized skin color correction.
  • Reusable mask template - a B-Spline with few points is easier to reuse across several similar shots (fewer adjustments).

Tips

  • Fewer points = better - place points at the major changes of direction in the shape. Don't add extra ones - the B-Spline curve smooths in between.
  • Tension with W + drag - to adjust the smoothing locally without touching the positions, hold W while dragging.
  • B-Spline for curved shapes only - if the shape has sharp corners (furniture, building), Polygon is more precise.
  • Done mode after drawing - locks the mask to avoid accidental changes. Switch back to Modify to edit it again.
  • Combine B-Spline + Polygon - if you need precise corners AND organic curves, combine both types with Paint Mode Add.
  • Gaussian Soft Edge for quality - a natural, soft feather. Essential for realistic grading integrations.
  • Planar Tracker modifier - for a moving subject, attach the B-Spline points to a Planar Tracker for automatic animation.