B-Spline

B-Spline belongs to Fusion's classic Mask Tools. Its primary result is single-channel 0-1 coverage rather than a normal RGBA frame. A typical Flow sends MediaIn through the image effect to MediaOut while B-Spline travels separately into that effect's blue Effect Mask input.

The defining difference from Polygon is the curve model. Polygon uses Bezier points with handles, while B-Spline uses a single control point plus tension. Rounded faces, shoulders, cloth and other organic silhouettes can therefore remain smooth with fewer points.

Blackmagic's historical Tool Reference describes adjusting the tension of a selected point with W plus drag. That is useful workflow evidence, but it is an interaction shortcut, so verify the exact shortcut in the target Resolve 21/21.1 build before publishing it as a current UI guarantee.

Shape geometry and common mask controls solve different problems. Control points define the contour; Level, Filter, Soft Edge, Border Width, Invert and Solid process the resulting matte. A large Soft Edge should not be used to conceal an inaccurate shape.

Show View Controls is a Viewer ergonomics control: it shows or hides editable overlay geometry. It does not perform an image or mask-processing operation.

Level scales final mask intensity. Level 1 preserves full authored coverage; lower values reduce downstream effect contribution, but this is not the same as bypassing the effect or lowering the effect's own Blend.

Filter defines edge-filter character. The current bundled capture confirms Box, Bartlett, Multi-box, Gaussian and Fast Gaussian. Differences become easiest to judge with a visible Soft Edge and a high-contrast boundary.

Soft Edge feathers the boundary between active and inactive coverage. For roto, temporarily keep Soft Edge at zero while refining geometry, then add only the feather the shot actually needs and inspect it at 100-200% Viewer zoom.

Border Width emphasizes a band around the contour rather than only a filled region. Thin borders make point instability and interpolation wobble especially easy to see.

Invert swaps active and inactive mask coverage. It does not invert RGB and does not turn B-Spline into an image-processing node.

Solid controls whether the closed shape produces a filled interior. For outline workflows, Solid, Border Width, Border Style, Position and Length become the relevant cluster, but exact names for icon-only Border Style choices should be verified in the target Inspector rather than invented.

Border Style is represented by icons in the current capture. This article deliberately avoids assigning unsupported textual labels to those icons; compare them on a simple test contour in the target Resolve build.

Position and Length belong to partial-border/outline behavior. They are easiest to understand with Solid disabled, where it becomes clear that they do not move the whole mask the way Center does.

Center X/Y moves the entire B-Spline in normalized frame space. Use it for broad mask translation instead of moving every control point and creating unnecessary shape keys.

Size scales the complete mask around its Center while preserving point relationships. It is the right first choice for a global scale change.

X Rotation and Y Rotation tilt the mask through the common mask transform controls; Z Rotation rotates it in the image plane. These are global transforms, not substitutes for local contour repair.

Fill Method controls how overlapping or self-intersecting regions are filled. Blackmagic's historical documentation specifically recommends comparing Alternate and Non-Zero Winding when overlapping segments produce unwanted holes.

The Right-click here for shape animation row is part of the Controls UI and reflects that B-Spline geometry can be animated as shape data. Decide whether a change needs a shape key or can be handled more cleanly by Center, Size or Rotation.

For shape animation, stable topology matters more than keyframe count. A contour can match perfectly on keyed frames yet twist, wobble or breathe between them if point count/order changes aggressively.

The current Image-tab capture confirms Output Size with Default/Custom and Clipping Mode with None/Frame. These are technical raster/domain controls, not spline-geometry controls.

With Output Size set to Default, the mask normally follows the expected composition/consumer dimensions. Use Custom only for a real domain requirement; hidden Custom fields are not claimed as current evidence until they are explicitly captured.

Clipping Mode becomes relevant when Soft Edge, Border Width or a transformed shape crosses the frame boundary. Compare None and Frame on a contour that actually reaches outside frame so the difference is visible.

The optional mask input is for combining another mask with the B-Spline, not for supplying the RGB image. This supports B-Spline plus Rectangle/Ellipse/Polygon/Bitmap combinations while the image path stays separate.

B-Spline is excellent for organic roto but not automatically the best primitive. Polygon is better for intentional corners and explicit Bezier handles; Ellipse/Rectangle are faster for regular geometry; Bitmap is appropriate when coverage comes from image data.

Production QC should inspect both the matte and the final driven effect. A subtle matte defect can be nearly invisible in a black-and-white mask Viewer yet obvious after Blur, Color Corrector, Glow or key cleanup.

The three bundled screenshots define the current UI evidence boundary: main Controls, the open Filter menu and the Image tab. Unshown Settings controls, hidden Custom Output Size fields and textual names for icon-only Border Style choices are not treated as confirmed by these captures.

A robust order of work is: build a simple closed contour with few points, refine tension/geometry, review interpolation, then add Soft Edge, and only afterward tune border/fill/domain controls. This order makes diagnosis much easier.

If roto requires dozens of tightly packed points and manual correction on almost every frame, the problem may not be a lack of B-Spline controls. Consider tracking/stabilizing the broad motion and reserve B-Spline for local shape deformation.