Masters and Interpolation

Open a .designspace file and Runebender opens every master it names. Open a single .ufo and you get one master.

The Masters section of the right panel lists them, and each row draws the current glyph as that master draws it, so you compare without switching. Click a row to switch. View → Next Master and View → Previous Master step through them.

View → Show All Masters draws every master at once over the current glyph.

Axis sliders sit above the master list, and in the Axes section in the grid view. Moving one shows the interpolated result as a ghost over the drawing, so you can see a weight between two masters while you edit either one.

Axes section

Sliders	One per designspace axis. The position they set is the preview location, which several commands below use.
Instances	Name a style at the preview location to add it, or rename the instance already there. The family name is inherited.
Mappings	An avar pair on the first axis: user-space input to design-space output, written into the designspace.

Interpolation needs matching outlines: the same number of contours, the same number of points in each, in the same order, starting at the same place. When a glyph does not interpolate, that is almost always why.

The usual fixes

Correct Path Direction	Outer contours one way, holes the other, by containment depth.
Set Start Point	Make each contour start in the same place.
Tidy Up Paths	Remove zero-length segments, which differ between masters without being visible.

The Compare section in the grid view reports structural incompatibilities across every master at once, along with glyph counts, differing advances, kerning pair counts, and vertical metrics that disagree. Use it before you go hunting glyph by glyph.

Glyph → Reinterpolate replaces the current glyph with what the surrounding masters say it should be at this position. It is the way back when a glyph has drifted out of step with the rest of the designspace.

Sometimes the straight interpolation between two masters is wrong in the middle, and adding a whole master is too much. An intermediate layer fixes one glyph at one point in the space.

In the Layers block, + Intermediate freezes the current interpolation of the open glyph at the preview location: a named UFO layer plus a sparse designspace source. Edit it through the swap arrows. The interpolation ghost and the master strip pick it up live.

This is a standard designspace construct, so every compiler understands the result.

An ease bakes timing into an intermediate layer. A positive ease means the change arrives late, so the lighter shape holds on longer; a negative ease means it arrives early.

Only the selected points ease. The rest stay on the straight interpolation, which keeps the layer point-compatible with its neighbours.

Between two masters, a point normally travels in a straight line. A trajectory bends that line.

Turn on Trajectories with the select tool and each node shows a knob at the middle of its path. Drag the knob and the node’s interpolation curves through that point instead of running straight. Use it to stop a stem bowing, or a joint pinching, partway along an axis.

The intermediate points are stored on the axis-minimum master so they stay editable. What compilers consume is the baked intermediate layer, so the result is standard designspace either way.

A shape switch swaps one drawing for another partway along an axis: the classic case is a two-storey letter that becomes single-storey at heavy weights.

Adding one creates an unencoded alternate, copied into every master, plus a designspace rule substituting it from a chosen axis position to the end of the axis. Design the alternate in the Regular master; the copies start marked so you can see which ones still need drawing.

A smart component carries its own axes, so one component holds a range of shapes rather than a single fixed one. See Components and Anchors.

Interpolation across several axes uses a corner-delta model: the default shape plus the contribution of each axis corner. One axis interpolates linearly, and two axes with every corner present interpolate bilinearly.

A contour can be marked as a mask, subtracting from the shapes behind it while you draw. Glyph → Bake Masks makes the subtraction real in every master, so the exported outline matches what the preview showed.