Interpolation and Compatibility
Interpolation combines corresponding data. Before judging the shape it produces, establish what FontLab is pairing between the masters. A numerical blend can run successfully while connecting the wrong parts of a letter.
Interpolation Mechanics
On a simple axis with Light at 300 and Bold at 700, a location of 500 is halfway between them. Each master contributes half of a corresponding point's coordinates. At 340, Light contributes 90% and Bold 10%.
If an intermediate master is added at 500, that drawing becomes a source in its own right. Locations between 300 and 500 follow the first interval, and those between 500 and 700 follow the second. The intermediate master is not forced to equal the former midpoint result.
With several axes, the interpolation model combines the relevant surrounding sources according to the location and their arrangement. Test combinations, such as a heavy condensed design, as well as movement along one axis at a time.
Pair the data before blending it
Pairing identifies which objects correspond. Within paired contours, point order provides the sequence of coordinates to combine. The structural pairing is indexed at each level: elements by their Elements-panel stack order, contours within a paired element by contour index, and points within a paired contour by point index. Named-element matching can establish the element pairs before those internal indices are compared.
Element names provide an additional matching mechanism. A named element can pair with its namesake in another master even when their raw stack positions differ. For repeated occurrences of a name, FontLab matches up to the smallest common count, using panel order within that group. Keep counts and order deliberate: a name alone does not identify which identical serif should correspond, and excess occurrences need inspection.
Unnamed or otherwise unmatched elements depend on structural ordering. Use meaningful names for reusable parts, and inspect the correspondence after adding or removing an occurrence. Do not assume that the visual resemblance of two shapes determines their pairing.
Compatibility and Matching Masters
Compatibility establishes that corresponding structures can be combined. Check element and contour counts, open or closed contour state, node/segment counts and types, and the correspondence between reusable parts. Automatic matching may reconcile some differences, so distinguish the original structures from the structures a corrected interpolation uses.
Compatibility is a pairwise structural test, independent of the number of axes or masters: compare a glyph in one source with its counterpart in another. A font with many masters needs consistency across the relevant sources, not only one successful comparison between two extremes.
Compatibility is not correct correspondence
Consider an o whose outer contour comes first in one master but whose inner contour comes first in another. Counts may agree, yet interpolation can connect the outside of one design to the counter of the other. The calculation runs, but the result is wrong.
The same problem occurs when starting points occupy different parts of otherwise similar closed contours, or when the contour directions disagree. Topological stability means that the intended parts continue to correspond throughout the blend. Equal counts do not establish it.
Matching masters is the work of establishing usable correspondence: aligning elements, contour order, starting nodes, directions, and segment structure. A matching operation can succeed fully, partly, or fail. Its status is a diagnostic result, not a design approval.
Use structural checks to identify whether a blend is possible, then inspect intermediate shapes to judge whether it is correct. Crossed relation lines, collapsing counters, or a corner appearing at a formerly smooth join need attention even if the masters pass a basic count test.
Interpolation beyond Point Coordinates
An instance also needs to be spaced and assembled correctly. Supported per-master data extends beyond the outline's node coordinates:
| Data | What to inspect in intermediate designs |
|---|---|
| Element placement and component offsets | Parts move to the intended relative positions. |
| Advance widths and sidebearings | Spacing remains appropriate as the drawing changes. |
| Kerning values | Pairs remain balanced between master locations. |
| Anchors and pins | Corresponding named attachment locations follow the design. |
| Vertical font dimensions | Caps height, x-height, and other varying dimensions reflect the intended design. |
Sidebearings are relationships between ink and advance width, so an apparently sensible width does not guarantee balanced spacing. Linked or expression-based metrics must resolve sensibly in each source. A cross-master link such as :Regular refers to the corresponding metric in the same glyph’s Regular layer; use it when that source relationship is intended, rather than expecting each master to have an independent value. Check the resulting instance rather than treating the expression text itself as a value to average.
Use consistent anchor and pin names. A top anchor in one master must correspond to the intended top in another, with the necessary points present where the construction needs them. Interpolated attachment positions let a mark follow a base as its proportions change. Missing or mismatched attachment data can break a composite even when its base outline is compatible.
Kerning is master-level data with its own matching and coverage requirements. Kerning does not support ranged intermediate masters. The across-master kerning workflow is taught in Kerning Visually.
Family dimensions are held constant across masters; they are not interpolated. Keep those values consistent across separately stored family members as well. Per-master font dimensions can vary and are reflected in the generated #instance; font-wide identity strings and every arbitrary setting are not simply numeric interpolation inputs.
Hints and zones tied to the design also need coordinated master data and suitable output handling. A successful outline preview is only one part of that work. Check a representative text string, composites, spacing, and the supported auxiliary data before treating an instance as complete.
An accent constructed from an auto layer illustrates why positions and metrics matter along with outlines: if only the master endpoint placements worked, an intermediate accent could drift relative to the blended base. Inspect attachment positions and linked or bound sidebearings throughout the range. FontLab’s matching and diagnostic tools are taught in Previewing, Diagnosing, and Fixing Interpolation. Hinting and alignment-zone workflows are developed in Alignment Zones and Standard Stems and Glyph-Level PostScript Hints.