How to Fix Gaps Between Mouth Keyforms in a 2D Character Rig

A keyform records an object's shape at a particular parameter value. Cubism automatically interpolates the shapes between keys, so a clean closed mouth and a clean open mouth do not establish that every position between them will look correct. The official About Parameters manual demonstrates this relationship using mouth opening and closing.

Cubism normally uses linear interpolation. Its extended interpolation documentation explains that points traveling along straight paths can produce unwanted shrinkage when the intended movement follows a curve. For mouth troubleshooting, this gives you a useful distinction: inspect both the poses you authored and the paths connecting them.

Treat the mouth as several cooperating shapes. A lip outline, the colored interior and a clipping mask may look aligned at a key while separating during the transition. Your first task is to find which boundary stops matching.

Save a separate working copy, then return unrelated parameters to their defaults. Start with the mouth facing forward so you can examine opening and shape changes without also adjusting head movement.

For a conventional Cubism setup, ParamMouthOpenY runs from 0 for closed to 1 for open, while ParamMouthForm commonly runs from −1 through 0 to +1. Custom rigs can use different ranges; inspect yours before changing it. These conventions appear in Live2D's Standard Parameter List.

Use this diagnostic sweep. The sample values are inspection points, not a required keyform layout:

1. Hold Mouth Form at 0 and move Mouth Open slowly through its full range.
2. Inspect opening values such as 0, 0.25, 0.5, 0.75 and 1.
3. Repeat with Mouth Form at each end of its range.
4. Hold the opening near the worst gap and sweep Mouth Form continuously.
5. Record the two values and the location of the exposed area.

This sequence is a practical diagnostic derived from Cubism's interpolation behavior. If opening alone produces the problem, investigate that transition first. If it appears only while opening and changing form together, preserve both values while investigating. Otherwise, you may repeatedly edit a pose that never reproduces the defect.

At the failing values, inspect the mouth components individually. Temporarily isolate them in the working copy, then restore their visibility before evaluating the complete result. Use the following table to choose your next check; its entries are diagnostic possibilities rather than confirmed causes.

What you observe	What to inspect next
A narrow exposed strip between an outline and the mouth interior	Interior coverage and the neighboring lip boundary
A triangular notch during a transition	Local vertex movement and triangle layout
A whole mouth component disappears	Key coverage on that object and its controlling hierarchy
A thin pale rim around clipped artwork	Mask edges and translucent texture pixels
The defect appears only with two mouth controls combined	The same parameter combination on all affected objects

Avoid changing every component together. Make one small correction, repeat the failing movement and compare it with the saved version. A localized test makes it easier to tell whether the adjustment repaired the cause or merely covered it at one position.

Begin with the boundary nearest the gap. Check whether the interior reaches behind the lip throughout the failing transition. Where your artwork permits it, adjust the interior to maintain a small overlap beneath the outline instead of depending on two independently deforming edges meeting exactly. This is a modeling recommendation, not a prescribed overlap measurement.

If moving the boundary produces a sharp notch or an uneven curve, inspect the mesh. Live2D recommends manually preparing suitable ArtMeshes for detailed facial deformation, including the mouth. Its Edit Mesh manually manual explains the triangular mesh structure and tools for editing vertices and edges.

Look for vertices that remain behind while neighboring points move, triangles that become extremely narrow, and corners whose movement pulls an unexpectedly large area. Adjust the existing keyform geometry first. If the topology itself prevents a clean shape, revise it in your saved copy and recheck every affected mouth pose afterward.

Original maker documentation provides a useful example: in Rigging complex mouth shapes in live2d, Sydney Cook describes coordinating the upper lip, lower lip and interior mask, with deliberately limited interior geometry to avoid leaving middle vertices behind. That construction illustrates why controlled vertex movement matters; it is not a universal mesh pattern to copy into every rig.

Add a corrective key when the neighboring poses are satisfactory but the shape between them needs explicit control. First confirm that the affected objects have appropriate keys across their intended range. Live2D notes that an object with only one keyform is displayed only at that key value; objects intended to remain displayed need keys at both parameter ends. See Add/Delete Keys to/from parameters.

For an illustrative repair, suppose the neutral mouth looks correct at opening values 0 and 1, but its interior pulls away from the lower outline near 0.4. Select the affected object, enter 0.4 in the parameter's numeric field and use the parameter adjustment pop-up's + control to add a key there. Correct its shape, then inspect both new intervals: 0–0.4 and 0.4–1. The official key-editing manual documents numeric positioning and adding keys through this pop-up.

Keep Mouth Form at the recorded failing value while making the correction, then check neighboring form values. A repair that works only at one exact combination still needs refinement. Add further keys only where testing reveals a distinct shaping need; a dense collection of keys makes later adjustments harder to assess.

If the interior appears correctly positioned but the edge has a pale rim, inspect clipping before reshaping the mouth again. Live2D explains that translucent mask boundaries can let underlying ArtMesh colors show through. Its Clipping Mask manual also documents selecting the mask through the Inspector's Clipping ID control.

Check that clipped mouth components reference the intended mask. Then examine that mask at the failing parameter combination. For the specific translucent-edge problem, the manual describes a duplicate, enlarged mask with opacity set to 0%, deformed to follow the original. Apply that method only after identifying the edge artifact.

Also verify mask key coverage and restore any temporarily hidden parts before export. The same manual explains that missing key ranges and hidden masks can produce differences between the Editor and an SDK application. Test the exported result in its actual destination when clipping is involved.

Extended interpolation is worth testing when the unwanted deformation comes from the trajectory between otherwise useful shapes. Compare it on the affected object and repeat your parameter sweep. Cubism offers curved interpolation through its Extended Interpolation settings, and the Interpolation Type manual states that at least three parameter keys are required.

A curved trajectory does not establish that the interior covers the opening or that the mask is correctly assigned. Keep the setting only if inspection shows an improvement across the relevant range. Live2D's extended interpolation manual also notes that generated interpolation keys can increase model size when multiple parameters are involved.

For rigs using overlapping vowel Blend Shapes, investigate their combined influence. Live2D specifically demonstrates mouth shapes breaking when multiple vowel Blend Shapes are active and explains how to constrain their contributions in Limit settings for blend shape weights. This applies to Blend Shape combinations; an ordinary mouth-opening correction does not require converting the rig.

Repeat the original sweep after each meaningful change. Inspect the mouth enlarged to locate small defects, then at its intended display size to judge the visible result. Include the positions immediately before and after every corrective key, rather than checking only the key itself.

Use this completion checklist:

• Sweep Mouth Open with Mouth Form at its center and both extremes.
• Sweep Mouth Form at closed, partial and full opening.
• Move both controls together through the previously failing region.
• Restore other controls and check any head positions used by the character.
• Confirm that temporary isolation settings are restored.
• If the mouth uses clipping, compare the exported model in its destination application.

A useful repair addresses a reproducible cause: missing key coverage, a boundary that separates, a mesh that deforms poorly or a mask that renders an unwanted edge. Keep the recorded failing values until the final check. They give you a precise way to confirm that the original gap is resolved while the surrounding mouth movement still works.

• About Parameters — Live2D Editor Manual
• Standard Parameter List — Live2D Editor Manual
• Add/Delete Keys to/from parameters — Live2D Editor Manual
• Edit Mesh manually — Live2D Editor Manual
• Rigging complex mouth shapes in live2d — Sydney Cook
• Clipping Mask — Live2D Editor Manual
• About Extended Interpolation — Live2D Editor Manual
• Interpolation Type — Live2D Editor Manual
• Limit settings for blend shape weights — Live2D Editor Manual