That's a great idea for a feature! Unfortunately, Spine's current clipping implementation works at the polygon/triangle level on the CPU — it clips geometry by cutting triangles against the clipping polygon's edges. This is already quite CPU-intensive, and the result is always a hard, crisp edge by nature of how the geometry clipping works.
A soft "airbrush" style fade-out at the clipping boundary would require a fundamentally different approach — most likely a shader-based solution (like using a gradient mask texture evaluated in the GPU fragment shader), rather than the CPU geometry clipping that Spine currently uses. This would be a significant architectural change to how clipping works in the runtimes.
What you can do in the meantime:
- You can manually create a mesh attachment with vertices that follow the edge you want to fade, and paint transparency/alpha into the texture itself so the edge fades out softly. This is a common workaround for achieving soft edges.
- You can also weight the clipping attachment's vertices to bones, which allows the clipping shape to deform and move with the skeleton, giving you more control — but the edge will still be hard.
For reference, the official docs note that clipping is already very expensive on the CPU, especially with many vertices:
"Use a clipping attachment with just 3 vertices whenever possible, which is the minimum number of vertices allowed."
See the Clipping attachments performance section for more details.
It's a worthwhile feature request, but given the current architecture, a soft airbrush-style clip would likely require a different rendering approach entirely.
