MotionSplicer: Part-Based Motion Editing for 4D Volumetric Videos

4D volumetric video via Gaussian Splatting (GS) enables free viewpoint control of real-world scenes but does not enable motion editing. One challenge is achieving part-based control in multi-object scenes; prior works focus on single objects, assume rigid parts, or require manual annotations. MotionSplicer is a template-free and annotation-free method that automatically discovers 3D parts in GS. To bridge the semanticto-motion domain gap, it extracts initial parts from temporal difference images and resolves multi-view inconsistencies via an adaptive 3D unifyand-split process. For robust tracking, it optimizes a time-invariant feature grid with temporal refinement to learn spatially and temporally coherent skinning weights. Experiments show that MotionSplicer can be applied more generally to objects and scenes than state-of-the-art methods, and enables creative motion edits like isolation, ghosting, and echo.