Guard4D: Robust Watermarking for 4D Gaussian Splatting via Decoupled Decoding

Abstract

4D Gaussian Splatting (4DGS) enables high-quality, real-time rendering of dynamic scenes and is becoming a popular format for sharing 4D assets. This trend calls for robust copyright watermarking that can be verified from rendered videos without access to the original scene or model parameters. However, watermarking 4DGS is challenging: verification relies on short multi-view clips whose frames vary with motion, viewpoint changes, and post-processing. More importantly, decoding from rendered frames is easily dominated by scene content (objects, textures, illumination) rather than subtle watermark traces, leading to unstable evidence across frames and poor generalization across scenes. We propose Guard4D, a decoupled watermarking framework for 4DGS. Guard4D pre-trains a general-purpose message decoder under text supervision and embeds a binary message by optimizing compact spherical-harmonic offsets while keeping geometry and opacity fixed. To improve extraction from short clips and suppress semantic interference, we introduce a temporal modeling and semantic decoupling (TMSD) module that temporally aggregates watermark information and uses clean and noise-perturbed control clips generated from the non-watermarked model to reduce the influence of semantics on watermark decoding. Extensive experiments on dynamic 4DGS scenes demonstrate the effectiveness of Guard4D. Code is available at https://github.com/shisyy/Guard4D.