3DIS: Real-Time Hybrid Relighting via 3D Importance Sampling from 3DGS Scenes

3D Gaussian Splatting (3DGS) supports high fidelity real time scene rendering, but its baked appearance representation makes it difficult to achieve harmonious illumination for inserted meshes. We propose a scene-level lighting estimation framework that recovers a reusable, spatially varying illumination representation from an existing 3DGS scene, enabling real-time and illumination-consistent rendering of inserted objects. Instead of predicting lighting independently for each view or object pose, our method estimates a compact virtual-point-light configuration from multi-location HDR observations of the 3DGS scene. To make this estimation both spatially stable and source-aware, we introduce a mixed 3D importance sampling strategy that allocates point lights across scene coverage, HDR radiance importance, and semantic light-source regions such as windows and lamps. The resulting point-light configuration captures both dominant direct illumination and distributed environmental light, enabling inserted meshes to be rendered consistently to both the 3DGS environment and the viewing direction (like specular reflection), even the inserted mesh is moved/manipulated. We also construct a paired synthetic benchmark comprising five scene–object configurations with matched object-free and object-present renderings for quantitative evaluation of insertion appearance. Experiments on multiple indoor scenes demonstrate that our method provides a practical solution for illumination-consistent mesh insertion and interactive relighting in reconstructed Gaussian environments.