Hybrid CPU–GPU Cutting Simulation Using Position-Based Dynamics and Internal Shape-Preserving Constraints in Unity 3D

Cutting simulation is a key component of surgical simulators, requiring real-time performance, numerical stability, and physically plausible deformation. Achieving these properties is challenging because cutting combines collision detection, dynamic topology modification, and computationally intensive physical simulation. In this work, deformable objects are modeled using Position-Based Dynamics (PBD), while volumetric behavior is approximated through internal shape-preserving constraints (ISPCs) applied to triangular surface meshes. The primary contribution is a hybrid CPU–GPU cutting architecture in Unity 3D, in which the computationally expensive triangle–triangle intersection test is parallelized using compute shaders, surface rendering is performed directly from GPU-resident geometry buffers, and CPU–GPU synchronization overhead is minimized. Compared with our previous implementation, which parallelized only deformation and collision handling, the proposed framework additionally accelerates the intersection detection stage of cutting. The method was evaluated with single- and multiple-cut experiments on eight triangular meshes of increasing complexity (20–30,000 triangles). Seven of the eight models sustained interactive frame rates, and the cutting stage contributed negligibly to steady-state cost—below 0.5 ms on average even under repeated free cutting—with frame time being dominated instead by deformation and inter-part collision handling. Relative to our previous work, average frame rates improved by up to 677% over the slicing-based approach and up to 395% over the CPU-based ISPC implementation. Volume was maintained to within 0.4% of the initial volume for models of low geometric complexity. Two limitations were observed: for the high-resolution Armadillo model, mean frame cost was dominated by deformation and collision, while rare cut-finalization frames produced large worst-case spikes, keeping it below interactive rates; the Octopus model exhibited markedly reduced volume preservation, stabilizing at 73.4% of its initial volume. These results indicate that the proposed hybrid method substantially improves the efficiency of cutting simulation, but scaling to large and geometrically complex models remains future work.