Sketch2BRep: Boundary-First Diffusion for Sketch-to-BRep CAD Generation

Generating B-Rep CAD models from sketches is challenging because sparse 2D drawings provide only partial boundary cues, while B-Reps require both continuous geometric primitives and discrete topological relations. Existing methods typically infer coupled geometry and topology from sparse sketches, often failing to produce B-Reps that are both geometrically precise and topologically valid. We observe that boundary curves naturally bridge sketches and B-Reps: sketches provide intuitive cues for object boundaries, while boundaries reveal faces of a B-Rep. Motivated by this, we propose Sketch2BRep, a boundary-first hierarchical latent diffusion framework for sketch-conditioned B-Rep generation. Sketch2BRep decomposes the generation process into three progressive stages: sketch-conditioned boundary generation, boundary-conditioned surface generation, and explicit boundary-surface relation modeling. Specifically, a Shape-Layout Coupled Sketch Diffusion employs interacting curve-shape and spatial-layout branches to infer a plausible structured 3D boundary conditioned on the partial boundary cues in the sketch. A Boundary-Conditioned Joint Surface Diffusion enables direct interaction between face and edge tokens to synthesize surfaces consistent with the recovered boundaries. A Geometry-Aware RelationNet models pairwise boundary-surface relationships to explicitly assemble the B-Rep topology. Experiments demonstrate that the proposed method, by following the intrinsic boundary–surface–topology construction logic of B-Reps, outperforms state-of-the-art methods in geometric fidelity, topological validity, and sketch consistency.