Beyond Pattern Matching in 3D MLLMs: Part-Level Functional Reasoning for Generalizable Recognition

Overview & Synopsis

3D MLLMs recognize objects via pattern matching, which fails when geometrically similar objects require functional reasoning to disambiguate (e.g., dresser vs. wardrobe). Our per-sample Geometric Confusion Score (GCS) exposes this failure: baseline accuracy is 33.7% on ModelNet40's most ambiguous 20% versus 84.0% on its least ambiguous 20%. We identify a function-as-annotation pattern, in which functional language decorates a known identity instead of supporting inference, and hypothesize that reversing this information flow should concentrate improvements on the most ambiguous samples. PFIR (Part-level Functional Inference for Recognition) reverses that flow. Built on PartNeXt part segmentation, its data pipeline produces 507K QA pairs with functional CoTs (geometry $\to$ function $\to$ identity), and its progressive curriculum culminates in dual-objective GRPO: strict functional reasoning and descriptive grounding for Objaverse's diverse distribution. The two RL objectives yield nearly orthogonal task vectors ($\cos = 0.056$), enabling composition via Task Arithmetic. Under a strict dual OOD protocol (Objaverse $\to$ ModelNet40; XYZRGB $\to$ XYZ-only), PFIR raises zero-shot accuracy from 60.5% to 66.7%, with gains concentrated $2.6\times$ more on geometrically ambiguous samples---supporting part-level functional reasoning as an effective route to more generalizable 3D recognition. Code: https://github.com/this233/pfir-3d.