GRAZE: Grounded Refinement and Motion-Aware Zero-Shot Event Localization
It addresses the need for reliable biomechanical analysis in sports by enabling automated event localization in unconstrained practice footage, though it is incremental as it builds on existing models like Grounding DINO and SAM2.
The paper tackles the problem of localizing the First Point of Contact (FPOC) in American football practice videos, where contact onset is brief and scenes are cluttered, and achieves 77.5% accuracy within ±10 frames and 82.7% within ±20 frames on 738 videos without task-specific training.
American football practice generates video at scale, yet the interaction of interest occupies only a brief window of each long, untrimmed clip. Reliable biomechanical analysis, therefore, depends on spatiotemporal localization that identifies both the interacting entities and the onset of contact. We study First Point of Contact (FPOC), defined as the first frame in which a player physically touches a tackle dummy, in unconstrained practice footage with camera motion, clutter, multiple similarly equipped athletes, and rapid pose changes around impact. We present GRAZE, a training-free pipeline for FPOC localization that requires no labeled tackle-contact examples. GRAZE uses Grounding DINO to discover candidate player-dummy interactions, refines them with motion-aware temporal reasoning, and uses SAM2 as an explicit pixel-level verifier of contact rather than relying on detection confidence alone. This separation between candidate discovery and contact confirmation makes the approach robust to cluttered scenes and unstable grounding near impact. On 738 tackle-practice videos, GRAZE produces valid outputs for 97.4% of clips and localizes FPOC within $\pm$ 10 frames on 77.5% of all clips and within $\pm$ 20 frames on 82.7% of all clips. These results show that frame-accurate contact onset localization in real-world practice footage is feasible without task-specific training.