TimeLens2: Generalist Video Temporal Grounding with Multimodal LLMs

• URL: http://arxiv.org/abs/2607.17423v1
• Date: Sun, 19 Jul 2026 22:04:33 GMT
• Title: TimeLens2: Generalist Video Temporal Grounding with Multimodal LLMs
• Abstract summary: Video multimodal large language models (MLLMs) can describe what happens in a video, but rarely identify when the supporting evidence occurs.<n>We study generalist video temporal grounding, in which one model predicts a variable-cardinality set of evidence intervals across video lengths, domains, query forms, and viewpoints.<n>TimeLens2 treats temporal evidence as an interval set throughout supervision and optimization.
• Score: 36.2604844032041
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: Video multimodal large language models (MLLMs) can describe what happens in a video, but rarely identify when the supporting evidence occurs. We study generalist video temporal grounding, in which one model predicts a variable-cardinality set of evidence intervals across video lengths, domains, query forms, and viewpoints. Existing training strategies are misaligned with this set-valued task: long-video labels often rely on brittle one-pass annotation, while reinforcement-learning rewards either fail to distinguish non-overlapping predictions or require fragile segment matching. TimeLens2 treats temporal evidence as an interval set throughout supervision and optimization. TimeLens2-93K constructs reliable multi-span supervision through caption-derived proposals, independent localization, cross-agent consensus, semantic verification, and boundary refinement. Our temporal Wasserstein reward computes exact one-dimensional \(W_1\) between uniform distributions over merged interval supports, providing dense, matching-free feedback under unequal cardinalities and equivalent fragmentation; temporal IoU complements it with precise-overlap feedback. Across seven benchmarks, TimeLens2-2B outperforms all size-matched baselines on every benchmark, while the 4B and 8B variants achieve state-of-the-art performance, surpassing open-source models with up to 397B parameters. The 2B, 4B, and 8B variants improve over their Qwen3-VL backbones by 14.2, 13.0, and 18.1 mIoU points, respectively.

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