OST-Bench: Evaluating the Capabilities of MLLMs in Online
  Spatio-temporal Scene Understanding
Authors: name: JingLi Lin; name: Chenming Zhu; name: Runsen Xu; name: Xiaohan Mao; name: Xihui Liu; name: Tai Wang; name: Jiangmiao Pang
Published: 2025-07-10 17:56:07

Abstract: Recent advances in multimodal large language models (MLLMs) have shown
remarkable capabilities in integrating vision and language for complex
reasoning. While most existing benchmarks evaluate models under offline
settings with a fixed set of pre-recorded inputs, we introduce OST-Bench, a
benchmark designed to evaluate Online Spatio-Temporal understanding from the
perspective of an agent actively exploring a scene. The Online aspect
emphasizes the need to process and reason over incrementally acquired
observations, while the Spatio-Temporal component requires integrating current
visual inputs with historical memory to support dynamic spatial reasoning.
OST-Bench better reflects the challenges of real-world embodied perception.
Built on an efficient data collection pipeline, OST-Bench consists of 1.4k
scenes and 10k question-answer pairs collected from ScanNet, Matterport3D, and
ARKitScenes. We evaluate several leading MLLMs on OST-Bench and observe that
they fall short on tasks requiring complex spatio-temporal reasoning. Under the
online setting, their accuracy declines as the exploration horizon extends and
the memory grows. Through further experimental analysis, we identify common
error patterns across models and find that both complex clue-based spatial
reasoning demands and long-term memory retrieval requirements significantly
drop model performance along two separate axes, highlighting the core
challenges that must be addressed to improve online embodied reasoning. To
foster further research and development in the field, our codes, dataset, and
benchmark are available. Our project page is:
https://rbler1234.github.io/OSTBench.github.io/