PRIMA: Multi-Image Vision-Language Models for Reasoning Segmentation

Despite significant advancements in Large Vision-Language Models (LVLMs)' capabilities, existing pixel-grounding models operate in single-image settings, limiting their ability to perform detailed, fine-grained comparisons across multiple images. Conversely, current multi-image understanding models lack pixel-level grounding. Our work addresses this gap by introducing the task of multi-image pixel-grounded reasoning segmentation alongside PRIMA, an LVLM that integrates pixel-level grounding with robust multi-image reasoning to produce contextually rich, pixel-grounded explanations. Central to PRIMA is SQuARE, a vision module that injects cross-image relational context into compact query-based visual tokens before fusing them with the language backbone. To support training and evaluation, we curate M⁴Seg, a new multi-image reasoning segmentation benchmark consisting of ~744K question-answer pairs that require fine-grained visual understanding across multiple images. PRIMA outperforms state-of-the-art baselines with 7.83% and 11.25% improvements in Recall and S-IoU, respectively. Ablation studies further demonstrate the effectiveness of the proposed SQuARE module in capturing cross-image relationships.

• Novel Task. We propose the novel task of multi-image pixel-grounded reasoning segmentation, which requires fine-grained comparison and contextual understanding across multiple images at pixel level and natural language reasoning.


• New Benchmark. We curate M⁴Seg, a new challenging benchmark with ~744K multi-image QA pairs, annotated with multiple object and part segmentation masks to train and evaluate multi-image pixel-grounding models.


• New Multi-Image Pixel-Grounded LVLM. We propose PRIMA, an LVLM designed to perform instruction-guided cross-image alignment of relevant visual features via a novel SQuARE module, enabling reasoning with contextually grounded segmentation masks across multiple images. Experiments demonstrate PRIMA's impressive performance compared to strong baselines across segmentation (+↑8.11% mIoU and +↑7.83% Recall) and text-based metrics (+↑6.45% Semantic Similarity and +↑11.25% S-IoU).

Overview of the proposed PRIMA architecture. Leveraging a LoRA-finetuned language model, a novel SQUARE vision encoder, and a SAM-based decoder, PRIMA dynamically generates segmentation masks corresponding to objects referenced in natural language queries, supporting pixel-grounded reasoning in complex multi-image tasks.

Our proposed SQuARE module. Learnable relational queries attend over the concatenated multi-image features to form a shared relational representation. This representation is injected into the query pathway for global feature extraction, producing enriched visual representations that capture cross-image interactions.

Qualitative Results. PRIMA exhibits strong qualitative performance in both segmentation and reasoning. Each example shows the posted question, the textual responses from each model, and the corresponding segmentation masks alongside the Ground Truth reference. For clarity, we use consistent colors between the segmentation masks and the highlighted text spans referring to each object or part. The results indicate that PRIMA produces high-quality textual responses together with crisp, well-localized segmentation masks, whereas GLaMM often yields noisy masks, attends to incorrect objects (e.g., treating the cabinet as a surface for placing small items), and hallucinates categories (e.g., misidentifying the horse as a truck or a dog). We also include a failure case in which PRIMA predicts the correct number of objects but fails to precisely distinguish individual instances, instead producing overlapping masks.

PRIMA's performance on unseen images found on the web. Notably, PRIMA generates superior segmentation masks compared to baselines, as exemplified by the fine-grained details of the hammers in Example 3 and the dining chairs in Example 4. Moreover, the baselines are more prone to hallucinations, e.g., "pizza is shown with multiple slices" in the first example (GLaMM), "watch" in the second example (both GLaMM and LISA). Beyond visual recognition, these examples underscore PRIMA's capability to retain compositional reasoning capabilities when transferring to diverse and noisy real-world images.