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GROUNDHOG: Grounding Large Language Models to Holistic Segmentation
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Introduces GROUNDHOG, an MLLM that grounds phrases to pixel-level holistic segmentation masks by retrieving class-agnostic entity mask proposals, trained on the 2.5M-pair M3G2 instruction dataset.
Problem
Box-based grounded MLLMs ground language via location tokens, which cannot represent amorphous stuff, parts, irregular regions, or groups, and their failures are hard to diagnose.
Approach
Convert class-agnostic entity mask proposals into visual entity tokens (CLIP+DINOv2 features pooled by mask), then have <GRD> phrase query embeddings score and retrieve entities, merging selected masks into a grounding mask; mask proposals are decoupled from the MLLM so failures are diagnosable.
Key results
- On RES benchmarks, GROUNDHOG-7B reaches 78.5 cIoU on RefCOCO val and 56.2 mIoU on ReasonSeg val, outperforming LISA-7B (74.1 and 44.0).
- On POPE adversarial split, GROUNDHOG-7B reports 86.28 F1 and 86.33 accuracy, with the paper citing a 4.0% accuracy gain over the best prior model on Adversarial and 5.2% on Popular.
- On Flickr30K-Entity grounded captioning, GROUNDHOG-7B achieves CIDEr 91.3 and F1_all 32.1, versus 76.1 and 15.1 for Ferret-13B.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Use Mask2Former+ trained on the combined multi-grained segmentation dataset instead of Mask2Former (ablation) | GROUNDHOG mask proposals · RefCOCO+, PhraseCut, ReasonSeg, TextVQA-X | Any-IoU: 0.867 -> 0.873; 0.563 -> 0.624; 0.602 -> 0.745; 0.137 -> 0.446 | Mixed |
| Use Mask2Former instead of Mask2Former+ (ablation) | GROUNDHOG · RefCOCO+, Flickr30K, TextVQA-X | cIoU / R@1 / mIoU: 66.6 -> 67.1; 77.2 -> 69.0; 34.0 -> 9.8 | Mixed |
| Add DINOv2 features to CLIP entity features (ablation) | GROUNDHOG · RefCOCO+, Flickr30K, TextVQA-X | cIoU / R@1 / mIoU: 59.8 -> 66.6; 75.0 -> 77.2; 32.0 -> 34.0 | Helped |
| Add CLIP features to DINOv2 entity features (ablation) | GROUNDHOG · RefCOCO+, Flickr30K, TextVQA-X | cIoU / R@1 / mIoU: 62.3 -> 66.6; 76.3 -> 77.2; 28.4 -> 34.0 | Mixed |
| Use the sum of <GRD> and </GRD> embeddings instead of <GRD> only (ablation) | GROUNDHOG grounding query · RefCOCO+, Flickr30K, TextVQA-X | cIoU / R@1 / mIoU: 64.4 -> 66.6; 67.5 -> 77.2; 34.2 -> 34.0 | Mixed |
| Use the sum of <GRD> and </GRD> embeddings instead of </GRD> only (ablation) | GROUNDHOG grounding query · RefCOCO+, Flickr30K, TextVQA-X | cIoU / R@1 / mIoU: 64.4 -> 66.6; 77.2 -> 77.2; 33.5 -> 34.0 | Mixed |
| Increase evaluation input resolution from 224--480 to 800--1024 (ablation) | GROUNDHOG · RefCOCO+, Flickr30K, TextVQA-X | cIoU / R@1 / mIoU: 54.7 -> 66.6; 67.2 -> 77.2; 27.6 -> 34.0 | Helped |
| Increase evaluation input resolution from 480--640 to 800--1024 (ablation) | GROUNDHOG · RefCOCO+, Flickr30K, TextVQA-X | cIoU / R@1 / mIoU: 65.5 -> 66.6; 76.7 -> 77.2; 27.6 -> 34.0 | Mixed |
| Train the generalist model on M3G2 rather than use LISA-7B | GROUNDHOG-7B · RefCOCO+, PhraseCut, ReasonSeg | cIoU / mIoU: 62.4 -> 70.5; - -> 54.5; 44.0 -> 56.2 | Helped |
| Train with M3G2 including negative question-answering samples | GROUNDHOG-7B · POPE Popular, POPE Adversarial | accuracy: +5.2%; +4.0% | Helped |
Surprising findings
- The original Mask2Former outperforms Mask2Former+ on RefCOCO+ (67.1 vs 66.6 in ablation), attributed to COCO-specific training.
- Using only CLIP or only DINOv2 entity features each underperforms the combined features on some datasets, but DINOv2 alone beats CLIP on RefCOCO+ (62.3 vs 59.8).
Limitations
- M3G2 relies on blended academic datasets with varying annotation quality and limited concept coverage.
- The model only processes single images; extension to 3D or video is untested.
- Performance depends on the coverage of the mask proposal model (e.g., Mask2Former+ fails to propose some exact masks).
Experimental setup
Datasets: M3G2 PNG Flickr30K-Entity RefCOCO RefCOCO+ RefCOCOg RefCLEF gRefCOCO PhraseCut D-Cube ReasonSeg RIO
Benchmarks: RefCOCO RefCOCO+ RefCOCOg gRefCOCO PhraseCut ReasonSeg RIO Flickr30K-Entities PNG TextVQA-X PointQA-Twice V7W
Metrics: cIoU mIoU Any-IoU box recall R@1 AR BLEU-4 METEOR CIDEr SPICE F1_all
Baselines: LISA PolyFormer UNINEXT-H Shikra Ferret PiGLET SAB mPLUG-Owl LLaVA MiniGPT-4 InstructBLIP MultiModal-GPT
Builds on: Mask2Former LLaVA-1.5 LISA Shikra Segment Anything
Closest papers by meaning
- GroundingME: Exposing the Visual Grounding Gap in MLLMs through Multi-Dimensional Evaluation (CVPR 2026 · similarity 0.91)
- Your Large Vision-Language Model Only Needs A Few Attention Heads For Visual Grounding (CVPR 2025 · similarity 0.90)
- PostAlign: Multimodal Grounding as a Corrective Lens for MLLMs (ICLR 2026 · similarity 0.90)
- Connecting the Dots: Training-Free Visual Grounding via Agentic Reasoning (AAAI 2026 · similarity 0.90)
- Enhancing Part-Level Point Grounding for Any Open-Source MLLMs (CVPR 2026 · similarity 0.90)
- GeM-VG: Towards Generalized Multi-image Visual Grounding with Multimodal Large Language Models (AAAI 2026 · similarity 0.90)
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