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A Picture is Worth More Than 77 Text Tokens: Evaluating CLIP-Style Models on Dense Captions
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Introduces the Densely Captioned Images (DCI) dataset of ~8k SA-1B images with human mask-aligned dense descriptions, an LLM-summarized 77-token version (sDCI), a subcrop-caption matching benchmark, and shows LoRA fine-tuning CLIP on sDCI improves ARO/VL-Checklist results.
Problem
Curated vision-language captions are too short to capture rich image detail, and existing compositional benchmarks (ARO, VL-Checklist) rely on hard negatives that can be solved by language priors, so VLMs lack reliable dense, mask-aligned evaluation and training data.
Approach
Collect human-written, submask-aligned dense descriptions (averaging over 1000 words per image), summarize them with Llama2 to fit CLIP's 77-token limit, and use the multiple-crop structure to build a subcrop-caption matching test that checks whether models distinguish captions of different regions within the same image.
Key results
- CLIP baseline scores 40.06% on All SCM and 60.79% on All Neg; NegCLIP improves SCM to 43.35%, while DAC-SAM reaches 84.17% on All Neg but only 37.90% on All SCM.
- CLIP fine-tuned on sDCI with negatives loss (sDCI P1) reaches 81.17%→88.37% on VL-Checklist Object without negatives and 76.23% on ARO VG-R.
- Fine-tuned sDCI model outperforms DAC trained on matched small image counts (10k and 100k images) on ARO and VL-Checklist.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Fine-tune CLIP on sDCI with negatives loss | ViT/32B CLIP · ARO and VL-Checklist | accuracy | Mixed |
| Remove negatives loss (ablation) | sDCI Pick1 fine-tuning of ViT/32B CLIP · ARO and VL-Checklist | accuracy: 76.23% -> 57.34%; 67.56% -> 61.98%; 88.58% -> 39.36%; 91.30% -> 44.62%; 80.71% -> 88.37%; 68.69% -> 70.42%; 70.12% -> 61 | Mixed |
| Fine-tune CLIP on sDCI rather than DAC LLM captions using a sample-limited dataset | CLIP · ARO and VL-Checklist | accuracy | Mixed |
| Fine-tune CLIP on sDCI with negatives loss | ViT/32B CLIP · VL-Checklist Object | accuracy: 81.17% -> 80.71% | No clear effect |
Surprising findings
- Models trained with CLIP-style objectives do not improve subcrop-caption matching, and DAC, despite strong negatives scores, performs worst on SCM.
- Fine-tuning without negatives loss can degrade ARO and VL-Checklist performance relative to CLIP baseline, while still improving VL-Object.
Limitations
- LLM summarization to 77 tokens may introduce noise, drop details, and remove distinguishing information between captions.
- Pixel-level masks are not used; only crops around masks are used in experiments.
- The dataset is released under CC-BY-NC, and the dense captions exceed current CLIP text context length.
Experimental setup
Datasets: DCI sDCI SA-1B ARO VL-Checklist DAC captions
Benchmarks: sDCI Subcrop-Caption Matching sDCI negatives tests ARO VL-Checklist
Metrics: accuracy
Baselines: CLIP NegCLIP BLIP Flava X-VLM DAC-LLM DAC-SAM BLIP2 SVLC
Builds on: CLIP NegCLIP DAC (Densely Aligned Captions) Segment Anything Model Llama 2
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