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Leveraging Vision-Language Models for Improving Domain Generalization in Image Classification
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Distills a black-box CLIP teacher into an ImageNet-pretrained vision student via align-distill-predict stages, improving OOD domain generalization on DG benchmarks.
Problem
Distilling a vision-language model into a small unimodal image classifier with limited labeled task data often improves in-domain accuracy but fails to transfer the VLM's out-of-distribution generalization, especially when the teacher is a black box.
Approach
Align the pretrained student's features to the CLIP image and text embeddings via a frozen-backbone linear projection head first, then distill the backbone with the same loss, and finally use CLIP text embeddings as the classifier head; a white-box self-distillation variant (VL2V-SD) distills text-encoder invariances into the CLIP image encoder.
Key results
- VL2V-ADiP with ViT-B/16 ImageNet-initialized student reaches 75.48% average OOD accuracy versus 73.97% for the best baseline RISE (S), about 1.5 points gain.
- VL2V-SD in the white-box setting reaches 77.73% average OOD accuracy versus 76.66% for MIRO (S) and 70.60% for CLIP zero-shot.
- Distillation with ViT-S/16, DeiT-S/16 and ResNet-50 students improves average OOD accuracy over KD (S) in all cases, e.g. 71.73% vs 69.08% for ViT-S/16.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Distill the VLM's text and image representations using Align, Distill, and Predict stages | ImageNet-initialized ViT-B/16 student · Average OOD accuracy across DG benchmarks | accuracy: 73.97 -> 75.48 | Helped |
| Self-distill the CLIP image encoder using its text embeddings | CLIP ViT-B/16 · Average OOD accuracy across DG benchmarks | accuracy: 76.66 -> 77.73 | Helped |
| Combine Align and Distill stages instead of aligning first (ablation) | VL2V-ADiP · OH, TI, VLCS, and PACS average | OOD accuracy: 79.50 -> 73.24 | Hurt |
| Do not freeze the projection head in Stage-2 (ablation) | VL2V-ADiP · OH, TI, VLCS, and PACS average | OOD accuracy: 79.50 -> 79.59 | No clear effect |
| Use only text-encoder distillation in Stages 1 and 2 (ablation) | VL2V-ADiP · OH, TI, VLCS, and PACS average | OOD accuracy: 79.50 -> 75.22 | Hurt |
| Use only image-encoder distillation in Stages 1 and 2 (ablation) | VL2V-ADiP · OH, TI, VLCS, and PACS average | OOD accuracy: 79.50 -> 70.04 | Hurt |
| Fine-tune the CLIP-initialized classifier head in Stage-3 using cross-entropy (ablation) | VL2V-ADiP · OH, TI, VLCS, and PACS average | OOD accuracy: 79.50 -> 77.15 | Hurt |
| Fine-tune the full network in Stage-3 with a CLIP-initialized classifier and cross-entropy (ablation) | VL2V-ADiP · OH, TI, VLCS, and PACS average | OOD accuracy: 79.50 -> 76.52 | Hurt |
| Fine-tune a randomly initialized classifier head in Stage-3 using cross-entropy (ablation) | VL2V-ADiP · OH, TI, VLCS, and PACS average | OOD accuracy: 79.50 -> 77.26 | Hurt |
| Fine-tune the full network in Stage-3 with a randomly initialized classifier and cross-entropy (ablation) | VL2V-ADiP · OH, TI, VLCS, and PACS average | OOD accuracy: 79.50 -> 76.28 | Hurt |
Surprising findings
- Combining the align and distill stages into one joint training step drops OOD accuracy by 6.3 points (79.50 to 73.24), showing alignment before distillation matters.
- CLIP zero-shot text embeddings outperform average image embeddings of the labeled source-domain data, so a generic text classifier is more robust than a classifier learned from limited images.
Limitations
- Evaluated only on standard DomainBed image classification benchmarks with CLIP-style teachers.
- Fine-tuning on limited in-domain data degrades OOD performance, so the method relies on fixed text-embedding classifiers.
- Requires text-prompt class names for the classifier head, so it depends on class-name availability.
Experimental setup
Datasets: OfficeHome PACS VLCS Terra-Incognita DomainNet
Benchmarks: OfficeHome PACS VLCS Terra-Incognita DomainNet
Metrics: OOD accuracy in-domain validation accuracy
Baselines: ERM KD SWAD MIRO DART SAGM LP-FT FLYP CLIPood WiSE-FT RISE SimKD
Builds on: CLIP Knowledge distillation (Hinton et al.) SWAD DomainBed RISE
Closest papers by meaning
- CLIPCEIL: Domain Generalization through CLIP via Channel rEfinement and Image-text aLignment (NeurIPS 2024 · similarity 0.90)
- PracticalDG: Perturbation Distillation on Vision-Language Models for Hybrid Domain Generalization (CVPR 2024 · similarity 0.89)
- Domain Generalization in CLIP via Learning with Diverse Text Prompts (CVPR 2025 · similarity 0.89)
- Language Models as Black-Box Optimizers for Vision-Language Models (CVPR 2024 · similarity 0.89)
- Overcoming the Pitfalls of Vision-Language Model Finetuning for OOD Generalization (ICLR 2024 · similarity 0.89)
- Bridge the Modality and Capability Gaps in Vision-Language Model Selection (NeurIPS 2024 · similarity 0.89)
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