src/retrieval/README.md

# Retrieval Module

FAISS-based similarity search with modality filtering.

## Files

| File | Description |
|------|-------------|
| `index.py` | FAISS index operations (build, search, save/load) |
| `engine.py` | Retrieval engine |
| `multimodal.py` | Multi-modal retrieval with same-modal and cross-modal queries |

## Index Type

| Parameter | Value |
|-----------|-------|
| Type | IndexFlatIP |
| Similarity | Inner Product (Cosine on L2-normalized vectors) |
| Build Time | O(N) |
| Search Time | O(N) |

## Usage

```python
from src.retrieval.multimodal import MultiModalRetrieval

# Initialize
retrieval = MultiModalRetrieval(embed_dim=768)

# Build index from modality embeddings
retrieval.build_index({
    "optical": optical_embeddings,
    "sar": sar_embeddings,
    "multispectral": ms_embeddings,
})

# Same-modal query
result = retrieval.same_modal_query(query, modality="optical", k=5)

# Cross-modal query
result = retrieval.cross_modal_query(
    query, source_modality="optical", target_modality="sar", k=5
)
```