Multimodal RAG and Cross-Modal Retrieval

> Vision-native document RAG is one slice. Production multimodal RAG goes wider — retrieving across text, images, audio, and video for workflows like trip planning ("find me a quiet vegan brunch with natural light"), medical triage ("what injury matches this photo + these notes"), e-commerce ("outfits similar to this selfie, in my size"), and field service ("diagnose this engine sound plus photo of the part"). Three 2025 surveys — Abootorabi et al., Mei et al., Zhao et al. — codified the sub-problems: cross-modal retrieval, retrieval fusion, generation grounding, multimodal evaluation. This lesson reads the surveys and designs a production pipeline.

Type: Build

Languages: Python (stdlib, cross-modal retriever with fusion + grounded generator)

Prerequisites: Phase 12 · 23 (ColPali), Phase 11 (RAG basics)

Time: ~180 minutes

Learning Objectives

• Design cross-modal retrieval: text → image, image → text, audio → video, etc.
• Compare three fusion strategies: score fusion, attention-based fusion, MoE fusion.
• Explain generation grounding: what "cite your sources" looks like when sources are a mix of modalities.
• Name the three canonical multimodal RAG surveys of 2025 and their sub-problem taxonomy.

The Problem

Single-modality RAG is a solved pattern: embed query, embed chunks, retrieve, stuff into LLM. Multimodal RAG requires:

1. Multiple retrieval heads (each modality needs embeddings in a compatible space).
2. Fusion of retrieval results across modalities.
3. Generation grounding that cites sources across modalities.
4. Evaluation metrics that cover cross-modal signal.

The 2025 surveys all arrive at the same taxonomy.

The Concept

Cross-modal retrieval

Retrieve documents of modality B given a query of modality A. Three patterns:

1. Shared embedding space. CLIP and CLAP produce text + image / text + audio embeddings in a shared space. Cosine similarity across modalities works directly. Limited to CLIP-trained pairs.

1. Per-modality encoder + translation. Text encoder + image encoder + a small translator module mapping between spaces. Sen2Sen by Gupta et al. and other 2024 designs. Flexible but adds complexity.

1. VLM as encoder. Use a VLM's hidden states as the retrieval representation. Any modality the VLM supports works. Higher quality, more expensive.

Choice: CLIP / SigLIP 2 for text+image; CLAP for text+audio; VLM-hidden-states for cross-modal at frontier quality.

Fusion strategies

You retrieved 10 results: 5 images, 3 text passages, 2 audio clips. How do you merge?

Score fusion (cheapest). Each modality has its own retriever, each returns scores. Normalize scores within-modality then sum. Simple, often works.

Attention-based fusion. Concatenate all retrieved items, let a small attention network weight them. Needs training.

MoE fusion. Gating network routes to modality-specific experts. Different query types route differently — a visual question weights images higher.

Production default: score fusion with a slight bias toward the query's dominant modality. Upgrade to MoE if A/B shows clear wins on your domain.

Generation grounding

The LLM should cite which retrieved item drove each claim. For multi-modal:

• Text source: standard citation [1].
• Image source: [img 3] with a short caption.
• Audio: [audio 2 at 0:34].

Train the generator with grounding-aware data: each claim in the training target is tagged with the source index. At inference, the model naturally emits citations.

The 2025 surveys

Abootorabi et al. (arXiv:2502.08826, "Ask in Any Modality"): taxonomy for multimodal RAG. Covers retrieval, fusion, generation. Broadest coverage.

Mei et al. (arXiv:2504.08748, "A Survey of Multimodal RAG"): focuses on sub-task benchmarks and failure modes. Useful for evaluation design.

Zhao et al. (arXiv:2503.18016): vision-focused survey. Strong on ColPali-family work.

Reading all three gives you the state of the art as of spring 2025. Most of the sub-problems are still open.

MuRAG — the foundational paper

MuRAG (Chen et al., 2022) was the first multimodal RAG. Retrieved image + text from a multimodal KB, generated answers. Showed feasibility before the VLM wave. Modern systems (REACT, VisRAG, M3DocRAG) build on it.

A production trip-planner example

Query: "find me a quiet vegan brunch with natural light."

Pipeline:

1. Decompose query. "quiet" → audio/review keyword; "vegan brunch" → menu item; "natural light" → image feature.
2. Retrieve per modality:

- Text retrieval on reviews: "vegan brunch, quiet ambiance."

- Image retrieval on restaurant photos: "natural light, airy."

- Audio retrieval on ambient-sound clips: "low decibel, no music."

1. Fuse scores. Each restaurant has a composite score.
2. Top-k restaurants → VLM generator with all evidence → answer with citations.

This is well beyond text-RAG. Each modality adds signal that text alone misses.

Agentic multimodal RAG

Multi-hop: if the first retrieval does not return high-confidence answers, the LLM reformulates and retrieves again. Agentic RAG patterns from Phase 14 apply here. Examples:

• Retrieve initial top-10 → LLM asks "too noisy, filter for <40 dB" → re-retrieve.
• Retrieve images → LLM sees one has a menu → retrieve the menu text → answer.

Adds complexity but handles queries that single-shot retrieval cannot.

Evaluation

Cross-modal evaluation is still immature. Common proxies:

• Recall@k per modality.
• Fused top-k accuracy.
• Human-judged end-to-end satisfaction.
• Task-specific (bookings completed, purchases made).

No standard benchmark spans all modalities. Most papers evaluate on domain-specific tasks.

Use It

code/main.py:

• Three mock retrievers (text, image, audio) operating on a shared corpus of restaurants.
• Score fusion that combines modality scores with configurable weights.
• A generator stub that emits a final answer with citations.
• A simple agentic loop that reformulates the query if confidence is low.

Ship It

This lesson produces outputs/skill-multimodal-rag-designer.md. Given a product spec with a multimodal query flow, designs retrievers, fusion, generator, and evaluation.

Exercises

1. Propose a medical-triage multimodal RAG: query = photo of injury + text symptoms. What modalities retrieve from what KB?

1. Score fusion is a simple weighted sum. What failure mode does it have that MoE fusion avoids?

1. Read Abootorabi et al.'s taxonomy (Section 3). What are the three canonical sub-problems and how do they map to your chosen product?

1. Design an eval spec for a trip-planner multimodal RAG. What metrics cover image recall, audio recall, and composite correctness?

1. Agentic multi-hop RAG has a latency tax per round-trip. At what query difficulty does the accuracy gain justify the latency?

Key Terms

Further Reading

• Abootorabi et al. — Ask in Any Modality (arXiv:2502.08826)
• Mei et al. — A Survey of Multimodal RAG (arXiv:2504.08748)
• Zhao et al. — Vision RAG Survey (arXiv:2503.18016)
• Chen et al. — MuRAG (arXiv:2210.02928)
• Liu et al. — REACT (arXiv:2301.10382)