Introduction: Semantic Video Retrieval and RAG

In today’s world, companies have massive video libraries—thousands or even millions of clips—that need to be searched efficiently. Traditional keyword search on transcripts isn’t enough; users want to search by scene, action, concept, or even images in the video, using natural language. This is where Retrieval-Augmented Generation (RAG) comes in. RAG augments large language models by fetching relevant external information at query time, improving accuracy and reducing hallucination. In a video retrieval context, RAG means indexing video clips (via embeddings) and then using those retrievals to ground the answer to the user’s query. By combining a vector database for fast similarity search with generative AI models, an AI-powered video clip retrieval system can deliver precise, context-aware results. (Amazon Web Services, Inc., arXiv, MongoDB)

Key idea: Break each video into segments, convert each segment’s content into a high-dimensional vector (“embedding”) that captures its meaning, and store these in a vector search index. At query time, convert the user’s text into an embedding, then perform a nearest-neighbor search in the vector index to find matching clips. Optionally, feed the retrieved clips (or their transcripts) into an LLM to compose a richer answer. This RAG-style pipeline enhances the video search experience for end users by making video libraries fully queryable via natural language. (arXiv, MongoDB)

Figure: An AI video search pipeline. Video files are analyzed (e.g., key frames, transcripts) and mapped to vector embeddings. These embeddings are indexed in a Vector Database. A user’s text query is also embedded and used to retrieve the most semantically similar video clips. (Diagram adapted from AWS). (Amazon Web Services, Inc.)

Key Components of an AI Video Search System

Media Ingestion & Processing: Upload or collect raw videos (to cloud storage like S3 or a media archive). Use tools (e.g., Amazon Transcribe, Amazon Rekognition, or other vision/ASR models) to extract context—transcripts of speech, scene segmentation, detected objects, or sampled key frames. Each video is broken into meaningful segments (shots or fixed time slices). (Amazon Web Services, Inc.)

Embedding Generation: For each video segment, generate one or more vector embeddings that capture its semantic content. Example: embed transcripts with OpenAI’s text-embedding-3 models, and embed sampled frames with a vision-language model like CLIP that maps images and text into a shared space. (OpenAI Platform, Medium)

Vector Database (Index): Store embeddings (plus metadata like timestamps/video IDs) in a vector search engine. Popular options include MongoDB Atlas Vector Search or PostgreSQL with pgvector. (MongoDB, Zilliz)

Query Pipeline: At search time, convert the user’s input (text, or even an image) to an embedding with the same model(s), run a similarity search, and return the best-matching segments. The system can then return clips with timestamps, or feed their transcripts into an LLM to produce a more polished answer (the “generation” in RAG). (MongoDB)

Orchestration / API Layer: Glue it together with serverless functions or microservices—e.g., AWS API Gateway + AWS Lambda (and Step Functions for multi-step flows). (Amazon Web Services, Inc.)

Typical Data Flow

Video Upload: New video lands in S3 → event triggers a function.
Transcription & Segmentation: Invoke services like Transcribe/Rekognition to get transcripts and scene boundaries.
Segment Embedding: Call your embedding model (OpenAI for text; CLIP for frames). (Medium)
Store in Vector DB: Write vectors + metadata to Atlas or Postgres/pgvector. (MongoDB, Zilliz)
Query Handling: API converts the user query to a vector and searches the index. (MongoDB)
(Optional) LLM Post-Processing: Feed retrieved transcripts to an LLM to summarize/answer naturally. (arXiv)

Note: Use the same embedding model for indexing and querying to ensure vector space compatibility. (See “querying your data” guidance in MongoDB’s tutorial.) (MongoDB)

Figure: Example indexing/query pipelines; serverless functions create embeddings and persist vectors in MongoDB Atlas; queries vectorize user text and perform vector search, then return matching clips. (MongoDB)

Building the Vector Index: MongoDB Atlas vs. pgvector

MongoDB Atlas Vector Search: Managed NoSQL with built-in vector similarity search; store embeddings alongside document metadata and filter with standard fields. (MongoDB)

PostgreSQL + pgvector: Open-source extension that adds vector columns/indexing to Postgres—great if you’re already on SQL and want relational + vector together. (Zilliz)

Each has trade-offs; benchmark with your data. (Zilliz’s comparison summarizes functional/scalability differences.) (Zilliz)

Generating Embeddings for Video Clips

Textual embeddings (ASR transcripts): Run ASR, then embed with OpenAI’s embeddings (e.g., text-embedding-3 family). (OpenAI Platform)

Visual embeddings (frames): Sample key frames and embed with CLIP to capture visual semantics that transcripts might miss. (Medium)

AWS shows this end-to-end pattern (extract frames + transcripts → create embeddings → store vectors) in their semantic video search guide. (Amazon Web Services, Inc.)

You can start simple (transcript-only, chunked into 10–30s) and later blend text + image embeddings (e.g., average or concatenate vectors) for higher recall. (Amazon Web Services, Inc.)

Where to run embedding jobs:
• Serverless on upload: S3 event → Lambda → ASR → Embeddings → DB write. (GitHub)
• DB triggers: Use Atlas Triggers to run a function when new docs arrive and back-fill vectors. (MongoDB)

Querying the Video Library (Semantic Search)

The server takes the user’s query, calls the same embedding model, then runs a nearest-neighbors search (cosine/Euclidean) in the vector DB to fetch top-K segments. Return clip IDs and timestamps—or send transcripts to an LLM for a natural-language summary. See MongoDB’s querying your data for a concrete flow. (MongoDB)

This also supports multimodal queries: users can upload an image and ask “find video clips like this frame”; the image is embedded and searched. AWS highlights this capability in its solution guidance and related demos. (Amazon Web Services, Inc.)

Serverless Orchestration on AWS

Use event-driven pipelines: S3 upload → Lambda/Step Functions for ASR, scene detection, embeddings → write to DB. For multi-step/branching workflows, prefer Step Functions over monolithic “Lambda as orchestrator.” (Amazon Web Services, Inc.)

Putting It All Together

A practical architecture: UI → API Gateway → Search Lambda (embeds query, hits Atlas/pgvector) → returns video IDs/timestamps → client streams from S3. Ingestion is a separate, automated flow on upload. AWS/MongoDB examples show this modular pattern working well at scale. (Amazon Web Services, Inc., MongoDB)

Why Use RAG for Video Retrieval?

RAG “incorporates external knowledge bases” to reduce hallucinations and add domain context—see VideoRAG for a canonical description adapted to long-context video. In video, your “knowledge base” is the indexed clip library; retrieval grounds LLM answers in actual footage. (arXiv)

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