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How we index images for RAG

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Why This Matters

This article highlights a breakthrough in indexing images for Retrieval-Augmented Generation (RAG) systems, enabling AI assistants to deliver more accurate and actionable answers from technical documentation. By describing images at indexing time with a vision model and retrieving these descriptions alongside text, the approach significantly enhances answer quality with minimal query overhead, benefiting both tech companies and end-users. This advancement underscores the importance of integrating visual context into AI-driven knowledge retrieval for more effective technical support.

Key Takeaways

Kapa builds AI assistants that answer questions from technical documentation. The knowledge bases we process hold millions of images: screenshots, architecture diagrams, circuit schematics, annotated UI walkthroughs. We spent several months working out how to make them useful in our RAG pipeline.

The short version: we don't send images to the model at query time. We describe each image once, at indexing time, with a cheap vision model, store the descriptions as text, and retrieve them alongside ordinary text chunks. Indexing is a one-time cost; after that, per-query overhead is 1% to 6% over text-only, and answers are measurably, statistically significantly better. This post explains how we got there.

Both answers are correct. The one that shows the screenshot is the one a user can act on without hunting for the setting.

What images actually do in technical documentation

We went through thousands of real customer questions across hardware, semiconductor, and developer-tooling accounts to see how images earn their place in an answer. They split into two kinds.

Most are illustrative. They show what the text already says, only more clearly: a guide says "click the settings icon," and the screenshot beside it shows which icon, where, and what it looks like. The words carry the fact; the picture makes it easy to act on.

Some are load-bearing. A wiring diagram, a spec table, a certification or color-availability matrix can hold a value that lives in the figure and essentially nowhere else. There the picture is not a convenience, it is the source of the answer.

We confirmed the lift either way: with image context available, an LLM judge preferred the answers across three customer projects and two models, by a statistically significant margin (McNemar's test, p < 0.05).

The improvement is the kind a user feels. Instead of "look for the configuration section that controls the setting," you get the specific path plus a screenshot showing exactly where to click. Same facts, far easier to act on. For a support assistant, that is the difference between a user who self-serves and one who opens a ticket.

Either way, images make answers materially better. The engineering question is the one the rest of this post is about: how to use them without paying a vision bill on every query.

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