Open-Source Whiteboard: Get AI Answers Right Next to Handwritten Formulas

Civil-Direction-6981 · reddit · 2026-07-15

The author built an open-source canvas called PenEcho, allowing users to handwrite formulas, draw diagrams, and paste notes in a whiteboard-style interface, with the model generating responses right alongside.

The interaction works as follows: when the user pauses, the system sends only the relevant canvas regions to the model. The model's reply then appears as an editable draft next to the current workspace, rather than redirecting to a separate chat window. The author envisions it being used to explain derivation steps, answer questions, continue lines of thought, or point out potential errors.

Technically, the canvas is logically 20,000 × 20,000, but it only allocates 512 × 512 tiles where there are pen strokes. Each request sends a cropped visual atlas and geometric information instead of the entire canvas. According to the author, typical daily usage involves only a few thousand input tokens and less than 1,000 output tokens per request, costing just a few cents.

The project supports local execution and can connect to OpenAI-compatible APIs or use existing Codex CLI logins. The code is licensed under AGPL-3.0. The author is currently testing primarily with GPT-5.6 Sol, Terra, and Luna, and is seeking feedback on the naturalness of the interaction and any misunderstandings of handwriting or spatial context.

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