Open-source canvas PenEcho lets Claude reply beside your handwriting
The idea started from a frustration with physics and math research: working out equations on a whiteboard with a stylus, then having to type a half-finished derivation into a chat box, which breaks the train of thought while explaining how connects. Seeing recent improvements in model vision, particularly from Opus 4.8 (especially fb5), raised the idea of letting the model meet the user on the whiteboard instead of in a chat window.
That led to PenEcho, an canvas where equations can be handwritten, diagrams sketched, and notes placed anywhere on the page. When the user pauses, the relevant region of the canvas is sent to the model, and the response appears beside the work as an editable draft — explaining a step, continuing an idea, or flagging a possible mistake, all without switching to a separate chat window.
The canvas is logically 20,000 x 20,000 in size, but only allocates 512 x 512 tiles where ink actually exists. Each request sends a cropped visual atlas and geometry data rather than the whole canvas, which in normal use amounts to a few thousand and fewer than 1,000 — usually costing a few cents or less depending on the model and provider used.
Key points
- canvas where handwritten equations and diagrams get replies from Claude placed right beside the work
- Only sends 512x512 tiles where ink exists, not the whole 20,000x20,000 canvas
- Typical use costs a few thousand and under 1,000 , often a few cents or less
- Responses appear as editable drafts next to the work, no separate chat window needed