Printing an image at a real-world size is harder than it should be. The print dialog scales the page without saying so, the image file carries no physical size of its own, and a hand cut needs a line to follow. Exactly Print takes an image and a size in millimetres and produces a one-page PDF with the image at that size, ready to print at 100% and cut.

Features

• The image is placed by physical millimetres, not pixels. Give one side and the other follows from the image's proportions
• A 2 mm bleed past the cut line and crop marks in the corners, so a slightly off cut still shows image rather than paper
• Two 100 mm rulers, along the bottom and up the left edge, so a test print shows whether the printer scaled the page, and by how much in each direction
• Per-printer calibration: measure the rulers once, type in what they came out as, and every later page is pre-scaled to cancel the printer's own scaling
• Nothing is stored. The server holds the image only for the length of the request

How it is built

FastAPI serves one page, an htmx preview and the PDF. Pillow reads the upload and draws the preview. The PDF is written by hand: one page, one image, a few lines and a bit of Helvetica. The geometry lives in a single pure module that both the preview and the PDF draw from, so the two cannot disagree.

Run it

docker compose up

Links

Source on GitHub.