Geometry first
Every sheet starts as numbers, not pixels. The engine works in millimetres with 0.001 mm precision: a quarter-inch square is 6.350 mm, a 14-count cross-stitch cell is 1.814 mm, a midi bead peg sits on a 5.000 mm pitch. Lines are placed at exact multiples of the spacing from the grid origin, so error never accumulates across the page, and grids fit whole cells only unless you ask otherwise.
One drawing, three outputs
The engine produces a single list of drawing instructions. The on-screen preview, the SVG export and the PDF export are all rendered from that same list, which is why the preview is a faithful picture of the printout. The PDF writer is our own: it declares the exact page size (for example 612 × 792 points for US Letter), uses vector lines and standard fonts only, and adds a PrintScaling None preference so PDF viewers default to actual size. Repeated pages share one drawing, keeping a 50-page pad under 50 KB.
Printer calibration
Home printers are rarely perfect. Our calibration wizard prints a test sheet with 100 mm and 4-inch bars, asks what you measured, and stores the ratio for that printer in your browser. Downloads then include a matching scale inside each page, without changing the declared paper size. Measurements in the 94–97% range are flagged as a "Fit to page" setting rather than silently corrected.
How we test
Automated tests parse every PDF the engine produces and check page sizes, coordinates and file structure, and check that ten quarter-inch squares measure exactly 63.5 mm in the file. Printed size still depends on your printer and print settings, so every sheet carries a simple check (for example, ten quarter-inch squares should measure 2.5 inches), and the calibration wizard corrects printers that run slightly long or short. Changes are listed in the changelog.
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