Screen PPI API

Screen and display pixel-density maths as an API, computed locally and deterministically. The ppi endpoint computes the pixels per inch of a display from its resolution and diagonal size — along with the pixels per centimetre, the dot pitch in millimetres, the diagonal in pixels, the total pixels and megapixels, the simplified aspect ratio, and the physical width and height. The size endpoint does the inverse: from a resolution and a known PPI it works out the physical dimensions and diagonal in inches and centimetres. The retina endpoint analyses a display at a viewing distance: it computes the pixels per degree, says whether the display is effectively "retina" (pixels indistinguishable to 20/20 vision, around 60 pixels per degree), and gives the distance at which it becomes retina. Everything is computed locally and deterministically, so it is instant and private. Ideal for display and monitor tools, AV and signage planning, UI and responsive-design work, and hardware comparison sites. Pure local computation — no key, no third-party service, instant. Live, nothing stored. 3 endpoints. This is screen pixel density; for print resolution and image-to-print sizing use a DPI API.