CSS pixels and physical pixels are different contracts
CSS defines fixed ratios among its absolute length units. A CSS pixel is exactly one ninety-sixth of a CSS inch, regardless of the number of hardware pixels used to render it.
A raster pixel has no physical size until a pixel density is supplied. CalcOS therefore activates the PPI input only when a raster pixel participates in the conversion.
Points measure the type body, not visible letter height
A nominal point size describes the font’s design body. The visible glyph height depends on the typeface’s metrics, so two fonts set to 12 pt can appear different in size.
| Unit | Definition used | Boundary |
|---|---|---|
| CSS pixel | 1/96 CSS inch | Not one hardware pixel |
| PostScript point | 1/72 inch | Traditional 72.27-point systems excluded |
| PostScript pica | 12 points = 1/6 inch | PostScript convention |
| Raster pixel | 1 ÷ selected PPI inches | User-supplied physical context |
DPI and PPI are related but not interchangeable labels
PPI describes sampled pixels per inch. DPI commonly describes printer dots per inch. A printer can use several ink dots to render one image pixel, so CalcOS does not treat printer DPI as raster PPI.
Relative font units are deliberately excluded
em, rem, ch, line-height, and percentage values require a font or layout reference. They are not fixed-length units and cannot be converted honestly without additional inputs.
Authoritative references: W3C CSS Values and Units Level 4, Adobe PostScript Language Reference, third edition, and Adobe ruler and point/pica definitions.