pdf

Overview ¶

Package pdf renders a chart to PDF using nothing but the standard library.

It is the second built-in emitter, and it exists for the same reason as backend/svg: PDF is a text format with a simple imaging model, and writing it directly costs a few hundred lines and no dependency at all. A report generator that wants a vector chart in a document links the same stdlib-only core it would have linked for SVG.

p := refract.New(refract.Title("Signal"))
p.Add(geom.Line(src, geom.X("t"), geom.Y("y")))
err := p.Render(pdf.File("signal.pdf"))

Coordinates ¶

PDF puts the origin at the bottom left with Y running up; refract's IR puts it at the top left with Y running down. The page's content stream opens with a flip, so every coordinate refract emits is written unchanged and the two backends' geometry agrees exactly. Text is placed with its own matrix, which undoes the flip for the glyphs alone so they read the right way up.

Text ¶

The output uses the base-14 Helvetica, which every PDF reader has and no document has to embed. That is also the metric set internal/fontmetrics carries, so this backend measures with exactly the font it draws with — margins, tick spacing and collision decisions are not approximations here.

Text is encoded as WinAnsi, which covers Latin-1 plus the usual typographic punctuation. A rune outside it is written as "?" rather than silently dropped, because a missing label is harder to notice than a wrong one.

Not here ¶

One page per document, no embedded fonts, no tagging or accessibility structure, no transparency groups. Alpha is expressed as a graphics-state constant, which is what a faded area fill needs and is not the same thing as a full transparency model.