OpsLog/internal/pdf/pdf.go

// Package pdf writes the one kind of PDF the label printer needs: a document
// whose every page is a single full-bleed image at an exact physical size.
//
// Written by hand rather than through a library for two reasons. The build is
// pure Go with no room for cgo, and the need is tiny: the pages arrive as
// PNGs rasterised by the SAME canvas renderer the designer's preview uses, so
// this file only has to carry pixels to paper without touching them. Fonts,
// vectors, compression profiles — all already decided upstream.
//
// The images are stored as 8-bit DeviceGray with FlateDecode: labels are
// monochrome, grey keeps antialiased text edges smooth on a 300 dpi thermal
// head, and flate is lossless — JPEG artefacts around small print are exactly
// what a QSL label cannot afford.
package pdf

import (
	"bytes"
	"compress/zlib"
	"fmt"
	"image/png"
)

const mmToPt = 72.0 / 25.4

// Doc accumulates pages; Bytes() renders the file.
type Doc struct {
	pages []pageData
}

type pageData struct {
	wPt, hPt float64
	imgW     int
	imgH     int
	gray     []byte // zlib-compressed 8-bit samples
}

// AddImagePage appends one page of wMm×hMm entirely covered by the PNG.
// The PNG's aspect ratio is not checked against the page's: the caller
// rasterised it AT this size, and a mismatch would be its bug to see.
func (d *Doc) AddImagePage(pngBytes []byte, wMm, hMm float64) error {
	img, err := png.Decode(bytes.NewReader(pngBytes))
	if err != nil {
		return fmt.Errorf("page image: %w", err)
	}
	b := img.Bounds()
	w, h := b.Dx(), b.Dy()
	if w <= 0 || h <= 0 {
		return fmt.Errorf("page image is empty")
	}
	// To 8-bit grey. Luminance weights, not an average: blue text on a designer
	// screen should darken the way a photocopier would darken it.
	gray := make([]byte, w*h)
	i := 0
	for y := b.Min.Y; y < b.Max.Y; y++ {
		for x := b.Min.X; x < b.Max.X; x++ {
			r, g, bb, _ := img.At(x, y).RGBA()
			gray[i] = byte((299*r + 587*g + 114*bb) / 1000 >> 8)
			i++
		}
	}
	var buf bytes.Buffer
	zw := zlib.NewWriter(&buf)
	if _, err := zw.Write(gray); err != nil {
		return err
	}
	if err := zw.Close(); err != nil {
		return err
	}
	d.pages = append(d.pages, pageData{
		wPt: wMm * mmToPt, hPt: hMm * mmToPt,
		imgW: w, imgH: h, gray: buf.Bytes(),
	})
	return nil
}

// Bytes renders the whole document.
func (d *Doc) Bytes() ([]byte, error) {
	if len(d.pages) == 0 {
		return nil, fmt.Errorf("no pages")
	}
	var out bytes.Buffer
	offsets := []int{0} // object 0 is the free-list head
	obj := func(body func()) int {
		offsets = append(offsets, out.Len())
		n := len(offsets) - 1
		fmt.Fprintf(&out, "%d 0 obj\n", n)
		body()
		out.WriteString("endobj\n")
		return n
	}

	out.WriteString("%PDF-1.4\n%\xe2\xe3\xcf\xd3\n")

	// Objects 1 (catalog) and 2 (pages) reference their children by number, so
	// the numbering is laid out first: 3 objects per page after the two roots.
	nPages := len(d.pages)
	pageObj := func(i int) int { return 3 + i*3 }

	obj(func() { out.WriteString("<< /Type /Catalog /Pages 2 0 R >>\n") }) // 1
	obj(func() {                                                           // 2
		out.WriteString("<< /Type /Pages /Kids [")
		for i := 0; i < nPages; i++ {
			fmt.Fprintf(&out, "%d 0 R ", pageObj(i))
		}
		fmt.Fprintf(&out, "] /Count %d >>\n", nPages)
	})
	for i, p := range d.pages {
		content := fmt.Sprintf("q %.4f 0 0 %.4f 0 0 cm /Im0 Do Q", p.wPt, p.hPt)
		obj(func() { // page
			fmt.Fprintf(&out, "<< /Type /Page /Parent 2 0 R /MediaBox [0 0 %.4f %.4f] /Contents %d 0 R /Resources << /XObject << /Im0 %d 0 R >> >> >>\n",
				p.wPt, p.hPt, pageObj(i)+1, pageObj(i)+2)
		})
		obj(func() { // contents
			fmt.Fprintf(&out, "<< /Length %d >>\nstream\n%s\nendstream\n", len(content), content)
		})
		obj(func() { // image
			fmt.Fprintf(&out, "<< /Type /XObject /Subtype /Image /Width %d /Height %d /ColorSpace /DeviceGray /BitsPerComponent 8 /Filter /FlateDecode /Length %d >>\nstream\n",
				p.imgW, p.imgH, len(p.gray))
			out.Write(p.gray)
			out.WriteString("\nendstream\n")
		})
	}

	xref := out.Len()
	fmt.Fprintf(&out, "xref\n0 %d\n0000000000 65535 f \n", len(offsets))
	for _, off := range offsets[1:] {
		fmt.Fprintf(&out, "%010d 00000 n \n", off)
	}
	fmt.Fprintf(&out, "trailer\n<< /Size %d /Root 1 0 R >>\nstartxref\n%d\n%%%%EOF\n", len(offsets), xref)
	return out.Bytes(), nil
}