52 lines
1.9 KiB
Go
52 lines
1.9 KiB
Go
package main
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import (
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"bytes"
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"fmt"
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"image/color"
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"image/jpeg"
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)
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// PDF writes a one-page PDF holding the JPEG as it is (DCTDecode), the page as large as the image really is:
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// pixels over dpi, in points (72 to the inch).
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func PDF(jpegBytes []byte, dpi int) ([]byte, error) {
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if dpi <= 0 {
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return nil, fmt.Errorf("resolution %d dpi is no resolution", dpi)
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}
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cfg, err := jpeg.DecodeConfig(bytes.NewReader(jpegBytes))
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if err != nil {
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return nil, fmt.Errorf("the image cannot be read: %w", err)
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}
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space := "DeviceRGB"
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if cfg.ColorModel == color.GrayModel {
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space = "DeviceGray"
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}
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wpt := float64(cfg.Width) / float64(dpi) * 72
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hpt := float64(cfg.Height) / float64(dpi) * 72
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content := fmt.Sprintf("q %.2f 0 0 %.2f 0 0 cm /Im0 Do Q\n", wpt, hpt)
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var b bytes.Buffer
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var offsets []int
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obj := func(body string) {
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offsets = append(offsets, b.Len())
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fmt.Fprintf(&b, "%d 0 obj\n%s\nendobj\n", len(offsets), body)
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}
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b.WriteString("%PDF-1.4\n%\xE2\xE3\xCF\xD3\n")
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obj("<< /Type /Catalog /Pages 2 0 R >>")
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obj("<< /Type /Pages /Kids [3 0 R] /Count 1 >>")
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obj(fmt.Sprintf("<< /Type /Page /Parent 2 0 R /MediaBox [0 0 %.2f %.2f] /Resources << /XObject << /Im0 4 0 R >> >> /Contents 5 0 R >>", wpt, hpt))
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offsets = append(offsets, b.Len())
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fmt.Fprintf(&b, "4 0 obj\n<< /Type /XObject /Subtype /Image /Width %d /Height %d /ColorSpace /%s /BitsPerComponent 8 /Filter /DCTDecode /Length %d >>\nstream\n",
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cfg.Width, cfg.Height, space, len(jpegBytes))
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b.Write(jpegBytes)
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b.WriteString("\nendstream\nendobj\n")
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obj(fmt.Sprintf("<< /Length %d >>\nstream\n%sendstream", len(content), content))
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xref := b.Len()
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fmt.Fprintf(&b, "xref\n0 %d\n0000000000 65535 f \n", len(offsets)+1)
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for _, o := range offsets {
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fmt.Fprintf(&b, "%010d 00000 n \n", o)
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}
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fmt.Fprintf(&b, "trailer\n<< /Size %d /Root 1 0 R >>\nstartxref\n%d\n%%%%EOF\n", len(offsets)+1, xref)
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return b.Bytes(), nil
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}
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