6. prepress:: 命名空间 — 印前检查

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涵盖印刷厂在上版前必须确认事项的 30 个函数：油墨总量、分色、字体、线宽、 页面框。

6.1 油墨总量（TAC）

TAC（Total Area Coverage）是某一点上四色油墨的总和。超过印刷机上限会导致 蹭脏、干燥不良和背面粘脏。铜版纸胶印通常以 300% 为上限。

测量方式有两种，差别很关键。

估算值会把深色中性灰（丰富黑）压缩到 100% 附近。

check "Ink limit" {
  // 校验上限时始终使用「准确的 TAC」
  doc.pages.each { |page|
    tac = prepress::calculate_exact_tac(page.number)
    assert tac <= 300, "page #{page.number}: #{tac}% ink"
  }

  print("exact (declared in the file):", prepress::calculate_exact_tac(), "%")
  print("estimated (by rendering):", prepress::calculate_tac(), "%")
  // 真实文件实测：准确值 324%，估算值 299%
  // — 只有准确值才能发现超标。

  // 折线处油墨过多会在加工时开裂
  fold = region(290, 0, 15, 842, "center fold")
  measured = prepress::calculate_tac_by_region(1, fold)
  assert measured.first() < 240, "TAC of #{measured.first()}% on the fold (max 240%)"
}

6.2 颜色与分色

颜色以列表传入：4 个值 = CMYK，3 个 = RGB，1 个 = 灰度。Delta-E 的经验值： 小于 1 无法察觉，3 以内印刷可接受，大于 5 明显不同。

保留的分色 All 和 None 不会被列出：All 是套准标记而非油墨。

check "Colors" {
  spots = prepress::detect_spot_colors()
  assert spots.length == 0, "file uses an unquoted special ink: #{spots.join(", ")}"

  mode = prepress::detect_color_mode()
  assert mode == "CMYK" || mode == "None",
    "document is #{mode} — offset printing requires CMYK"

  // 品牌色的允差
  difference = prepress::compare_colors_delta_e([1.0, 0.6, 0.0, 0.1], [1.0, 0.62, 0.0, 0.12])
  assert difference < 3.0, "brand color out of tolerance (ΔE #{difference})"

  // 小字用丰富黑会让套印不准更明显
  assert !prepress::detect_rich_black(),
    "rich black found — use flat black (0/0/0/100) for text"

  // 意外的叠印会让元素消失
  assert prepress::validate_overprint_settings(),
    "overprint is enabled — verify that it is intentional"

  assert prepress::validate_output_intent(),
    "PDF has no Output Intent — the shop cannot know the target color profile"
}

6.3 线宽

线宽 0 是 PostScript 的经典发丝线，设备会以最小可能宽度渲染（不可预测）。

check "Strokes" {
  assert !prepress::detect_hairlines(0.25),
    "there are strokes below 0.25 pt — they will disappear in print"
  assert !prepress::detect_hairlines_exact(),
    "there is a stroke with 0 width — set a real thickness"
  assert prepress::validate_minimum_stroke_width(0.5),
    "the shop contract requires strokes of at least 0.5 pt"
}

6.4 字体

check "Fonts" {
  print("fonts:", prepress::list_fonts().join(", "))

  missing = prepress::detect_text_substitution()
  assert missing.length == 0,
    "fonts not embedded (text will change at the RIP): #{missing.join(", ")}"

  problems = prepress::detect_missing_glyphs()
  assert problems.length == 0,
    "fonts without a widths table: #{problems.join(", ")}"

  assert prepress::subset_fonts(),
    "a full font is embedded — the file is larger than it needs to be"

  risky = prepress::check_font_licensing()
  assert risky.length == 0, "fonts with licensing risk: #{risky.join(", ")}"

  // 说明书和合同对最小字号有法规要求
  assert prepress::validate_font_size(6),
    "there is text below 6 pt — illegible once printed"
}

6.5 页面与页面框

PDF 的页面框定义了工作区域：MediaBox（纸张）、BleedBox（出血）、 TrimBox（成品）、CropBox（显示）、ArtBox（内容）。

check "Geometry" {
  size = prepress::get_page_size(1)
  assert abs(size.first() - 595.0) < 5, "width is outside A4"
  prepress::get_page_boxes(1).each { |box| print(box) }

  assert prepress::validate_trim_box(),
    "no TrimBox — the shop cannot know where to trim"
  assert prepress::validate_bleed_box(),
    "no BleedBox — no bleed area is defined"

  // 使用单位字面量既好读，换算也自动完成
  assert prepress::check_page_geometry(3mm),
    "bleed smaller than 3 mm on some page"
}

6.6 完整示例

// offset_magazine.pdfl — 胶印的完整印前检查
// 用法: pdfl run offset_magazine.pdfl magazine.pdf --output html --output-file report.html
profile "offset-magazine" {

  const TAC_LIMIT = 300%
  const BLEED = 3mm
  const MIN_DPI = 300

  check "Ink coverage" tags: ["prepress", "colors"] {
    doc.pages.each { |page|
      tac = prepress::calculate_exact_tac(page.number)
      assert tac <= TAC_LIMIT,
        "page #{page.number}: #{tac}% ink (limit #{TAC_LIMIT}%)"
    }
    print("average coverage:", prepress::calculate_ink_coverage(), "%")
  }

  check "Colors" tags: ["prepress", "colors"] {
    assert prepress::detect_color_mode() != "RGB", "document is in RGB"
    spots = prepress::detect_spot_colors()
    assert spots.length == 0, "unquoted special ink: #{spots.join(", ")}"
    assert !prepress::detect_rich_black(), "rich black in text"
    assert prepress::validate_output_intent(), "no Output Intent"
  }

  check "Fonts" tags: ["fonts"] {
    missing = prepress::detect_text_substitution()
    assert missing.length == 0, "fonts not embedded: #{missing.join(", ")}"
    assert prepress::validate_font_size(6), "text below 6 pt"
  }

  check "Strokes" tags: ["prepress"] {
    assert !prepress::detect_hairlines(0.25), "strokes below 0.25 pt"
    assert !prepress::detect_hairlines_exact(), "stroke with 0 width"
  }

  check "Geometry" tags: ["prepress", "boxes"] {
    require prepress::validate_trim_box()
    require prepress::validate_bleed_box()
    assert prepress::check_page_geometry(BLEED), "bleed smaller than 3 mm"
  }

  check "Images" tags: ["images"] {
    doc.images.each { |img|
      assert img.dpi >= MIN_DPI,
        "image on page #{img.page_number}: #{round(img.dpi)} DPI"
      assert img.color_space != "DeviceRGB", "RGB image on page #{img.page_number}"
    }
  }
}

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