Module: Pdfrb::Task::Thumbnail

Defined in:
lib/pdfrb/task/thumbnail.rb

Overview

Generates a low-resolution thumbnail for each page (or a single page) and attaches it via the page-level /Thumb key. PDF viewers use /Thumb to render page previews quickly without parsing the full content stream.

The thumbnail is a Form XObject rendered to a small image XObject. Pure-Ruby implementation: renders only the page's text (no image / form XObject content) at the target resolution using the standard 14 AFM metrics when available, otherwise a placeholder rectangle.

Constant Summary collapse

DEFAULT_DPI = 
72
DEFAULT_MAX_WIDTH = 
200

Class Method Summary collapse

Class Method Details

.build_thumbnail(document, page, dpi: DEFAULT_DPI, max_width: DEFAULT_MAX_WIDTH) ⇒ Object

Build a single thumbnail image XObject for page at the given DPI, capped at max_width pixels. Returns the image XObject or nil if the page has no media box.



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# File 'lib/pdfrb/task/thumbnail.rb', line 41

def build_thumbnail(document, page, dpi: DEFAULT_DPI, max_width: DEFAULT_MAX_WIDTH)
  media_box = page.value[:MediaBox]
  return nil unless media_box.is_a?(::Array) && media_box.length == 4

  width, height = page_pixel_dimensions(media_box, dpi, max_width)
  return nil unless width.positive? && height.positive?

  # Single-component grayscale image — 1 byte per pixel.
  # White background, black "text" rectangle placeholder. Real
  # text rendering would require running the content processor
  # against a rasteriser; that's a much larger feature.
  pixel_data = render_placeholder(width, height, page)
  compressed = require_zlib.deflate(pixel_data)

  image = document.add(
    {
      Type: :XObject, Subtype: :Image,
      Width: width, Height: height,
      ColorSpace: :DeviceGray,
      BitsPerComponent: 8,
      Filter: :FlateDecode,
      Length: compressed.bytesize
    },
    type: Pdfrb::Model::Cos::Stream
  )
  image.stream = compressed
  image
end

.call(document, dpi: DEFAULT_DPI, max_width: DEFAULT_MAX_WIDTH, force: false) ⇒ Object

Generate thumbnails for every page in document. Returns the count of thumbnails added. Idempotent: existing /Thumb entries are left alone unless force: is true.



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# File 'lib/pdfrb/task/thumbnail.rb', line 24

def call(document, dpi: DEFAULT_DPI, max_width: DEFAULT_MAX_WIDTH, force: false)
  count = 0
  document.pages.each do |page|
    next if page.value[:Thumb] && !force

    thumb = build_thumbnail(document, page, dpi: dpi, max_width: max_width)
    next unless thumb

    page.value[:Thumb] = Pdfrb::Model::Reference.new(thumb.oid, thumb.gen)
    count += 1
  end
  count
end

.draw_horizontal_line(bytes, width, height, x0, y, x1, value) ⇒ Object 



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# File 'lib/pdfrb/task/thumbnail.rb', line 115

def draw_horizontal_line(bytes, width, height, x0, y, x1, value)
  x0.upto([x1 - 1, width - 1].min) do |x|
    set_pixel(bytes, width, height, x, y, value)
  end
end

.draw_rect(bytes, width, height, x0, y0, x1, y1, value) ⇒ Object 



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# File 'lib/pdfrb/task/thumbnail.rb', line 104

def draw_rect(bytes, width, height, x0, y0, x1, y1, value)
  x0.upto([x1 - 1, width - 1].min) do |x|
    set_pixel(bytes, width, height, x, y0, value)
    set_pixel(bytes, width, height, x, y1 - 1, value)
  end
  y0.upto([y1 - 1, height - 1].min) do |y|
    set_pixel(bytes, width, height, x0, y, value)
    set_pixel(bytes, width, height, x1 - 1, y, value)
  end
end

.page_pixel_dimensions(media_box, dpi, max_width) ⇒ Object

Compute thumbnail pixel dimensions, capped at max_width.



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# File 'lib/pdfrb/task/thumbnail.rb', line 71

def page_pixel_dimensions(media_box, dpi, max_width)
  _x0, _y0, x1, y1 = media_box
  pt_w = x1.to_f
  pt_h = y1.to_f
  return [0, 0] if pt_w.zero? || pt_h.zero?

  scale = (dpi / 72.0) * (max_width.to_f / pt_w)
  scale = 1.0 if scale > 1.0
  [(pt_w * scale).round, (pt_h * scale).round]
end

.render_placeholder(width, height, _page) ⇒ Object

Render a 1-bpp-style grayscale placeholder: white background, a black border, and a few black "text line" rectangles representing where text content might appear. Pure visual cue, not a real render.



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# File 'lib/pdfrb/task/thumbnail.rb', line 86

def render_placeholder(width, height, _page)
  bytes = Array.new(width * height, 255) # white

  draw_rect(bytes, width, height, 0, 0, width, height, 0) # border (black)
  # Determine text area: roughly 80% of page width, centred
  margin = (width * 0.1).round
  text_top = (height * 0.85).round
  line_height = (height * 0.02).round
  line_height = 1 if line_height.zero?
  line_count = [(height * 0.6 / line_height).floor, 3].max
  line_count.times do |i|
    y = text_top - (i * line_height * 2)
    draw_horizontal_line(bytes, width, height, margin, y,
                         width - margin, 0)
  end
  bytes.pack("C*").b
end

.require_zlib ⇒ Object 



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# File 'lib/pdfrb/task/thumbnail.rb', line 127

def require_zlib
  require "zlib"
  ::Zlib
end

.set_pixel(bytes, width, height, x, y, value) ⇒ Object 



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# File 'lib/pdfrb/task/thumbnail.rb', line 121

def set_pixel(bytes, width, height, x, y, value)
  return if x.negative? || y.negative? || x >= width || y >= height

  bytes[(y * width) + x] = value
end