Class: LogoSoup::Core::BackgroundDetector

Inherits:
Object
  • Object
show all
Defined in:
lib/logosoup/core/background_detector.rb

Overview

Detects whether background should be treated as transparent and estimates background RGB from the image perimeter.

Constant Summary collapse

QUANTIZATION_SHIFT = 
5
ALPHA_TRANSPARENCY_THRESHOLD = 
128
TRANSPARENT_PERIMETER_RATIO_THRESHOLD = 
0.1

Class Method Summary collapse

Class Method Details

.call(bytes, width, height) ⇒ Array(Boolean, Integer, Integer, Integer)

Returns [alpha_only, bg_r, bg_g, bg_b].

Parameters:

  • bytes (Array<Integer>) —

    RGBA bytes

  • width (Integer)
  • height (Integer)

Returns:

  • (Array(Boolean, Integer, Integer, Integer)) —
    alpha_only, bg_r, bg_g, bg_b


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# File 'lib/logosoup/core/background_detector.rb', line 16

def self.call(bytes, width, height)
  levels = 1 << (8 - QUANTIZATION_SHIFT)
  bucket_count = levels * levels * levels

  bucket_counts = Array.new(bucket_count, 0)
  bucket_r = Array.new(bucket_count, 0)
  bucket_g = Array.new(bucket_count, 0)
  bucket_b = Array.new(bucket_count, 0)

  opaque_count = 0
  transparent_count = 0

  sample = lambda do |x, y|
    idx = ((y * width) + x) * 4
    a = bytes[idx + 3]
    return if a.nil?

    if a < ALPHA_TRANSPARENCY_THRESHOLD
      transparent_count += 1
      return
    end

    opaque_count += 1
    r = bytes[idx]
    g = bytes[idx + 1]
    b = bytes[idx + 2]

    key = ((((r >> QUANTIZATION_SHIFT) * levels) + (g >> QUANTIZATION_SHIFT)) * levels) + (b >> QUANTIZATION_SHIFT)
    bucket_counts[key] += 1
    bucket_r[key] += r
    bucket_g[key] += g
    bucket_b[key] += b
  end

  width.times do |x|
    sample.call(x, 0)
    sample.call(x, height - 1) if height > 1
  end

  (1...(height - 1)).each do |y|
    sample.call(0, y)
    sample.call(width - 1, y) if width > 1
  end

  total_perimeter = opaque_count + transparent_count
  transparent = total_perimeter.positive? && transparent_count > total_perimeter * TRANSPARENT_PERIMETER_RATIO_THRESHOLD
  return [true, 0, 0, 0] if transparent

  best_idx = 0
  best_count = 0
  bucket_counts.each_with_index do |count, i|
    next unless count > best_count

    best_count = count
    best_idx = i
  end

  if best_count.positive?
    bg_r = (bucket_r[best_idx].to_f / best_count).round
    bg_g = (bucket_g[best_idx].to_f / best_count).round
    bg_b = (bucket_b[best_idx].to_f / best_count).round
    [false, bg_r, bg_g, bg_b]
  else
    [false, 255, 255, 255]
  end
end