Module: Okmain::Sampler

Defined in:
lib/okmain/sampler.rb

Constant Summary collapse

MAX_SAMPLE_SIZE = 
250_000

Class Method Summary collapse

Class Method Details

.compute_block_size(total) ⇒ Object 



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# File 'lib/okmain/sampler.rb', line 64

def compute_block_size(total)
  return 1 if total <= MAX_SAMPLE_SIZE

  bs = Math.sqrt(total.to_f / MAX_SAMPLE_SIZE).ceil
  # Round up to multiple of 4
  remainder = bs % 4
  bs += (4 - remainder) if remainder != 0
  bs
end

.sample(input) ⇒ Object

Returns [pixels, width, height] where pixels is an Array of [L, a, b] triples.



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# File 'lib/okmain/sampler.rb', line 12

def sample(input)
  image = if input.is_a?(Vips::Image)
            input
          else
            Vips::Image.new_from_file(input.to_s, access: :sequential)
          end

  # Flatten alpha to white background
  image = image.flatten(background: [255, 255, 255]) if image.has_alpha?

  # Convert to scRGB (linear float) for accurate block averaging
  image = image.colourspace(:scrgb)

  total = image.width * image.height
  block_size = compute_block_size(total)

  if block_size > 1
    # Claude wanted to do this in Ruby for a more-exact rust match, because apparently image.shrink
    # uses a slightly different algorithm, but an occasional 1 or 2 RGB value difference is a small
    # price for the huge speedup in letting VIPS do it for us.
    image = image.shrink(block_size, block_size)
  end

  width = image.width
  height = image.height

  # Claude's approach: Doing the colourspace translation ourselves
  # ==============================================================
  # Extract float pixel data [r, g, b] in linear space
  # floats = image.write_to_memory.unpack("f*")
  # bands = image.bands
  # pixel_count = width * height

  # pixels = Array.new(pixel_count)
  # i = 0
  # while i < pixel_count
  #   offset = i * bands
  #   r = floats[offset]
  #   g = floats[offset + 1]
  #   b = floats[offset + 2]
  #   pixels[i] = Oklab.linear_rgb_to_oklab(r, g, b)
  #   i += 1
  # end

  # My approach: more than twice as fast to let VIPS do it for us
  # =============================================================
  image = image.colourspace(:oklab)
  pixels = image.write_to_memory.unpack("f*").each_slice(3).to_a

  [pixels, width, height]
end