Pixelizing images with ChunkyPNG

output.png

simple_output.png

simple_solution.rb

require 'chunky_png'

image = ChunkyPNG::Image.from_file('input.png')

width, height = image.width, image.height
image.resample_nearest_neighbor!(width/10, height/10)
image.resample_nearest_neighbor!(width, height)

image.save('output.png')

solution.rb

require 'chunky_png'

module ChunkyPNG
  class Canvas
    module BilinearResampling
      def resample_bilinear!(new_width, new_height)
        width_ratio  = width.to_f / new_width.to_f
        height_ratio = height.to_f / new_height.to_f

        pixels = []
        1.upto(new_height) do |y|
          1.upto(new_width) do |x|
            pixels << get_bilinear_pixel((x-0.5)*width_ratio, (y-0.5)*height_ratio, width_ratio, height_ratio)
          end
        end
        replace_canvas!(new_width, new_height, pixels)
      end

    private
      def get_bilinear_pixel(x, y, sample_width, sample_height)
        x = x - sample_width/2
        y = y - sample_height/2
        pixel_weights = []
        (y.floor..[(y+sample_height).ceil, height-1].min).each do |image_y|
          weight_y = 1
          offset = y - image_y
          weight_y = offset if offset > 0
          offset = image_y - (y + sample_height)
          weight_y = offset if offset > 0
          (x.floor..[(x+sample_width).ceil, width-1].min).each do |image_x|
            weight = weight_y
            offset = x - image_x
            weight = (weight_y ** 2 + offset ** 2) ** 0.5 if offset > 0
            offset = image_x - (x + sample_width)
            weight = (weight_y ** 2 + offset ** 2) ** 0.5 if offset > 0
            pixel_weights << [get_pixel(image_x, image_y), weight]
          end
        end
        total_weight = pixel_weights.inject(0){|acc,(_,w)| acc+w }
        ChunkyPNG::Color.rgba(
          (pixel_weights.inject(0){|acc,(p,w)| acc+ChunkyPNG::Color.r(p)*w }/total_weight).round,
          (pixel_weights.inject(0){|acc,(p,w)| acc+ChunkyPNG::Color.g(p)*w }/total_weight).round,
          (pixel_weights.inject(0){|acc,(p,w)| acc+ChunkyPNG::Color.b(p)*w }/total_weight).round,
          (pixel_weights.inject(0){|acc,(p,w)| acc+ChunkyPNG::Color.a(p)*w }/total_weight).round
        )
      end
    end

    include BilinearResampling
  end
end

image = ChunkyPNG::Image.from_file('input.png')
width, height = image.width, image.height
image.resample_bilinear!(width/10, height/10)
image.resample_nearest_neighbor!(width, height)
image.save('output.png')