Code Brawl #2

README

Pixelise!
=========

This has two pixelising function, a straight average of the colour channels in a given block, or a weighted mean based
upon a two dimensional normal distribution.

The code uses the weighted version, but the other is kept on for display purposes.

Name your image "input.png" and run the program.

output.png

pixelise.rb

require 'chunky_png'

# 2d gaussian
#
# @params  x    x coord
# @params  y    y coord
# @params  w    width of sample
# @params  h    height of sample
def twodguass(x,y,w,h)

  #distro constants
  big_a = 1
  a = 0.5
  b = 0
  c = 0.5

  x_0 = w/2
  y_0 = h/2

  p1 = a*((x-x_0)**2)
  p2 = 2*b*((x-x_0)*(y-y_0))
  p3 = c*((y-y_0)**2)

  p = (p1+p2+p3)*-1
  return big_a*(Math.exp(p))
end

# Pixelise functions

# Return the average colour of a given area
#
# @params  image  image to manipulate
# @params  x      x coord
# @params  y      y coord 
# @params  w      width of area
# @params  h      height of area
def av_area(image, x, y, w, h)

    # image meta for error checking
    i_w = image.width
    i_h = image.height
    
    # Total area
    tot = w*h

    # average bins
    av_r = 0
    av_g = 0
    av_b = 0

    # fill bins
    (0...w).each do |i|
      (0...h).each do |j|
        if(x+i >= i_w || y+j >= i_h)
          tot -=1
        else
          raw = image[x+i,y+j]
          av_r += ChunkyPNG::Color.r(raw)
          av_g += ChunkyPNG::Color.g(raw)
          av_b += ChunkyPNG::Color.b(raw)
        end
      end
    end

    # divide bins
    av_r /= tot
    av_g /= tot
    av_b /= tot

    return ChunkyPNG::Color.rgb(av_r, av_g, av_b)
end

# Return the weighted average colour of a given area
#
# @params  image  image to manipulate
# @params  x      x coord
# @params  y      y coord 
# @params  w      width of area
# @params  h      height of area
def w_av_area(image, x, y, w, h)

    # image meta for error checking
    i_w = image.width
    i_h = image.height
    
    # Total weight 
    tot = 0

    # average bins
    av_r = 0
    av_g = 0
    av_b = 0

    # fill bins
    (0...w).each do |i|
      (0...h).each do |j|
        if(x+i >= i_w || y+j >= i_h)
        else
          weight = twodguass(i,j,w,h)
          tot += weight
          raw = image[x+i,y+j]
          av_r += ChunkyPNG::Color.r(raw) * weight
          av_g += ChunkyPNG::Color.g(raw) * weight
          av_b += ChunkyPNG::Color.b(raw) * weight
        end
      end
    end

    # divide bins
    av_r /= tot
    av_g /= tot
    av_b /= tot


    return ChunkyPNG::Color.rgb(av_r.to_i, av_g.to_i, av_b.to_i)
end

# Pixelise function
#
# @params  image		image to pixelise
# @params  b_w      block width
# @params  b_h      block height
#
# @return						pixelised image
def pixelise(image, b_w, b_h)

  # Get image size
  w = image.width
  h = image.height

  # Number of blocks
  nbx = (w/b_w.to_f).ceil
  nby = (h/b_h.to_f).ceil

  # Go through each "block" in the image
  (0...nbx).each do |p_x|
    (0...nby).each do |p_y|

      # Top left coord
      x = p_x * b_w
      y = p_y * b_h

      # Get the average colour of the block
      p_colour = w_av_area(image, x, y, b_w, b_h);

      # Fill in block
      (0...b_w).each do |x1|
        (0...b_h).each do |y1|
          # Fill in up to edges
          image[x+x1,y+y1] = p_colour if !(x+x1 >= w || y+y1 >= h) 
        end
      end
    end
  end
  return image
end

### Script

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

# Size of pixeled area
b_w = 10
b_h = 10


# Run the pixelisation
image = pixelise(image, b_w, b_h)

# save
image.save('output.png')