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I am trying to find the contrast of an image using ChunkyPNG. Is there any way to get the standard deviation of an image using ChunkyPNG?

Hommer Smith
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1 Answers1

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Looking at the ChunkyPNG code, I couldn't find any stats module.

But you can use the following method:

image = ChunkyPNG::Image.from_file('any PNG image file')

# @return [Hash] some statistics based on image pixels
def compute_image_stats(image, &pixel_transformation)
  # compute pixels values
  data  = image.pixels.map {|pixel| yield(pixel)} # apply the pixel convertion

  # compute stats
  n         = data.size # sum of pixels
  mean      = data.inject(:+).to_f / n
  variance  = data.inject(0) {|sum, item| sum += (item - mean)**2} / n
  sd        = Math.sqrt(variance) # standard deviation

  {mean: mean, variance: variance, sd: sd}
end

# compute stats for grayscale image version
compute_image_stats(image) {|pixel| ChunkyPNG::Color.grayscale_teint(pixel)}
# compute stats for blue channel
compute_image_stats(image) {|pixel| ChunkyPNG::Color.b(pixel)}

I included all stats in the return, because they were computed for standard deviation (sd) calculation.

Thiago Lewin
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  • Thanks for taking the time on doing this. Really useful. I will try it out – Hommer Smith Nov 27 '13 at 19:59
  • I have a question. Is there any way I could get the standard deviation normalized in a value between 0 and 1. When using ImageMagick standard deviation, I get ```standard deviation: 105.973 (0.415581)``` and the parentheses value is always in the range 0 to 1. So if multiplied by 100, it is percent of max value – Hommer Smith Nov 27 '13 at 21:21
  • ImageMagick divides by the maximum that the variable can reach. In the grayscale image the maximum is 255, so: `105.973/255 = 0.41558`. – Thiago Lewin Nov 27 '13 at 21:43