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POGet, or calculate the entropy of an image with Ruby and imagemagick
 

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  1. POGet, or calculate the entropy of an image with Ruby and imagemagick
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    <p>How to find the "entropy" with imagemagick, preferably mini_magic, in Ruby? I need this as part of a larger project, <em>finding "interestingness" in an image so to crop it</em>.</p> <p>I found a good <a href="https://stackoverflow.com/questions/1516736/django-sorl-thumbnail-crop-picture-head">example in Python/Django</a>, which gives the following pseudo-code:</p> <pre><code>image = Image.open('example.png') histogram = image.histogram() # Fetch a list of pixel counts, one for each pixel value in the source image #Normalize, or average the result. for each histogram as pixel histogram_recalc &lt;&lt; pixel / histogram.size endfor #Place the pixels on a logarithmic scale, to enhance the result. for each histogram_recalc as pixel if pixel != 0 entropy_list &lt;&lt; log2(pixel) endif endfor #Calculate the total of the enhanced pixel-values and invert(?) that. entropy = entroy_list.sum * -1 </code></pre> <p>This would translate to the formula <code>entropy = -sum(p.*log2(p))</code>. </p> <p>My questions: Did I interprete the Django/Python code correct? How can I fetch a histogram in ruby's mini_magick if at all? </p> <p>Most important question: is this algorithm any good in the first place? Would you suggest a better one to find the "entropy" or "amount of changing pixels" or "gradient depth" in (parts of) images? </p> <p><strong>Edit</strong>: Using a.o. the resources provided by the answer below, I came up with the working code:</p> <pre><code># Compute the entropy of an image slice. def entropy_slice(image_data, x, y, width, height) slice = image_data.crop(x, y, width, height) entropy = entropy(slice) end # Compute the entropy of an image, defined as -sum(p.*log2(p)). # Note: instead of log2, only available in ruby &gt; 1.9, we use # log(p)/log(2). which has the same effect. def entropy(image_slice) hist = image_slice.color_histogram hist_size = hist.values.inject{|sum,x| sum ? sum + x : x }.to_f entropy = 0 hist.values.each do |h| p = h.to_f / hist_size entropy += (p * (Math.log(p)/Math.log(2))) if p != 0 end return entropy * -1 end </code></pre> <p>Where image_data is an <code>RMagick::Image</code>. </p> <p>This is used in the <a href="http://smartcropper.berk.es/" rel="nofollow noreferrer">smartcropper gem</a>, which allows smart slicing and cropping for images with e.g. paperclip.</p>
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