switch from MEPP to PSNR for comparing image after optimization

Author: toy

spec/image_optim_spec.rb

@@ -61,9 +61,9 @@ def temp_copy(image)
 
       base_options = {skip_missing_workers: false}
       [
-        ['lossless', base_options, 0],
-        ['lossy', base_options.merge(allow_lossy: true), 0.00101],
-      ].each do |type, options, max_difference|
+        ['lossless', base_options, Float::INFINITY],
+        ['lossy', base_options.merge(allow_lossy: true), 30],
+      ].each do |type, options, psnr_min|
         it "does it #{type}" do
           image_optim = ImageOptim.new(options)
           copies = test_images.map{ |image| temp_copy(image) }
@@ -78,7 +78,7 @@ def temp_copy(image)
             expect(optimized).not_to have_same_data_as(original)
 
             compare_to = rotate_images.include?(original) ? rotated : original
-            expect(optimized).to be_similar_to(compare_to, max_difference)
+            expect(optimized).to be_similar_to(compare_to, psnr_min)
           end
         end
       end

spec/images/invisiblepixels/generate

@@ -7,6 +7,8 @@ require 'shellwords'
 
 side = 64
 
+colors = Array.new(256){ [rand(256), rand(256), rand(256)] }
+
 IO.popen(%W[
   convert
   -depth 8
@@ -18,7 +20,7 @@ IO.popen(%W[
   side.times do |a|
     side.times do |b|
       alpha = [0, 1, 0x7f, 0xff][((a / 8) + (b / 8)) % 4]
-      f << [rand(256), rand(256), rand(256), alpha].pack('C*')
+      f << [*colors.sample, alpha].pack('C*')
     end
   end
 end

spec/images/invisiblepixels/image.png

@@ -42,12 +42,12 @@ def flatten_animation(image)
   end
 end
 
-def mepp(image_a, image_b)
+def psnr(image_a, image_b)
   coalesce_a = flatten_animation(image_a)
   coalesce_b = flatten_animation(image_b)
   output = ImageOptim::Cmd.capture((IMAGEMAGICK_PREFIX + %W[
     compare
-    -metric MEPP
+    -metric PSNR
     -alpha Background
     #{coalesce_a.to_s.shellescape}
     #{coalesce_b.to_s.shellescape}
@@ -59,21 +59,21 @@ def mepp(image_a, image_b)
   end
 
   num_r = '\d+(?:\.\d+(?:[eE][-+]?\d+)?)?'
-  output[/\((#{num_r}), #{num_r}\)/, 1].to_f
+  num = output[/\A(#{num_r})/, 1].to_f
+  num == 0 ? Float::INFINITY : num
 end
 
 RSpec::Matchers.define :be_smaller_than do |expected|
   match{ |actual| actual.size < expected.size }
 end
 
-RSpec::Matchers.define :be_similar_to do |expected, max_difference|
+RSpec::Matchers.define :be_similar_to do |expected, psnr_min|
   match do |actual|
-    @diff = mepp(actual, expected)
-    @diff <= max_difference
+    @diff = psnr(actual, expected)
+    @diff >= psnr_min
   end
   failure_message do |actual|
-    "expected #{actual} to have at most #{max_difference} difference from " \
-      "#{expected}, got mean error per pixel of #{@diff}"
+    "expected peaks signal to noise ratio between #{actual} and #{expected} to be #{psnr_min}, got #{@diff}"
   end
 end