FEATURE: introduce lossy color optimization on resized pngs

This feature ensures optimized images run via pngquant, this results extreme amounts of savings for resized images. Effectively the only impact is that the color palette on small resized images is reduced to 256.

To ensure safety we only apply this optimisation to images smaller than 500k.

This commit also makes a bunch of image specs less fragile.

app/models/optimized_image.rb

@@ -318,9 +318,13 @@ class OptimizedImage < ActiveRecord::Base
     convert_with(instructions, to, opts)
   end
 
+  MAX_PNGQUANT_SIZE = 500_000
+
   def self.convert_with(instructions, to, opts = {})
     Discourse::Utils.execute_command(*instructions)
-    FileHelper.optimize_image!(to)
+
+    allow_pngquant = to.downcase.ends_with?(".png") && File.size(to) < MAX_PNGQUANT_SIZE
+    FileHelper.optimize_image!(to, allow_pngquant: allow_pngquant)
     true
   rescue => e
     if opts[:raise_on_error]

lib/file_helper.rb

@@ -82,7 +82,12 @@ class FileHelper
     tmp
   end
 
-  def self.optimize_image!(filename)
+  def self.optimize_image!(filename, allow_pngquant: false)
+    pngquant_options = false
+    if allow_pngquant
+      pngquant_options = { allow_lossy: true }
+    end
+
     ImageOptim.new(
       # GLOBAL
       timeout: 15,
@@ -92,7 +97,7 @@ class FileHelper
       advpng: false,
       pngcrush: false,
       pngout: false,
-      pngquant: false,
+      pngquant: pngquant_options,
       # JPG
       jpegoptim: { strip: SiteSetting.strip_image_metadata ? "all" : "none" },
       jpegtran: false,

spec/lib/upload_creator_spec.rb

@@ -98,6 +98,27 @@ RSpec.describe UploadCreator do
       end
     end
 
+    describe 'pngquant' do
+      let(:filename) { "pngquant.png" }
+      let(:file) { file_from_fixtures(filename) }
+
+      it 'should apply pngquant to optimized images' do
+        upload = UploadCreator.new(file, filename,
+          pasted: true,
+          force_optimize: true
+        ).create_for(user.id)
+
+        # no optimisation possible without losing details
+        expect(upload.filesize).to eq(9558)
+
+        thumbnail_size = upload.get_optimized_image(upload.width, upload.height, {}).filesize
+
+        # pngquant will lose some colors causing some extra size reduction
+        expect(thumbnail_size).to be < 7500
+      end
+
+    end
+
     describe 'converting to jpeg' do
       let(:filename) { "should_be_jpeg.png" }
       let(:file) { file_from_fixtures(filename) }

spec/models/optimized_image_spec.rb

@@ -6,7 +6,7 @@ describe OptimizedImage do
 
   unless ENV["TRAVIS"]
     describe '.crop' do
-      it 'should work correctly (requires correct version of image optim)' do
+      it 'should produce cropped images' do
         tmp_path = "/tmp/cropped.png"
 
         begin
@@ -17,12 +17,15 @@ describe OptimizedImage do
             5
           )
 
-          fixture_path = "#{Rails.root}/spec/fixtures/images/cropped.png"
-          fixture_hex = Digest::MD5.hexdigest(File.read(fixture_path))
+          # we don't want to deal with something new here every time image magick
+          # is upgraded or pngquant is upgraded, lets just test the basics ...
+          # cropped image should be less than 120 bytes
 
-          cropped_hex = Digest::MD5.hexdigest(File.read(tmp_path))
+          cropped_size = File.size(tmp_path)
+
+          expect(cropped_size).to be < 120
+          expect(cropped_size).to be > 50
 
-          expect(cropped_hex).to eq(fixture_hex)
         ensure
           File.delete(tmp_path) if File.exists?(tmp_path)
         end
@@ -128,7 +131,7 @@ describe OptimizedImage do
     end
 
     describe '.downsize' do
-      it 'should work correctly (requires correct version of image optim)' do
+      it 'should downsize logo' do
         tmp_path = "/tmp/downsized.png"
 
         begin
@@ -138,12 +141,10 @@ describe OptimizedImage do
             "100x100\>"
           )
 
-          fixture_path = "#{Rails.root}/spec/fixtures/images/downsized.png"
-          fixture_hex = Digest::MD5.hexdigest(File.read(fixture_path))
+          info = FastImage.new(tmp_path)
+          expect(info.size).to eq([100, 27])
+          expect(File.size(tmp_path)).to be < 2300
 
-          downsized_hex = Digest::MD5.hexdigest(File.read(tmp_path))
-
-          expect(downsized_hex).to eq(fixture_hex)
         ensure
           File.delete(tmp_path) if File.exists?(tmp_path)
         end