When generating thumbnails for animated images, pass the -coalesce

option to convert.

lib/paperclip/thumbnail.rb

@@ -76,6 +76,7 @@ module Paperclip
     def transformation_command
       scale, crop = @current_geometry.transformation_to(@target_geometry, crop?)
       trans = []
+      trans << "-coalesce" if animated?
       trans << "-resize" << %["#{scale}"] unless scale.nil? || scale.empty?
       trans << "-crop" << %["#{crop}"] << "+repage" if crop
       trans

test/thumbnail_test.rb

@@ -289,6 +289,10 @@ class ThumbnailTest < Test::Unit::TestCase
         cmd = %Q[identify -format "%wx%h" "#{dst.path}"]
         assert_equal "50x50"*12, `#{cmd}`.chomp
       end
+
+      should "use the -coalesce option" do
+	assert_equal @thumb.transformation_command.first, "-coalesce"
+      end
     end
 
     context "with omitted output format" do
@@ -301,6 +305,10 @@ class ThumbnailTest < Test::Unit::TestCase
         cmd = %Q[identify -format "%wx%h" "#{dst.path}"]
         assert_equal "50x50"*12, `#{cmd}`.chomp
       end
+
+      should "use the -coalesce option" do
+	assert_equal @thumb.transformation_command.first, "-coalesce"
+      end
     end
   end
 end