added the possibility to use imagemagick's -alpha option

Author: rubikan

lib/grim.rb

@@ -16,6 +16,9 @@ module Grim
 
   # Default colorspace
   COLORSPACE = "RGB"
+  
+  #Default Alpha
+  ALPHA = "Set"
 
   # Default exception class for Grim.
   class Exception < ::StandardError

lib/grim/image_magick_processor.rb

@@ -25,7 +25,8 @@ def save(pdf, index, path, options)
       density = options.fetch(:density, Grim::DENSITY)
       quality = options.fetch(:quality, Grim::QUALITY)
       colorspace = options.fetch(:colorspace, Grim::COLORSPACE)
-      command = [@imagemagick_path, "-resize", width.to_s, "-antialias", "-render",
+      alpha   = options.fetch(:alpha, Grim::ALPHA)
+      command = [@imagemagick_path, "-resize", width.to_s, "-alpha", alpha, "-antialias", "-render",
         "-quality", quality.to_s, "-colorspace", colorspace,
         "-interlace", "none", "-density", density.to_s,
         "#{Shellwords.shellescape(pdf.path)}[#{index}]", path]

spec/fixtures/remove_alpha.pdf

@@ -112,4 +112,20 @@
       expect(file1_size).to_not eq(file2_size)
     end
   end
+  
+  describe "#save with alpha option" do
+    before(:each) do
+      @path1 = tmp_path("to_png_spec-1.png")
+      @path2 = tmp_path("to_png_spec-2.png")
+      @pdf  = Grim::Pdf.new(fixture_path("remove_alpha.pdf"))
+    end
+    
+    it "should use alpha" do
+      Grim::ImageMagickProcessor.new.save(@pdf, 0, @path1, {:alpha => 'Set'})
+      Grim::ImageMagickProcessor.new.save(@pdf, 0, @path2, {:alpha => 'Remove'})
+
+      expect(`convert #{@path1} -verbose info:`.include?("alpha: 8-bit")).to be(true)
+      expect(`convert #{@path2} -verbose info:`.include?("alpha: 1-bit")).to be(true)
+    end
+  end
 end

spec/lib/grim/image_magick_processor_spec.rb

@@ -25,6 +25,10 @@
   it "should have COLORSPACE constant set to 'RGB'" do
     expect(Grim::COLORSPACE).to eq('RGB')
   end
+  
+  it "should have ALPHA constant set to 'Set'" do
+    expect(Grim::ALPHA).to eq('Set')
+  end
 
   describe "#reap" do
     it "should return an instance of Grim::Pdf" do