improving PNG transparency support

- add support for transparency in Greyscale and RGB images
- Just need to add transparency support for indexed images now

lib/prawn/images.rb

@@ -185,13 +185,35 @@ def build_png_object(data, png)
                                  :DeviceRGB,
                                  (png.palette.size / 3) -1,
                                  palette_obj]
+      end
+
+      # *************************************
+      # add transparency data if necessary
+      # *************************************
 
-        # add transparency data if necessary
-        #if png.transparency && png.transparency[:type] == 'indexed'
-        #  obj.data[:Mask] = png.transparency[:data]
-        #end
+      # For PNG color types 0, 2 and 3, the transparency data is stored in
+      # a dedicated PNG chunk, and is exposed via the transparency attribute
+      # of the PNG class.
+      if png.transparency[:grayscale]
+        # Use Color Key Masking (spec section 4.8.5)
+        # - An array with N elements, where N is two times the number of color
+        #   components.
+        obj.data[:Mask] = png.transparency[:grayscale].collect { |val| [val,val] }
+      elsif png.transparency[:rgb]
+        # Use Color Key Masking (spec section 4.8.5)
+        # - An array with N elements, where N is two times the number of color
+        #   components.
+        obj.data[:Mask] = png.transparency[:rgb].collect { |val| [val,val] }.flatten
+      elsif png.transparency[:indexed]
+        # TODO: broken. I was attempting to us Color Key Masking, but I think
+        #       we need to construct an SMask i think. Maybe do it inside
+        #       the PNG class, and store it in alpha_channel
+        #obj.data[:Mask] = png.transparency[:indexed]
       end
 
+      # For PNG color types 4 and 6, the transparency data is stored as a alpha
+      # channel mixed in with the main image data. The PNG class seperates
+      # it out for us and makes it available via the alpha_channel attribute
       if png.alpha_channel
         smask_obj = ref(:Type             => :XObject,
                         :Subtype          => :Image,

lib/prawn/images/png.rb

@@ -30,6 +30,7 @@ def initialize(data)
 
         @palette  = ""
         @img_data = ""
+        @transparency = {}
 
         loop do
           chunk_size  = data.read(4).unpack("N")[0]
@@ -61,16 +62,16 @@ def initialize(data)
               # the palette index in the PLTE ("palette") chunk up until the
               # last non-opaque entry. Set up an array, stretching over all
               # palette entries which will be 0 (opaque) or 1 (transparent).
-              @transparency[:type]  = 'indexed'
-              @transparency[:data]  = data.read(chunk_size).unpack("C*")
+              @transparency[:indexed]  = data.read(chunk_size).unpack("C*")
+              short = 255 - @transparency[:indexed].size
+              @transparency[:indexed] += ([255] * short) if short > 0
             when 0
               # Greyscale. Corresponding to entries in the PLTE chunk.
               # Grey is two bytes, range 0 .. (2 ^ bit-depth) - 1
-              @transparency[:grayscale] = data.read(2).unpack("n")
-              @transparency[:type]      = 'indexed'
+              @transparency[:grayscale] = data.read(chunk_size).unpack("n")
             when 2
               # True colour with proper alpha channel.
-              @transparency[:rgb] = data.read(6).unpack("nnn")
+              @transparency[:rgb] = data.read(chunk_size).unpack("nnn")
             end
           when 'IEND'
             # we've got everything we need, exit the loop
@@ -89,7 +90,7 @@ def initialize(data)
 
       def pixel_bytes
         case @color_type
-        when 0, 4    then 1
+        when 0, 3, 4 then 1
         when 1, 2, 6 then 3
         end
       end