lib/chunky_png/chunk.rb in chunky_png-1.1.2 vs lib/chunky_png/chunk.rb in chunky_png-1.2.0

@@ -175,15 +175,49 @@
     # @see ChunkyPNG::Chunk::Transparency
     # @see ChunkyPNG::Palette 
     class Palette < Generic
     end
 
-    # A transparency (tRNS) chunk contains the alpha channel for the colors 
-    # defined in the Palette (PLTE) chunk
+    # A transparency (tRNS) chunk defines the transparency for an image.
     #
+    # * For indexed images, it contains the alpha channel for the colors defined in the Palette (PLTE) chunk.
+    # * For grayscale images, it contains the grayscale teint that should be considered fully transparent.
+    # * For truecolor images, it contains the color that should be considered fully transparent.
+    #
+    # Images having a color mode that already includes an alpha channel, this chunk should not be included.
+    #
     # @see ChunkyPNG::Chunk::Palette
     # @see ChunkyPNG::Palette
     class Transparency < Generic
+      
+      # Returns the alpha channel for the palette of an indexed image.
+      #
+      # This method should only be used for images having color mode ChunkyPNG::COLOR_INDEXED (3).
+      #
+      # @return [Array<Integer>] Returns an array of alpha channel values [0-255].
+      def palette_alpha_channel
+        content.unpack('C*')
+      end
+      
+      # Returns the truecolor entry to be replaced by transparent pixels,
+      #
+      # This method should only be used for images having color mode ChunkyPNG::COLOR_TRUECOLOR (2).
+      #
+      # @return [Integer] The color to replace with fully transparent pixels.
+      def truecolor_entry(bit_depth)
+        values = content.unpack('nnn').map { |c| ChunkyPNG::Canvas.send(:"decode_png_resample_#{bit_depth}bit_value", c) }
+        ChunkyPNG::Color.rgb(*values)
+      end
+
+      # Returns the grayscale entry to be replaced by transparent pixels.
+      #
+      # This method should only be used for images having color mode ChunkyPNG::COLOR_GRAYSCALE (0).
+      #
+      # @return [Integer] The (grayscale) color to replace with fully transparent pixels.      
+      def grayscale_entry(bit_depth)
+        value = ChunkyPNG::Canvas.send(:"decode_png_resample_#{bit_depth}bit_value", content.unpack('n')[0])
+        ChunkyPNG::Color.grayscale(value)
+      end
     end
 
     class ImageData < Generic
       
       def self.read(type, content)