chunk.rb in chunky_png-0.5.4

- old
+ new

@@ -7,10 +7,12 @@
   # All chunck types are instances of the {ChunkyPNG::Chunk::Base} class. For
   # some chunk types a specialized class is available, e.g. the IHDR chunk is
   # represented by the {ChunkyPNG::Chunk::Header} class. These specialized
   # classes help accessing the content of the chunk. All other chunks are
   # represented by the {ChunkyPNG::Chunk::Generic} class.
+  #
+  # @see ChunkyPNG::Datastream
   module Chunk
 
     # Reads a chunk from an IO stream.
     #
     # @param [IO, #read] io The IO stream to read from.
@@ -35,41 +37,83 @@
     def self.verify_crc!(type, content, found_crc)
       expected_crc = Zlib.crc32(content, Zlib.crc32(type))
       raise "Chuck CRC mismatch!" if found_crc != expected_crc
     end
 
+    # The base chunk class is the superclass for every chunk type. It contains
+    # methods to write the chunk to an output stream.
+    #
+    # A subclass should implement the +content+ method, which gets called when
+    # the chunk gets written to a PNG datastream
+    #
+    # @abstract
     class Base
+      
+      # The four-character type indicator for the chunk. This field is used to
+      # find the correct class for a chunk when it is loaded from a PNG stream.
+      # @return [String]
       attr_accessor :type
 
+      # Initializes the chunk instance.
+      # @param [String] type The four character chunk type indicator.
+      # @param [Hash] attributes A hash of attributes to set on this chunk.
       def initialize(type, attributes = {})
         self.type = type
         attributes.each { |k, v| send("#{k}=", v) }
       end
 
+      # Writes the chunk to the IO stream, using the provided content.
+      # The checksum will be calculated and appended to the stream.
+      # @param [IO] io The IO stream to write to.
+      # @param [String] content The content for this chunk.
       def write_with_crc(io, content)
         io << [content.length].pack('N') << type << content
         io << [Zlib.crc32(content, Zlib.crc32(type))].pack('N')
       end
 
+      # Writes the chunk to the IO stream.
+      #
+      # It will call te +content+ method to get the content for this chunk,
+      # and will calculate and append the checksum automatically.
+      # @param [IO] io The IO stream to write to.
       def write(io)
         write_with_crc(io, content || '')
       end
     end
 
+    # The Generic chunk type will read the content from the chunk as it, 
+    # and will write it back as it was read.
     class Generic < Base
 
+      # The attribute to store the content from the chunk, which gets 
+      # written by the +write+ method.
       attr_accessor :content
 
+      
       def initialize(type, content = '')
         super(type, :content => content)
       end
 
+      # Creates an instance, given the chunk's type and content.
+      # @param [String] type The four character chunk type indicator.
+      # @param [String] content The content read from the chunk.
+      # @return [ChunkyPNG::Chunk::Generic] The new chunk instance.
       def self.read(type, content)
         self.new(type, content)
       end
     end
 
+    # The header (IHDR) chunk is the first chunk of every PNG image, and 
+    # contains information about the image: i.e. its width, height, color 
+    # depth, color mode, compression method, filtering method and interlace
+    # method.
+    #
+    # ChunkyPNG supports all values for these variables that are defined in
+    # the PNG spec, except for color depth: Only 8-bit depth images are
+    # supported. Note that it is still possible to access the chunk for such
+    # an image, but ChunkyPNG will raise an exception if you try to access
+    # the pixel data.
     class Header < Base
 
       attr_accessor :width, :height, :depth, :color, :compression, :filtering, :interlace
 
       def initialize(attrs = {})
@@ -79,39 +123,67 @@
         @compression ||= ChunkyPNG::COMPRESSION_DEFAULT
         @filtering   ||= ChunkyPNG::FILTERING_DEFAULT
         @interlace   ||= ChunkyPNG::INTERLACING_NONE
       end
 
+      # Reads the 13 bytes of content from the header chunk to set the image attributes.
+      # @param [String] type The four character chunk type indicator (= "IHDR").
+      # @param [String] content The 13 bytes of content read from the chunk.
+      # @return [ChunkyPNG::Chunk::End] The new Header chunk instance with the 
+      #    variables set to the values according to the content.
       def self.read(type, content)
         fields = content.unpack('NNC5')
         self.new(:width => fields[0],  :height => fields[1], :depth => fields[2], :color => fields[3],
                        :compression => fields[4], :filtering => fields[5], :interlace => fields[6])
       end
 
+      # Returns the content for this chunk when it gets written to a file, by packing the
+      # image information variables into the correct format.
+      # @return [String] The 13-byte content for the header chunk.
       def content
         [width, height, depth, color, compression, filtering, interlace].pack('NNC5')
       end
     end
 
+    # The End (IEND) chunk indicates the last chunk of a PNG stream. It does not
+    # contain any data.
     class End < Base
+      
       def initialize
         super('IEND')
       end
-
+      
+      # Reads the END chunk. It will check if the content is empty.
+      # @param [String] type The four character chunk type indicator (= "IEND").
+      # @param [String] content The content read from the chunk. Should be empty.
+      # @return [ChunkyPNG::Chunk::End] The new End chunk instance.
+      # @raise [RuntimeError] Raises an exception if the content was not empty.
       def self.read(type, content)
         raise 'The IEND chunk should be empty!' if content != ''
         self.new
       end
 
+      # Returns an empty string, because this chunk should always be empty.
+      # @return [""] An empty string.
       def content
         ''
       end
     end
 
+    # The Palette (PLTE) chunk contains the image's palette, i.e. the
+    # 8-bit RGB colors this image is using.
+    #
+    # @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
+    #
+    # @see ChunkyPNG::Chunk::Palette
+    # @see ChunkyPNG::Palette
     class Transparency < Generic
     end
 
     class ImageData < Generic
       
@@ -124,10 +196,18 @@
         # TODO: Split long streamdata over multiple chunks
         [ ChunkyPNG::Chunk::ImageData.new('IDAT', streamdata) ]
       end
     end
 
+    # The Text (tEXt) chunk contains keyword/value metadata about the PNG stream.
+    # In this chunk, the value is stored uncompressed.
+    #
+    # The tEXt chunk only supports Latin-1 encoded textual data. If you need UTF-8 
+    # support, check out the InternationalText chunk type.
+    #
+    # @see ChunkyPNG::Chunk::CompressedText
+    # @see ChunkyPNG::Chunk::InternationalText
     class Text < Base
 
       attr_accessor :keyword, :value
 
       def initialize(keyword, value)
@@ -138,40 +218,70 @@
       def self.read(type, content)
         keyword, value = content.unpack('Z*a*')
         new(keyword, value)
       end
 
+      # Creates the content to write to the stream, by concatenating the keyword
+      # with the value, joined by a null character.
+      #
+      # @return The content that should be written to the datastream.
       def content
         [keyword, value].pack('Z*a*')
       end
     end
 
+    # The CompressedText (zTXt) chunk contains keyword/value metadata about 
+    # the PNG stream. In this chunk, the value is compressed using Deflate
+    # compression.
+    #
+    # @see ChunkyPNG::Chunk::CompressedText
+    # @see ChunkyPNG::Chunk::InternationalText
     class CompressedText < Base
 
       attr_accessor :keyword, :value
 
       def initialize(keyword, value)
-        super('tEXt')
+        super('zTXt')
         @keyword, @value = keyword, value
       end
 
       def self.read(type, content)
         keyword, compression, value = content.unpack('Z*Ca*')
         raise "Compression method #{compression.inspect} not supported!" unless compression == ChunkyPNG::COMPRESSION_DEFAULT
         new(keyword, Zlib::Inflate.inflate(value))
       end
 
+      # Creates the content to write to the stream, by concatenating the keyword
+      # with the deflated value, joined by a null character.
+      #
+      # @return The content that should be written to the datastream.
       def content
         [keyword, ChunkyPNG::COMPRESSION_DEFAULT, Zlib::Deflate.deflate(value)].pack('Z*Ca*')
       end
     end
 
+    # The Text (iTXt) chunk contains keyword/value metadata about the PNG stream.
+    # The metadata in this chunk can be encoded using UTF-8 characters. Moreover,
+    # it is possible to define the language of the metadata, and give a translation
+    # of the keyword name. Finally, it supports bot compressed and uncompressed
+    # values.
+    # 
+    # @todo This chunk is currently not implemented, but merely read and written 
+    #    back intact.
+    #
+    # @see ChunkyPNG::Chunk::Text
+    # @see ChunkyPNG::Chunk::CompressedText
     class InternationalText < Generic
+      
       # TODO
     end
 
-    # Maps chunk types to classes.
-    # If a chunk type is not given in this hash, a generic chunk type will be used.
+    # Maps chunk types to classes, based on the four byte chunk type indicator at the
+    # beginning of a chunk.
+    #
+    # If a chunk type is not specified in this hash, the Generic chunk type will be used.
+    #
+    # @see ChunkyPNG::Chunk.read
     CHUNK_TYPES = {
       'IHDR' => Header, 'IEND' => End, 'IDAT' => ImageData, 'PLTE' => Palette, 'tRNS' => Transparency,
       'tEXt' => Text, 'zTXt' => CompressedText, 'iTXt' => InternationalText
     }
   end