class ChunkyPNG::Datastream

The Datastream class represents a PNG formatted datastream. It supports both reading from and writing to strings, streams and files.

A PNG datastream begins with the PNG signature, and then contains multiple chunks, starting with a header (IHDR) chunk and finishing with an end (IEND) chunk.

@see ChunkyPNG::Chunk

Constants

SIGNATURE

The signature that each PNG file or stream should begin with.

Attributes

data_chunks[RW]

The chunks that together compose the images pixel data. @return [Array<ChunkyPNG::Chunk::ImageData>]

end_chunk[RW]

The empty chunk that signals the end of this datastream @return [ChunkyPNG::Chunk::Header]

header_chunk[RW]

The header chunk of this datastream. @return [ChunkyPNG::Chunk::Header]

other_chunks[RW]

All other chunks in this PNG file. @return [Array<ChunkyPNG::Chunk::Generic>]

palette_chunk[RW]

The chunk containing the image’s palette. @return [ChunkyPNG::Chunk::Palette]

physical_chunk[RW]

The chunk containing the physical dimensions of the PNG’s pixels. @return [ChunkyPNG::Chunk::Physical]

transparency_chunk[RW]

The chunk containing the transparency information of the palette. @return [ChunkyPNG::Chunk::Transparency]

Public Class Methods

from_blob(str) click to toggle source

Reads a PNG datastream from a string. @param [String] str The PNG encoded string to load from. @return [ChunkyPNG::Datastream] The loaded datastream instance.

   # File lib/chunky_png/datastream.rb
58 def from_blob(str)
59   from_io(StringIO.new(str, "rb"))
60 end
Also aliased as: from_string
from_file(filename) click to toggle source

Reads a PNG datastream from a file. @param [String] filename The path of the file to load from. @return [ChunkyPNG::Datastream] The loaded datastream instance.

   # File lib/chunky_png/datastream.rb
67 def from_file(filename)
68   ds = nil
69   File.open(filename, "rb") { |f| ds = from_io(f) }
70   ds
71 end
from_io(io) click to toggle source

Reads a PNG datastream from an input stream @param [IO] io The stream to read from. @return [ChunkyPNG::Datastream] The loaded datastream instance.

   # File lib/chunky_png/datastream.rb
76 def from_io(io)
77   io.set_encoding(::Encoding::BINARY)
78   verify_signature!(io)
79 
80   ds = new
81   while ds.end_chunk.nil?
82     chunk = ChunkyPNG::Chunk.read(io)
83     case chunk
84       when ChunkyPNG::Chunk::Header       then ds.header_chunk = chunk
85       when ChunkyPNG::Chunk::Palette      then ds.palette_chunk = chunk
86       when ChunkyPNG::Chunk::Transparency then ds.transparency_chunk = chunk
87       when ChunkyPNG::Chunk::ImageData    then ds.data_chunks << chunk
88       when ChunkyPNG::Chunk::Physical     then ds.physical_chunk = chunk
89       when ChunkyPNG::Chunk::End          then ds.end_chunk = chunk
90       else ds.other_chunks << chunk
91     end
92   end
93   ds
94 end
from_string(str)
Alias for: from_blob
new() click to toggle source

Initializes a new Datastream instance.

   # File lib/chunky_png/datastream.rb
45 def initialize
46   @other_chunks = []
47   @data_chunks  = []
48 end
verify_signature!(io) click to toggle source

Verifies that the current stream is a PNG datastream by checking its signature.

This method reads the PNG signature from the stream, setting the current position of the stream directly after the signature, where the IHDR chunk should begin.

@param [IO] io The stream to read the PNG signature from. @raise [RuntimeError] An exception is raised if the PNG signature is not found at

the beginning of the stream.
    # File lib/chunky_png/datastream.rb
104 def verify_signature!(io)
105   signature = io.read(ChunkyPNG::Datastream::SIGNATURE.length)
106   unless signature == ChunkyPNG::Datastream::SIGNATURE
107     raise ChunkyPNG::SignatureMismatch, "PNG signature not found, found #{signature.inspect} instead of #{ChunkyPNG::Datastream::SIGNATURE.inspect}!"
108   end
109 end

Public Instance Methods

chunks() click to toggle source

Returns an enumerator instance for this datastream’s chunks. @return [Enumerable::Enumerator] An enumerator for the :each_chunk method. @see ChunkyPNG::Datastream#each_chunk

    # File lib/chunky_png/datastream.rb
137 def chunks
138   enum_for(:each_chunk)
139 end
each_chunk() { |header_chunk| ... } click to toggle source

Enumerates the chunks in this datastream.

This will iterate over the chunks using the order in which the chunks should appear in the PNG file.

@yield [chunk] Yields the chunks in this datastream, one by one in the correct order. @yieldparam [ChunkyPNG::Chunk::Base] chunk A chunk in this datastream. @see ChunkyPNG::Datastream#chunks

    # File lib/chunky_png/datastream.rb
124 def each_chunk
125   yield(header_chunk)
126   other_chunks.each { |chunk| yield(chunk) }
127   yield(palette_chunk)      if palette_chunk
128   yield(transparency_chunk) if transparency_chunk
129   yield(physical_chunk)     if physical_chunk
130   data_chunks.each { |chunk| yield(chunk) }
131   yield(end_chunk)
132 end
imagedata() click to toggle source

Returns the uncompressed image data, combined from all the IDAT chunks @return [String] The uncompressed image data for this datastream

    # File lib/chunky_png/datastream.rb
153 def imagedata
154   ChunkyPNG::Chunk::ImageData.combine_chunks(data_chunks)
155 end
metadata() click to toggle source

Returns all the textual metadata key/value pairs as hash. @return [Hash] A hash containing metadata fields and their values.

    # File lib/chunky_png/datastream.rb
143 def metadata
144   metadata = {}
145   other_chunks.each do |chunk|
146     metadata[chunk.keyword] = chunk.value if chunk.respond_to?(:keyword) && chunk.respond_to?(:value)
147   end
148   metadata
149 end
save(filename) click to toggle source

Saves this datastream as a PNG file. @param [String] filename The filename to use.

    # File lib/chunky_png/datastream.rb
170 def save(filename)
171   File.open(filename, "wb") { |f| write(f) }
172 end
to_blob() click to toggle source

Encodes this datastream into a string. @return [String] The encoded PNG datastream.

    # File lib/chunky_png/datastream.rb
176 def to_blob
177   str = StringIO.new
178   str.set_encoding("ASCII-8BIT")
179   write(str)
180   str.string
181 end
Also aliased as: to_string, to_s
to_s()
Alias for: to_blob
to_string()
Alias for: to_blob
write(io) click to toggle source

Writes the datastream to the given output stream. @param [IO] io The output stream to write to.

    # File lib/chunky_png/datastream.rb
163 def write(io)
164   io << SIGNATURE
165   each_chunk { |c| c.write(io) }
166 end