Module: ChunkyPNG::Chunk

Defined in:
lib/chunky_png/chunk.rb

Overview

A PNG datastream consists of multiple chunks. This module, and the classes contained within, help with handling these chunks. It supports both reading and writing chunks.

All chunck types are instances of the Base class. For some chunk types a specialized class is available, e.g. the IHDR chunk is represented by the Header class. These specialized classes help accessing the content of the chunk. All other chunks are represented by the Generic class.

Defined Under Namespace

Classes: Base, CompressedText, End, Generic, Header, ImageData, InternationalText, Palette, Text, Transparency

Constant Summary collapse

CHUNK_TYPES =

Maps chunk types to classes. If a chunk type is not given in this hash, a generic chunk type will be used.

{
  'IHDR' => Header, 'IEND' => End, 'IDAT' => ImageData, 'PLTE' => Palette, 'tRNS' => Transparency,
  'tEXt' => Text, 'zTXt' => CompressedText, 'iTXt' => InternationalText
}

Class Method Summary collapse

Class Method Details

.read(io) ⇒ ChunkyPNG::Chung::Base

Reads a chunk from an IO stream.

Parameters:

  • io (IO, #read) —

    The IO stream to read from.

Returns:

  • (ChunkyPNG::Chung::Base) —

    The loaded chunk instance.



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# File 'lib/chunky_png/chunk.rb', line 18

def self.read(io)

  length, type = io.read(8).unpack('Na4')
  content      = io.read(length)
  crc          = io.read(4).unpack('N').first

  verify_crc!(type, content, crc)

  CHUNK_TYPES.fetch(type, Generic).read(type, content)
end

.verify_crc!(type, content, found_crc) ⇒ Object

Verifies the CRC of a chunk.

Parameters:

  • type (String) —

    The chunk’s type.

  • content (String) —

    The chunk’s content.

  • content (Fixnum) —

    The chunk’s content.

Raises:

  • (RuntimeError) —

    An exception is raised if the found CRC value is not equal to the expected CRC value.



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# File 'lib/chunky_png/chunk.rb', line 35

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