module ChunkyPNG::Chunk
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 chunk 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.
Constants
- CHUNK_TYPES
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.
Public Class Methods
Reads a chunk from an IO stream.
@param io [IO, read] The IO stream to read from. @return [ChunkyPNG::Chung::Base] The loaded chunk instance.
# File lib/chunky_png/chunk.rb 20 def self.read(io) 21 length, type = read_bytes(io, 8).unpack("Na4") 22 23 content = read_bytes(io, length) 24 crc = read_bytes(io, 4).unpack("N").first 25 verify_crc!(type, content, crc) 26 27 CHUNK_TYPES.fetch(type, Generic).read(type, content) 28 end
Reads an exact number of bytes from an IO stream. @param io [IO, read] The IO stream to read from. @param length [Integer] The IO exact number of bytes to read. @return [String] A binary string of exactly length bytes. @raise [ChunkyPNG::ExpectationFailed] If not exactly length
bytes could be read from the IO stream.
# File lib/chunky_png/chunk.rb 36 def self.read_bytes(io, length) 37 data = io.read(length) 38 raise ExpectationFailed, "Couldn't read #{length} bytes from IO stream." if data.nil? || data.bytesize != length 39 data 40 end
Verifies the CRC of a chunk. @param type [String] The chunk’s type. @param content [String] The chunk’s content. @param found_crc [Integer] The chunk’s found CRC value. @raise [ChunkyPNG::CRCMismatch] An exception is raised if
the found CRC value is not equal to the expected CRC value.
# File lib/chunky_png/chunk.rb 48 def self.verify_crc!(type, content, found_crc) 49 expected_crc = Zlib.crc32(content, Zlib.crc32(type)) 50 raise ChunkyPNG::CRCMismatch, "Chuck CRC mismatch!" if found_crc != expected_crc 51 end