Class: ChunkyPNG::Chunk::Header

Inherits:
Base
  • Object
show all
Defined in:
lib/chunky_png/chunk.rb

Overview

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.

Instance Attribute Summary collapse

Attributes inherited from Base

#type

Class Method Summary collapse

Instance Method Summary collapse

Methods inherited from Base

#write, #write_with_crc

Constructor Details

#initialize(attrs = {}) ⇒ Header

Returns a new instance of Header.



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# File 'lib/chunky_png/chunk.rb', line 123

def initialize(attrs = {})
  super("IHDR", attrs)
  @depth       ||= 8
  @color       ||= ChunkyPNG::COLOR_TRUECOLOR
  @compression ||= ChunkyPNG::COMPRESSION_DEFAULT
  @filtering   ||= ChunkyPNG::FILTERING_DEFAULT
  @interlace   ||= ChunkyPNG::INTERLACING_NONE
end

Instance Attribute Details

#color ⇒ Object

Returns the value of attribute color.



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# File 'lib/chunky_png/chunk.rb', line 121

def color
  @color
end

#compression ⇒ Object

Returns the value of attribute compression.



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# File 'lib/chunky_png/chunk.rb', line 121

def compression
  @compression
end

#depth ⇒ Object

Returns the value of attribute depth.



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# File 'lib/chunky_png/chunk.rb', line 121

def depth
  @depth
end

#filtering ⇒ Object

Returns the value of attribute filtering.



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# File 'lib/chunky_png/chunk.rb', line 121

def filtering
  @filtering
end

#height ⇒ Object

Returns the value of attribute height.



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# File 'lib/chunky_png/chunk.rb', line 121

def height
  @height
end

#interlace ⇒ Object

Returns the value of attribute interlace.



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# File 'lib/chunky_png/chunk.rb', line 121

def interlace
  @interlace
end

#width ⇒ Object

Returns the value of attribute width.



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# File 'lib/chunky_png/chunk.rb', line 121

def width
  @width
end

Class Method Details

.read(type, content) ⇒ ChunkyPNG::Chunk::End

Reads the 13 bytes of content from the header chunk to set the image attributes.

Parameters:

  • type (String) —

    The four character chunk type indicator (= “IHDR”).

  • content (String) —

    The 13 bytes of content read from the chunk.

Returns:

  • (ChunkyPNG::Chunk::End) —

    The new Header chunk instance with the variables set to the values according to the content.



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# File 'lib/chunky_png/chunk.rb', line 138

def self.read(type, content)
  fields = content.unpack("NNC5")
  new(
    width: fields[0],
    height: fields[1],
    depth: fields[2],
    color: fields[3],
    compression: fields[4],
    filtering: fields[5],
    interlace: fields[6]
  )
end

Instance Method Details

#content ⇒ String

Returns the content for this chunk when it gets written to a file, by packing the image information variables into the correct format.

Returns:

  • (String) —

    The 13-byte content for the header chunk.



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# File 'lib/chunky_png/chunk.rb', line 154

def content
  [
    width,
    height,
    depth,
    color,
    compression,
    filtering,
    interlace,
  ].pack("NNC5")
end