Class: Color::CMYK

Inherits:
Object
  • Object
show all
Defined in:
lib/color/cmyk.rb,
lib/color.rb

Overview

An CMYK colour object. CMYK (cyan, magenta, yellow, and black) colours are based on additive percentages of ink. A CMYK colour of (0.3, 0, 0.8, 0.3) would be mixed from 30% cyan, 0% magenta, 80% yellow, and 30% black.

Constant Summary collapse

PDF_FORMAT_STR =

The format of a DeviceCMYK colour for PDF. In color-tools 2.0 this will be removed from this package and added back as a modification by the PDF::Writer package.

"%.3f %.3f %.3f %.3f %s"

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(c = 0, m = 0, y = 0, k = 0) ⇒ CMYK

Creates a CMYK colour object from percentages. Internally, the colour is managed as fractional values 0..1.

Color::CMYK.from_fraction(30, 0, 80, 30)



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# File 'lib/color/cmyk.rb', line 54

def initialize(c = 0, m = 0, y = 0, k = 0)
  @c = c / 100.0
  @m = m / 100.0
  @y = y / 100.0
  @k = k / 100.0
end

Instance Attribute Details

#c ⇒ Object

Returns the value of attribute c.



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# File 'lib/color/cmyk.rb', line 160

def c
  @c
end

#k ⇒ Object

Returns the value of attribute k.



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# File 'lib/color/cmyk.rb', line 160

def k
  @k
end

#m ⇒ Object

Returns the value of attribute m.



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# File 'lib/color/cmyk.rb', line 160

def m
  @m
end

#y ⇒ Object

Returns the value of attribute y.



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# File 'lib/color/cmyk.rb', line 160

def y
  @y
end

Class Method Details

.from_fraction(c = 0, m = 0, y = 0, k = 0) ⇒ Object

Creates a CMYK colour object from fractional values 0..1.

Color::CMYK.from_fraction(0.3, 0, 0.8, 0.3)



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# File 'lib/color/cmyk.rb', line 41

def self.from_fraction(c = 0, m = 0, y = 0, k = 0)
  colour = Color::CMYK.new
  colour.c = c
  colour.m = m
  colour.y = y
  colour.k = k
  colour
end

Instance Method Details

#==(other) ⇒ Object

Compares the other colour to this one. The other colour will be converted to CMYK before comparison, so the comparison between a CMYK colour and a non-CMYK colour will be approximate and based on the other colour’s #to_cmyk conversion. If there is no #to_cmyk conversion, this will raise an exception. This will report that two CMYK colours are equivalent if all component values are within 1e-4 (0.0001) of each other.



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# File 'lib/color/cmyk.rb', line 29

def ==(other)
  other = other.to_cmyk
  other.kind_of?(Color::CMYK) and
  ((@c - other.c).abs <= 1e-4) and
  ((@m - other.m).abs <= 1e-4) and
  ((@y - other.y).abs <= 1e-4) and
  ((@k - other.k).abs <= 1e-4)
end

#html ⇒ Object

Present the colour as an RGB HTML/CSS colour string. Note that this will perform a #to_rgb operation using the default conversion formula.



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# File 'lib/color/cmyk.rb', line 75

def html
  to_rgb.html
end

#pdf_fill ⇒ Object

Present the colour as a DeviceCMYK fill colour string for PDF. This will be removed from the default package in color-tools 2.0.



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# File 'lib/color/cmyk.rb', line 63

def pdf_fill
  PDF_FORMAT_STR % [ @c, @m, @y, @k, "k" ]
end

#pdf_stroke ⇒ Object

Present the colour as a DeviceCMYK stroke colour string for PDF. This will be removed from the default package in color-tools 2.0.



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# File 'lib/color/cmyk.rb', line 69

def pdf_stroke
  PDF_FORMAT_STR % [ @c, @m, @y, @k, "K" ]
end

#to_cmyk ⇒ Object 



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# File 'lib/color/cmyk.rb', line 146

def to_cmyk
  self
end

#to_grayscale ⇒ Object Also known as: to_greyscale

Converts the CMYK colour to a single greyscale value. There are undoubtedly multiple methods for this conversion, but only a minor variant of the Adobe conversion method will be used:

g = 1.0 - min(1.0, 0.299 * c + 0.587 * m + 0.114 * y + k)

This treats the CMY values similarly to YIQ (NTSC) values and then adds the level of black. This is a variant of the Adobe version because it uses the more precise YIQ (NTSC) conversion values for Y (intensity) rather than the approximates provided by Adobe (0.3, 0.59, and 0.11).



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# File 'lib/color/cmyk.rb', line 137

def to_grayscale
  c = 0.299 * @c.to_f
  m = 0.587 * @m.to_f
  y = 0.114 * @y.to_f
  g = 1.0 - [1.0, c + m + y + @k].min
  Color::GrayScale.from_fraction(g)
end

#to_hsl ⇒ Object

Converts to RGB then HSL.



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# File 'lib/color/cmyk.rb', line 156

def to_hsl
  to_rgb.to_hsl
end

#to_rgb(use_adobe_method = false) ⇒ Object

Converts the CMYK colour to RGB. Most colour experts strongly suggest that this is not a good idea (some even suggesting that it’s a very bad idea). CMYK represents additive percentages of inks on white paper, whereas RGB represents mixed colour intensities on a black screen.

However, the colour conversion can be done, and there are two different methods for the conversion that provide slightly different results. Adobe PDF conversions are done with the first form.

  # Adobe PDF Display Formula
r = 1.0 - min(1.0, c + k)
g = 1.0 - min(1.0, m + k)
b = 1.0 - min(1.0, y + k)

  # Other
r = 1.0 - (c * (1.0 - k) + k)
g = 1.0 - (m * (1.0 - k) + k)
b = 1.0 - (y * (1.0 - k) + k)

If we have a CMYK colour of [33% 66% 83% 25%], the first method will give an approximate RGB colour of (107, 23, 0) or #6b1700. The second method will give an approximate RGB colour of (128, 65, 33) or #804121. Which is correct? Although the colours may seem to be drastically different in the RGB colour space, they are very similar colours, differing mostly in intensity. The first is a darker, slightly redder brown; the second is a lighter brown.

Because of this subtlety, both methods are now offered for conversion in color-tools 1.2 or later. The Adobe method is not used by default; to enable it, pass true to #to_rgb.

Future versions of color-tools may offer other conversion mechanisms that offer greater colour fidelity.



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# File 'lib/color/cmyk.rb', line 113

def to_rgb(use_adobe_method = false)
  if use_adobe_method
    r = 1.0 - [1.0, @c + @k].min
    g = 1.0 - [1.0, @m + @k].min
    b = 1.0 - [1.0, @y + @k].min
  else
    r = 1.0 - (@c.to_f * (1.0 - @k.to_f) + @k.to_f)
    g = 1.0 - (@m.to_f * (1.0 - @k.to_f) + @k.to_f)
    b = 1.0 - (@y.to_f * (1.0 - @k.to_f) + @k.to_f)
  end
  Color::RGB.from_fraction(r, g, b)
end

#to_yiq ⇒ Object

Converts to RGB then YIQ.



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# File 'lib/color/cmyk.rb', line 151

def to_yiq
  to_rgb.to_yiq
end