Method: Color::CMYK#to_rgb

Defined in:

lib/color/cmyk.rb

#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. The Adobe method is not used by default; to enable it, pass true to #to_rgb.

Future versions of Color may offer other conversion mechanisms that offer greater colour fidelity, including recognition of ICC colour profiles.

# File 'lib/color/cmyk.rb', line 125

def to_rgb(use_adobe_method = false)
  if use_adobe_method
    Color::RGB.from_fraction(*adobe_cmyk_rgb)
  else
    Color::RGB.from_fraction(*standard_cmyk_rgb)
  end
end