conversions from L*a*b to RGB

lab_to_rgb.rb

# port of David Dalrymple's GNU C code
# the original C code can be found at http://davidad.net/colorviz/

def lab_to_xyz(lab)

  l   = lab[0]
  a   = lab[1]
  b   = lab[2]

  ill = [0.9643, 1.0, 0.8251]
  sl  = (l + 0.16) / 1.16
  x   = ill[0] * finv(sl + (a/5.0))
  y   = ill[1] * finv(sl)
  z   = ill[2] * finv(sl = (b/2.0))

  return x, y, z

end


def finv(t) 

  ratio = (6.0/29.0)
  t > ratio ? (t*t*t) : (3.0*ratio*ratio*(t - (4.0/29.0)))

end


def xyz_to_rgb(xyz)

  x  = lab[0]
  y  = lab[1]
  z  = lab[2]

  rl = (3.2406*x) - (1.5372*y) - (0.4986*z)
  gl = (-0.9689*x) + (1.8758*y) + (0.0415*z)
  bl = (0.0557*x ) - (0.2040*y) + (1.0570*z)

  clip = (rl<0.0 || rl>1.0 || gl<0.0 || gl>1.0 || bl<0.0 || bl>1.0)

  if clip
    rl = (rl<0.0) ? 0.0 : ((rl>1.0) ? 1.0 : rl )
    gl = (gl<0.0) ? 0.0 : ((gl>1.0) ? 1.0 : gl )
    bl = (bl<0.0) ? 0.0 : ((bl>1.0) ? 1.0 : bl )
  end

  r = 255.0 * correct(rl)
  g = 255.0 * correct(gl)
  b = 255.0 * correct(bl)

  return r, g, b

end


def correct(c)

  a = 0.055

  return (c<=0.0031308) ? (12.92*c) : ((1 + a) * (c**(1/2.4)) - a)

end


def lab_to_rgb(lab)

  lab_to_xyz(lab)

  xyz_to_rgb(xyz)

end


def cl_to_pix(rgb, quant, qual)

  tau = 2*Math::PI

  ptr = rgb

  l    = qual*0.61 + 0.09
  angle = (tau/6.0) - (quant*tau)

  r = qual * 0.311 + 0.125
  a = Math.sin(angle) * r
  b = Math.cos(angle) * r

  lab_to_rgb([l,a,b])

end