colorsys.py

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"""Conversion functions between RGB and other color systems.

This modules provides two functions for each color system ABC:

  rgb_to_abc(r, g, b) --> a, b, c
  abc_to_rgb(a, b, c) --> r, g, b

All inputs and outputs are triples of floats in the range [0.0...1.0].
Inputs outside this range may cause exceptions or invalid outputs.

Supported color systems:
RGB: Red, Green, Blue components
YIQ: used by composite video signals
HLS: Hue, Luminance, Saturation
HSV: Hue, Saturation, Value
"""
# References:
# XXX Where's the literature?

__all__ = ["rgb_to_yiq","yiq_to_rgb","rgb_to_hls","hls_to_rgb",
           "rgb_to_hsv","hsv_to_rgb"]

# Some floating point constants

ONE_THIRD = 1.0/3.0
ONE_SIXTH = 1.0/6.0
TWO_THIRD = 2.0/3.0


# YIQ: used by composite video signals (linear combinations of RGB)
# Y: perceived grey level (0.0 == black, 1.0 == white)
# I, Q: color components

def rgb_to_yiq(r, g, b):
    y = 0.30*r + 0.59*g + 0.11*b
    i = 0.60*r - 0.28*g - 0.32*b
    q = 0.21*r - 0.52*g + 0.31*b
    return (y, i, q)

def yiq_to_rgb(y, i, q):
    r = y + 0.948262*i + 0.624013*q
    g = y - 0.276066*i - 0.639810*q
    b = y - 1.105450*i + 1.729860*q
    if r < 0.0: r = 0.0
    if g < 0.0: g = 0.0
    if b < 0.0: b = 0.0
    if r > 1.0: r = 1.0
    if g > 1.0: g = 1.0
    if b > 1.0: b = 1.0
    return (r, g, b)


# HLS: Hue, Luminance, S???
# H: position in the spectrum
# L: ???
# S: ???

def rgb_to_hls(r, g, b):
    maxc = max(r, g, b)
    minc = min(r, g, b)
    # XXX Can optimize (maxc+minc) and (maxc-minc)
    l = (minc+maxc)/2.0
    if minc == maxc: return 0.0, l, 0.0
    if l <= 0.5: s = (maxc-minc) / (maxc+minc)
    else: s = (maxc-minc) / (2.0-maxc-minc)
    rc = (maxc-r) / (maxc-minc)
    gc = (maxc-g) / (maxc-minc)
    bc = (maxc-b) / (maxc-minc)
    if r == maxc: h = bc-gc
    elif g == maxc: h = 2.0+rc-bc
    else: h = 4.0+gc-rc
    h = (h/6.0) % 1.0
    return h, l, s

def hls_to_rgb(h, l, s):
    if s == 0.0: return l, l, l
    if l <= 0.5: m2 = l * (1.0+s)
    else: m2 = l+s-(l*s)
    m1 = 2.0*l - m2
    return (_v(m1, m2, h+ONE_THIRD), _v(m1, m2, h), _v(m1, m2, h-ONE_THIRD))

def _v(m1, m2, hue):
    hue = hue % 1.0
    if hue < ONE_SIXTH: return m1 + (m2-m1)*hue*6.0
    if hue < 0.5: return m2
    if hue < TWO_THIRD: return m1 + (m2-m1)*(TWO_THIRD-hue)*6.0
    return m1


# HSV: Hue, Saturation, Value(?)
# H: position in the spectrum
# S: ???
# V: ???

def rgb_to_hsv(r, g, b):
    maxc = max(r, g, b)
    minc = min(r, g, b)
    v = maxc
    if minc == maxc: return 0.0, 0.0, v
    s = (maxc-minc) / maxc
    rc = (maxc-r) / (maxc-minc)
    gc = (maxc-g) / (maxc-minc)
    bc = (maxc-b) / (maxc-minc)
    if r == maxc: h = bc-gc
    elif g == maxc: h = 2.0+rc-bc
    else: h = 4.0+gc-rc
    h = (h/6.0) % 1.0
    return h, s, v

def hsv_to_rgb(h, s, v):
    if s == 0.0: return v, v, v
    i = int(h*6.0) # XXX assume int() truncates!
    f = (h*6.0) - i
    p = v*(1.0 - s)
    q = v*(1.0 - s*f)
    t = v*(1.0 - s*(1.0-f))
    if i%6 == 0: return v, t, p
    if i == 1: return q, v, p
    if i == 2: return p, v, t
    if i == 3: return p, q, v
    if i == 4: return t, p, v
    if i == 5: return v, p, q
    # Cannot get here