Wavelength to RGB in Python

Wavelength to RGB in Python

This class converts a given wavelength in nanometers to a 24-bit R,G,B value. This is obviously a very approximate conversion and will not appear the same on every display device.

The input wavelength range is 380 nm through 750 nm (violet through red).

For testing see PNG canvas in Python.

wl_to_rgb.py:

#!/usr/bin/env python
# vim:set ft=python fileencoding=utf-8 sr et ts=4 sw=4 : See help 'modeline'

'''
    == A few notes about color ==

    Color   Wavelength(nm) Frequency(THz)
    Red     620-750        400-484
    Orange  590-620        484-508
    Yellow  570-590        508-526
    Green   495-570        526-606
    Blue    450-495        606-668
    Violet  380-450        668-789

    f is frequency (cycles per second)
    l (lambda) is wavelength (meters per cycle)
    e is energy (Joules)
    h (Plank's constant) = 6.6260695729 x 10^-34 Joule*seconds
                         = 6.6260695729 x 10^-34 m^2*kg/seconds
    c = 299792458 meters per second
    f = c/l
    l = c/f
    e = h*f
    e = c*h/l

    Human eye response:
        S cone: 437 nm
        M cone: 533 nm
        L cone: 564 nm
        rod:    550 nm daytime, light adapted 498 nm at night
'''

import sys
import os
import traceback
import optparse
import time
import logging


def wavelength_to_rgb(wavelength, gamma=0.8):

    '''This converts a given wavelength into an approximate RGB value.
    The given wavelength is in nanometers.
    The range of wavelength is 380 nm through 750 nm.

    Based on code by Dan Bruton
    http://www.physics.sfasu.edu/astro/color/spectra.html
    '''

    wavelength = float(wavelength)
    if wavelength >= 380 and wavelength <= 440:
        attenuation = 0.3 + 0.7 * (wavelength - 380) / (440 - 380)
        R = ((-(wavelength - 440) / (440 - 380)) * attenuation) ** gamma
        G = 0.0
        B = (1.0 * attenuation) ** gamma
    elif wavelength >= 440 and wavelength <= 490:
        R = 0.0
        G = ((wavelength - 440) / (490 - 440)) ** gamma
        B = 1.0
    elif wavelength >= 490 and wavelength <= 510:
        R = 0.0
        G = 1.0
        B = (-(wavelength - 510) / (510 - 490)) ** gamma
    elif wavelength >= 510 and wavelength <= 580:
        R = ((wavelength - 510) / (580 - 510)) ** gamma
        G = 1.0
        B = 0.0
    elif wavelength >= 580 and wavelength <= 645:
        R = 1.0
        G = (-(wavelength - 645) / (645 - 580)) ** gamma
        B = 0.0
    elif wavelength >= 645 and wavelength <= 750:
        attenuation = 0.3 + 0.7 * (750 - wavelength) / (750 - 645)
        R = (1.0 * attenuation) ** gamma
        G = 0.0
        B = 0.0
    else:
        R = 0.0
        G = 0.0
        B = 0.0
    R *= 255
    G *= 255
    B *= 255
    return (int(R), int(G), int(B))


def main(options=None, args=None):

#    import ppm_dump
#    import png_canvas
    import canvas
    if options.ppm:
        canvas = canvas.ppm_canvas(371, 278)
        canvas.is_ascii = True
    else:
        canvas = canvas.png_canvas(371, 278)
    for wl in range(380, 751):
        r, g, b = wavelength_to_rgb(wl)
        for yy in range(0, 278):
            canvas.pixel(wl - 380, yy, r, g, b)
    sys.stdout.write(str(canvas))

if __name__ == '__main__':
    try:
        start_time = time.time()
        parser = optparse.OptionParser(
            formatter=optparse.TitledHelpFormatter(),
            usage=globals()['__doc__'],
            version='1'
        )
        parser.add_option(
            '-v', '--verbose', action='store_true',
            default=False, help='verbose output'
        )
        parser.add_option(
            '--png', action='store_true',
            default=True, help='Output as PNG.'
        )
        parser.add_option(
            '--ppm', action='store_true',
            default=False, help='Output as PPM ASCII (Portable Pixmap).'
        )
        (options, args) = parser.parse_args()
        #if len(args) < 1:
        #    parser.error ('missing argument')
        if options.verbose:
            print(time.asctime())
        exit_code = main(options, args)
        if exit_code is None:
            exit_code = 0
        if options.verbose:
            print(time.asctime())
            print('TOTAL TIME IN MINUTES: %f'
                  % ((time.time() - start_time) / 60.0))
        sys.exit(exit_code)
    except KeyboardInterrupt as e:  # The user pressed Ctrl-C.
        raise e
    except SystemExit as e:  # The script called sys.exit() somewhere.
        raise e
    except Exception as e:
        print('ERROR: Unexpected Exception')
        print(str(e))
        traceback.print_exc()
        os._exit(2)