图像处理: 可见光波长(ｗavelength)与RGB之间的转换

关于颜色转换，有时间看标准当然最好，不过我手头一时没有相关标准，所以参考了一些网上的资料，

380nm 到 760nm的可见光 对应的RGB大概是这个样子的，

C语言版本程序，

//指定波长转换成RGBA颜色
std::vector lambdaToColor(double lambda,double gamma = 0.8,double intensityMax = 255.0)
{
    double r, g, b, alpha;
    if (lambda >= 380.0 && lambda < 440.0) {
        r = -1.0 * (lambda - 440.0) / (440.0 - 380.0);
        g = 0.0;
        b = 1.0;
    }else if (lambda >= 440.0 && lambda < 490.0) {
        r = 0.0;
        g = (lambda - 440.0) / (490.0 - 440.0);
        b = 1.0;
    }else if (lambda >= 490.0 && lambda < 510.0) {
        r = 0.0;
        g = 1.0;
        b = -1.0 * (lambda - 510.0) / (510.0 - 490.0);
    }else if (lambda >= 510.0 && lambda < 580.0) {
        r = (lambda - 510.0) / (580.0 - 510.0);
        g = 1.0;
        b = 0.0;
    }else if (lambda >= 580.0 && lambda < 645.0) {
        r = 1.0;
        g = -1.0 * (lambda - 645.0) / (645.0 - 580.0);
        b = 0.0;
    }else if (lambda >= 645.0 && lambda = 380.0 && lambda < 420.0) {
        alpha = 0.30 + 0.70 * (lambda - 380.0) / (420.0 - 380.0);
    }else if (lambda >= 420.0 && lambda < 701.0) {
        alpha = 1.0;
    }else if (lambda >= 701.0 && lambda < 780.0) {
        alpha = 0.30 + 0.70 * (780.0 - lambda) / (780.0 - 700.0);
    }else {
        alpha = 0.0;
    }

    //1953年在引入NTSC电视时,计算具有荧光体的监视器的亮度公式如下
    int Y = static_cast(0.212671*r + 0.715160*g + 0.072169*b);

    //伽马射线 gamma
    //照明强度 intensityMax
    int R = r == 0.0 ? 0 : static_cast(std::round(intensityMax * std::pow(r * alpha, gamma)));
    int G = g == 0.0 ? 0 : static_cast(std::round(intensityMax * std::pow(g * alpha, gamma)));
    int B = b == 0.0 ? 0 : static_cast(std::round(intensityMax * std::pow(b * alpha, gamma)));
    int A = static_cast(alpha);

    return std::vector{R, G, B, A, Y};
}

另一个转换是通过建立一个事先计算好的表来计算的，这个还没细看，不过感觉通过建立查找表，牺牲一点点空间，可以极大减少运算量，这也是高速程序的一般处理方式。

static class RgbCalculator {

    const int
         LEN_MIN = 380,
         LEN_MAX = 780,
         LEN_STEP = 5;

    static readonly double[]
        X = {
                0.000160, 0.000662, 0.002362, 0.007242, 0.019110, 0.043400, 0.084736, 0.140638, 0.204492, 0.264737,
                0.314679, 0.357719, 0.383734, 0.386726, 0.370702, 0.342957, 0.302273, 0.254085, 0.195618, 0.132349,
                0.080507, 0.041072, 0.016172, 0.005132, 0.003816, 0.015444, 0.037465, 0.071358, 0.117749, 0.172953,
                0.236491, 0.304213, 0.376772, 0.451584, 0.529826, 0.616053, 0.705224, 0.793832, 0.878655, 0.951162,
                1.014160, 1.074300, 1.118520, 1.134300, 1.123990, 1.089100, 1.030480, 0.950740, 0.856297, 0.754930,
                0.647467, 0.535110, 0.431567, 0.343690, 0.268329, 0.204300, 0.152568, 0.112210, 0.081261, 0.057930,
                0.040851, 0.028623, 0.019941, 0.013842, 0.009577, 0.006605, 0.004553, 0.003145, 0.002175, 0.001506,
                0.001045, 0.000727, 0.000508, 0.000356, 0.000251, 0.000178, 0.000126, 0.000090, 0.000065, 0.000046,
                0.000033
            },

        Y = {
                0.000017, 0.000072, 0.000253, 0.000769, 0.002004, 0.004509, 0.008756, 0.014456, 0.021391, 0.029497,
                0.038676, 0.049602, 0.062077, 0.074704, 0.089456, 0.106256, 0.128201, 0.152761, 0.185190, 0.219940,
                0.253589, 0.297665, 0.339133, 0.395379, 0.460777, 0.531360, 0.606741, 0.685660, 0.761757, 0.823330,
                0.875211, 0.923810, 0.961988, 0.982200, 0.991761, 0.999110, 0.997340, 0.982380, 0.955552, 0.915175,
                0.868934, 0.825623, 0.777405, 0.720353, 0.658341, 0.593878, 0.527963, 0.461834, 0.398057, 0.339554,
                0.283493, 0.228254, 0.179828, 0.140211, 0.107633, 0.081187, 0.060281, 0.044096, 0.031800, 0.022602,
                0.015905, 0.011130, 0.007749, 0.005375, 0.003718, 0.002565, 0.001768, 0.001222, 0.000846, 0.000586,
                0.000407, 0.000284, 0.000199, 0.000140, 0.000098, 0.000070, 0.000050, 0.000036, 0.000025, 0.000018,
                0.000013
            },

        Z = {
                0.000705, 0.002928, 0.010482, 0.032344, 0.086011, 0.197120, 0.389366, 0.656760, 0.972542, 1.282500,
                1.553480, 1.798500, 1.967280, 2.027300, 1.994800, 1.900700, 1.745370, 1.554900, 1.317560, 1.030200,
                0.772125, 0.570060, 0.415254, 0.302356, 0.218502, 0.159249, 0.112044, 0.082248, 0.060709, 0.043050,
                0.030451, 0.020584, 0.013676, 0.007918, 0.003988, 0.001091, 0.000000, 0.000000, 0.000000, 0.000000,
                0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000,
                0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000,
                0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000,
                0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000, 0.000000,
                0.000000
            };

    static readonly double[]
        MATRIX_SRGB_D65 = {
             3.2404542, -1.5371385, -0.4985314,
            -0.9692660,  1.8760108,  0.0415560,
             0.0556434, -0.2040259,  1.0572252
        };

    public static byte[] Calc(double len) {
        if(len < LEN_MIN || len > LEN_MAX)
            return new byte[3];

        len -= LEN_MIN;
        var index = (int)Math.Floor(len / LEN_STEP);
        var offset = len - LEN_STEP * index;

        var x = Interpolate(X, index, offset);
        var y = Interpolate(Y, index, offset);
        var z = Interpolate(Z, index, offset);

        var m = MATRIX_SRGB_D65;

        var r = m[0] * x + m[1] * y + m[2] * z;
        var g = m[3] * x + m[4] * y + m[5] * z;
        var b = m[6] * x + m[7] * y + m[8] * z;

        r = Clip(GammaCorrect_sRGB(r));
        g = Clip(GammaCorrect_sRGB(g));
        b = Clip(GammaCorrect_sRGB(b));

        return new[] { 
            (byte)(255 * r),
            (byte)(255 * g),
            (byte)(255 * b)
        };
    }

    static double Interpolate(double[] values, int index, double offset) {
        if(offset == 0)
            return values[index];

        var x0 = index * LEN_STEP;
        var x1 = x0 + LEN_STEP;
        var y0 = values[index];
        var y1 = values[1 + index];

        return y0 + offset * (y1 - y0) / (x1 - x0);
    }

    static double GammaCorrect_sRGB(double c) {
        if(c  1)
            return 1;
        return c;
    }
}

另外有一个python版本在这里，暂时用不到，挂个号在这里吧

#!/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        484-400
    Orange  590-620        508-484
    Yellow  570-590        526-508
    Green   495-570        606-526
    Blue    450-495        668-606
    Violet  380-450        789-668

    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

    List of peak frequency responses for each type of 
    photoreceptor cell in the human eye:
        S cone: 437 nm
        M cone: 533 nm
        L cone: 564 nm
        rod:    550 nm in bright daylight, 498 nm when dark adapted. 
                Rods adapt to low light conditions by becoming more sensitive.
                Peak frequency response shifts to 498 nm.

'''

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 of light to an 
    approximate RGB color value. The wavelength must be given
    in nanometers in the range from 380 nm through 750 nm
    (789 THz through 400 THz).

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

    wavelength = float(wavelength)
    if wavelength >= 380 and wavelength = 440 and wavelength = 490 and wavelength = 510 and wavelength = 580 and wavelength = 645 and wavelength