RGB转LAB

RGB不能直接转成LAB，需要先转成XYZ，然后在从XYZ转成LAB。

RGB和XYZ和LAB可以理解为同一类颜色空间。

/**
     * Convert RGB components to its CIE Lab representative components.
     *
     * <ul>
     * <li>outLab[0] is L [0 ...1)</li>
     * <li>outLab[1] is a [-128...127)</li>
     * <li>outLab[2] is b [-128...127)</li>
     * </ul>
     *
     * @param r      red component value [0..255]
     * @param g      green component value [0..255]
     * @param b      blue component value [0..255]
     * @param outLab 3-element array which holds the resulting LAB components
     */
    public static void RGBToLAB(@IntRange(from = 0x0, to = 0xFF) int r,
            @IntRange(from = 0x0, to = 0xFF) int g, @IntRange(from = 0x0, to = 0xFF) int b,
            @NonNull double[] outLab) {

        //outLab就是要输出的内存地址
        RGBToXYZ(r, g, b, outLab);
        // outLab now contains XYZ
        XYZToLAB(outLab[0], outLab[1], outLab[2], outLab);
        // outLab now contains LAB representation
    }

RGB的数值范围都是0~255

这一步就是将RGB归一化为XYZ：
0 ~ 255归一化为0 ~ 1
就是i/255（i= [0,255]）

   /**
     * Convert RGB components to it's CIE XYZ representative components.
     *
     * 
     * <p>The resulting XYZ representation will use the D65 illuminant and the CIE
     * 2° Standard Observer (1931).</p>
     *
     * <ul>
     * <li>outXyz[0] is X [0 ...95.047)</li>
     * <li>outXyz[1] is Y [0...100)</li>
     * <li>outXyz[2] is Z [0...108.883)</li>
     * </ul>
     *
     * @param r      red component value [0..255]
     * @param g      green component value [0..255]
     * @param b      blue component value [0..255]
     * @param outXyz 3-element array which holds the resulting XYZ components
     */
    public static void RGBToXYZ(@IntRange(from = 0x0, to = 0xFF) int r,
            @IntRange(from = 0x0, to = 0xFF) int g, @IntRange(from = 0x0, to = 0xFF) int b,
            @NonNull double[] outXyz) {
        // 要保证RGB三通道不为空
        if (outXyz.length != 3) {
            throw new IllegalArgumentException("outXyz must have a length of 3.");
        }
        
        //下面的可以理解为固定值，有固有算法
        double sr = r / 255.0;
        sr = sr < 0.04045 ? sr / 12.92 : Math.pow((sr + 0.055) / 1.055, 2.4);
        double sg = g / 255.0;
        sg = sg < 0.04045 ? sg / 12.92 : Math.pow((sg + 0.055) / 1.055, 2.4);
        double sb = b / 255.0;
        sb = sb < 0.04045 ? sb / 12.92 : Math.pow((sb + 0.055) / 1.055, 2.4);
        
        //分别写入RGB三通道内存地址
        outXyz[0] = 100 * (sr * 0.4124 + sg * 0.3576 + sb * 0.1805);
        outXyz[1] = 100 * (sr * 0.2126 + sg * 0.7152 + sb * 0.0722);
        outXyz[2] = 100 * (sr * 0.0193 + sg * 0.1192 + sb * 0.9505);
    }

然后再从XYZ转成LAB，因为是直接写进内存的，所以不需要通过返回值传递，而是直接通过内存地址拿值

    private static final double XYZ_WHITE_REFERENCE_X = 95.047;
    private static final double XYZ_WHITE_REFERENCE_Y = 100;
    private static final double XYZ_WHITE_REFERENCE_Z = 108.883;
    /**
     * Converts a color from CIE XYZ to CIE Lab representation.
     *
     * <p>This method expects the XYZ representation to use the D65 illuminant and the CIE
     * 2° Standard Observer (1931).</p>
     *
     * <ul>
     * <li>outLab[0] is L [0 ...1)</li>
     * <li>outLab[1] is a [-128...127)</li>
     * <li>outLab[2] is b [-128...127)</li>
     * </ul>
     *
     * @param x      X component value [0...95.047)
     * @param y      Y component value [0...100)
     * @param z      Z component value [0...108.883)
     * @param outLab 3-element array which holds the resulting Lab components
     */
    public static void XYZToLAB(@FloatRange(from = 0f, to = XYZ_WHITE_REFERENCE_X) double x,
            @FloatRange(from = 0f, to = XYZ_WHITE_REFERENCE_Y) double y,
            @FloatRange(from = 0f, to = XYZ_WHITE_REFERENCE_Z) double z,
            @NonNull double[] outLab) {
        if (outLab.length != 3) {
            throw new IllegalArgumentException("outLab must have a length of 3.");
        }
        //归一化组件
        x = pivotXyzComponent(x / XYZ_WHITE_REFERENCE_X);
        y = pivotXyzComponent(y / XYZ_WHITE_REFERENCE_Y);
        z = pivotXyzComponent(z / XYZ_WHITE_REFERENCE_Z);
        //写RGB到内存地址
        outLab[0] = Math.max(0, 116 * y - 16);
        outLab[1] = 500 * (x - y);
        outLab[2] = 200 * (y - z);
    }

csdn上这篇也不错
http://blog.csdn.net/scarecrow_wiscom/article/details/9795715