+ (NSArray *)rgbToHsv:(NSArray *)rgb {
    double r = [[rgb objectAtIndex:0] doubleValue]/255.0;
    double g = [[rgb objectAtIndex:1] doubleValue]/255.0;
    double b = [[rgb objectAtIndex:2] doubleValue]/255.0;

    double h,s,v;

    float min, max, delta;
    min = MIN(MIN( r, g), b);
    max = MAX(MAX( r, g), b);
    v = max;				// v
    delta = max - min;
    if( max != 0 )
        s = delta / max;		// s
    else {
        // r = g = b = 0		// s = 0, v is undefined
        s = 0;
        h = -1;
        return [NSArray arrayWithObjects:[NSNumber numberWithDouble:h],[NSNumber numberWithDouble:s],[NSNumber numberWithDouble:v], nil];
    }
    if( r == max )
        h = ( g - b ) / delta;		// between yellow & magenta
    else if( g == max )
        h = 2 + ( b - r ) / delta;	// between cyan & yellow
    else
        h = 4 + ( r - g ) / delta;	// between magenta & cyan
    h *= 60;                        // degrees
    if( h < 0 )
        h += 360;
    return [NSArray arrayWithObjects:[NSNumber numberWithDouble:h],[NSNumber numberWithDouble:s],[NSNumber numberWithDouble:v], nil];
}

+ (NSArray *)hsvToRgb:(NSArray *)hsv {
    double h = [[hsv objectAtIndex:0] doubleValue];
    double s = [[hsv objectAtIndex:1] doubleValue];
    double v = [[hsv objectAtIndex:2] doubleValue];

    double r,g,b;

    int i;
    float f, p, q, t;
    if( s == 0 ) {
        // achromatic (grey)
        r = g = b = v;
        return [NSArray arrayWithObjects:[NSNumber numberWithDouble:r],[NSNumber numberWithDouble:g],[NSNumber numberWithDouble:b], nil];
    }
    h /= 60;			// sector 0 to 5
    i = floor( h );
    f = h - i;			// factorial part of h
    p = v * ( 1 - s );
    q = v * ( 1 - s * f );
    t = v * ( 1 - s * ( 1 - f ) );
    switch( i ) {
        case 0:
            r = v;
            g = t;
            b = p;
            break;
        case 1:
            r = q;
            g = v;
            b = p;
            break;
        case 2:
            r = p;
            g = v;
            b = t;
            break;
        case 3:
            r = p;
            g = q;
            b = v;
            break;
        case 4:
            r = t;
            g = p;
            b = v;
            break;
        default:		// case 5:
            r = v;
            g = p;
            b = q;
            break;
    }
    return [NSArray arrayWithObjects:[NSNumber numberWithDouble:r*255],[NSNumber numberWithDouble:g*255],[NSNumber numberWithDouble:b*255], nil];
}