Question

I have a problem with converting a color from HSL to RGB. I’ve written the following function:

struct RGB {
    float r, g, b;
};
RGB hslToRgb( float hue, float saturation, float lightness ) {

    RGB rgb1, rgbResult;

    float chroma = ( 1.0 - (float) abs( 2.0 * lightness - 1.0 ) ) * saturation;
    float h1 = hue / 60.0;
    float x = chroma * ( 1.0 - (float) abs( (float) ( (int) h1 % 2 ) - 1.0 ) );

    if ( ( 0 <= h1 ) && ( h1 < 1 ) ) {

        rgb1.r = chroma;
        rgb1.g = x;
        rgb1.b = 0.0;

    } else if ( ( 1 <= h1 ) && ( h1 < 2 ) ) {

        rgb1.r = x;
        rgb1.g = chroma;
        rgb1.b = 0.0;

    } else if ( ( 2 <= h1 ) && ( h1 < 3 ) ) {

        rgb1.r = 0.0;
        rgb1.g = chroma;
        rgb1.b = x;

    } else if ( ( 3 <= h1 ) && ( h1 < 4 ) ) {

        rgb1.r = 0.0;
        rgb1.g = x;
        rgb1.b = chroma;

    } else if ( ( 4 <= h1 ) && ( h1 < 5 ) ) {

        rgb1.r = x;
        rgb1.g = 0.0;
        rgb1.b = chroma;

    } else if ( ( 5 <= h1 ) && ( h1 < 6 ) ) {

        rgb1.r = chroma;
        rgb1.g = 0;
        rgb1.b = x;

    } else {

        rgb1.r = 0.0;
        rgb1.g = 0.0;
        rgb1.b = 0.0;

    }

    float m = lightness - 0.5 * chroma;

    rgbResult.r = rgb1.r + m;
    rgbResult.g = rgb1.g + m;
    rgbResult.b = rgb1.b + m;

    return rgbResult;

}

here it is its test:

float cHue = 0.0;
while ( cHue < 360 ) {
    RGB rgb1 = hslToRgb( (int) cHue, 1.0, 0.5 ); // max on saturation and a middle value for lightness
    printf( "r = %f, g = %f, b = %f\n", rgb1.r, rgb1.g, rgb1.b );
    cHue += 1.0;
}

but I get only 1.0 and 0.0 when I need to get all range between this “integers”.

r = 1.000000, g = 0.000000, b = 1.000000
r = 1.000000, g = 0.000000, b = 1.000000
r = 1.000000, g = 0.000000, b = 1.000000
r = 1.000000, g = 0.000000, b = 0.000000
r = 1.000000, g = 0.000000, b = 0.000000
r = 1.000000, g = 0.000000, b = 0.000000

Can anyone help me with this code?
Formulas from: http://en.wikipedia.org/wiki/HSL_and_HSV

Answer 1

Use floating point modulo fmodf as:

float x = chroma * ( 1.0 - (float) abs( fmodf(h1, 2.0) - 1.0 ));