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RGB to HSV & HSV to RGB

The Hue/Saturation/Value model was created by A. R. Smith in 1978. It is based on such intuitive color characteristics as tint, shade and tone (or family, purety and intensity). The coordinate system is cylindrical, and the colors are defined inside a hexcone. The hue value H runs from 0 to 360º. The saturation S is the degree of strength or purity and is from 0 to 1. Purity is how much white is added to the color, so S=1 makes the purest color (no white). Brightness V also ranges from 0 to 1, where 0 is the black.

There is no transformation matrix for RGB/HSV conversion, but the algorithm follows:

// r,g,b values are from 0 to 1
// h = [0,360], s = [0,1], v = [0,1]
//      if s == 0, then h = -1 (undefined)
void RGBtoHSV( float r, float g, float b, float *h, float *s, float *v )
{
    float min, max, delta;
    min = MIN( r, g, b );
    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;
    }
    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;
}
void HSVtoRGB( float *r, float *g, float *b, float h, float s, float v )
{
    int i;
    float f, p, q, t;
    if( s == 0 ) {
        // achromatic (grey)
        *r = *g = *b = v;
        return;
    }
    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;
    }
}

'assumes lReturnValue contains the RGB color from GetSysColor

    Dim lBlue As Long
    Dim lGreen As Long
    Dim lRed As Long
    
    lBlue = (lReturnValue \ 65536) And &HFF&
    lGreen = (lReturnValue \ 256) And &HFF&
    lRed = lReturnValue And &HFF&

Source: <a href="http://www.xtremevbtalk.com/interface-and-graphics/164878-getsyscolor-rgb.html" rel="noopener">http://www.xtremevbtalk.com/interface-and-graphics/164878-getsyscolor-rgb.html</a>

#include "fivewin.ch"

Function Main()
   local nRGBColor := rgb( 204,232,255 )
   
   msgInfo( nAnd( nRGBColor , 255 ), "Red" )
   msgInfo( nAnd( nRGBColor/256, 255), "Green" )
   msgInfo( nAnd( nRGBColor/65536, 255), "Blue" )
   
Return nil

aHSL  := RGBTOHSL( nRGBColor )
aHSL[ 3 ] := Max( 0, aHSL[ 3 ] - 20 )
nRGBNew := HSLTORGB( aHSL )[ 4 ]