UIColor:RGB和HSV互相轉換

UIColor:RGB和HSV互相轉換

關於顏色的種種屬性，一直感覺挺神祕，不曾花時間來研究，今天揭開神祕面紗的一角。

RGB:三原色

        Red, Green, Blue

Alpha:不透明度

HSV :

       H  -   Hue            色度

       S  -   Saturation   彩度

       V  -   Brightness   亮度

關於RGB和HSV互相轉換，iOS編程中的顏色處理中，時常會用到。

參考資料：

How do i get the hue, saturation and brightness from a UIColor?

例子代碼：

static void RGBtoHSV( float r, float g, float b, float *h, float *s, float *v )
{
    float min, max, delta;
    min = MIN( r, MIN( g, b ));
    max = MAX( r, MAX( 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;
}

static 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;
    }
}

...

CGFloat r, g, b, a, h, s, v;

const CGFloat *comp = CGColorGetComponents([myUIColor CGColor]);

r = comp[0];
g = comp[1];
b = comp[2];
a = comp[3];

RGBtoHSV(r, g, b, &h, &s, &v);