C Code — Function To Change Color Saturation
©2006 Darel Rex Finley.  This complete article, unmodified, may be freely distributed for educational purposes.

This function will change the saturation of an RGB color.  It employs the HSP color system.

#define  Pr  .299
#define  Pg  .587
#define  Pb  .114

//  public-domain function by Darel Rex Finley
//  The passed-in RGB values can be on any desired scale, such as 0 to
//  to 1, or 0 to 255.  (But use the same scale for all three!)
//  The "change" parameter works like this:
//    0.0 creates a black-and-white image.
//    0.5 reduces the color saturation by half.
//    1.0 causes no change.
//    2.0 doubles the color saturation.
//  Note:  A "change" value greater than 1.0 may project your RGB values
//  beyond their normal range, in which case you probably should truncate
//  them to the desired range before trying to use them in an image.

void changeSaturation(double *R, double *G, double *B, double change) {

  double  P=sqrt(
  (*B)*(*B)*Pb ) ;

  *B=P+((*B)-P)*change; }

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