I76 Answer ― Spiral Cycles

Here is the relevant part of the solution:

  for ( r=0; r<nrows; r++ )
    for ( c=0; c<ncols; c++ )
    {
      dist    = sqrt( (r-cntR)*(r-cntR) + (c-cntC)*(c-cntC)) ;
      thetaCC = atan2( (c-cntC), (r-cntR) );
      radians = 2*M_PI*(dist/radius) ;
      red     = 125 + 125*cos( radians + thetaCC*redCycles );
      grn     = 125 + 125*cos( radians + thetaCC*grnCycles );
      blu     = 125 + 125*cos( radians + thetaCC*bluCycles );
      fputc( red, image );  fputc( grn, image );  fputc( blu, image );
    }

Comments: Most of the work in this program is in rearanging the calculation for the color values.