I10 Answer

washerboard( FILE *image, int nrows, int ncols, int nripples )
{
  int r, c, gray ;

  /* write out the pixel data */
  for ( r=0; r<nrows; r++ )
  {
    for ( c=0; c<ncols; c++ )
    {
      gray = 127 + 127*cos( 2*M_PI*nripples*r/nrows );
      fputc( gray, image );
    }
  }
}