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            png_bytep sp = row + (png_size_t)row_width * 2 - 1;
            png_bytep dp = sp  + (png_size_t)row_width * 4;
            for (i = 0; i < row_width; i++)
            {
               *(dp--) = *sp;
               *(dp--) = *(sp - 1);
               *(dp--) = *sp;
               *(dp--) = *(sp - 1);
               *(dp--) = *(sp--);
               *(dp--) = *(sp--);
            }
         }
      }

      else if (row_info->color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
      {
         if (row_info->bit_depth == 8)
         {
            /* This changes GA to RGBA */
            png_bytep sp = row + (png_size_t)row_width * 2 - 1;
            png_bytep dp = sp  + (png_size_t)row_width * 2;
            for (i = 0; i < row_width; i++)
            {
               *(dp--) = *(sp--);
               *(dp--) = *sp;
               *(dp--) = *sp;
               *(dp--) = *(sp--);
            }
         }

         else
         {
            /* This changes GGAA to RRGGBBAA */
            png_bytep sp = row + (png_size_t)row_width * 4 - 1;
            png_bytep dp = sp  + (png_size_t)row_width * 4;
            for (i = 0; i < row_width; i++)
            {
               *(dp--) = *(sp--);
               *(dp--) = *(sp--);
               *(dp--) = *sp;
               *(dp--) = *(sp - 1);
               *(dp--) = *sp;
               *(dp--) = *(sp - 1);
               *(dp--) = *(sp--);
               *(dp--) = *(sp--);
            }
         }
      }
      row_info->channels = (png_byte)(row_info->channels + 2);
      row_info->color_type |= PNG_COLOR_MASK_COLOR;
      row_info->pixel_depth = (png_byte)(row_info->channels *
          row_info->bit_depth);
      row_info->rowbytes = PNG_ROWBYTES(row_info->pixel_depth, row_width);
   }
}
#endif

#ifdef PNG_READ_RGB_TO_GRAY_SUPPORTED
/* Reduce RGB files to grayscale, with or without alpha
 * using the equation given in Poynton's ColorFAQ of 1998-01-04 at
 * <http://www.inforamp.net/~poynton/>  (THIS LINK IS DEAD June 2008 but
 * versions dated 1998 through November 2002 have been archived at
 * http://web.archive.org/web/20000816232553/http://www.inforamp.net/
 * ~poynton/notes/colour_and_gamma/ColorFAQ.txt )
 * Charles Poynton poynton at poynton.com
 *
 *     Y = 0.212671 * R + 0.715160 * G + 0.072169 * B
 *
 *  which can be expressed with integers as
 *
 *     Y = (6969 * R + 23434 * G + 2365 * B)/32768
 *
 * Poynton's current link (as of January 2003 through July 2011):
 * <http://www.poynton.com/notes/colour_and_gamma/>
 * has changed the numbers slightly:
 *
 *     Y = 0.2126*R + 0.7152*G + 0.0722*B
 *
 *  which can be expressed with integers as
 *
 *     Y = (6966 * R + 23436 * G + 2366 * B)/32768
 *
 *  Historically, however, libpng uses numbers derived from the ITU-R Rec 709
 *  end point chromaticities and the D65 white point.  Depending on the
 *  precision used for the D65 white point this produces a variety of different
 *  numbers, however if the four decimal place value used in ITU-R Rec 709 is
 *  used (0.3127,0.3290) the Y calculation would be:
 *
 *     Y = (6968 * R + 23435 * G + 2366 * B)/32768
 *
 *  While this is correct the rounding results in an overflow for white, because
 *  the sum of the rounded coefficients is 32769, not 32768.  Consequently
 *  libpng uses, instead, the closest non-overflowing approximation:
 *
 *     Y = (6968 * R + 23434 * G + 2366 * B)/32768
 *
 *  Starting with libpng-1.5.5, if the image being converted has a cHRM chunk
 *  (including an sRGB chunk) then the chromaticities are used to calculate the
 *  coefficients.  See the chunk handling in pngrutil.c for more information.
 *
 *  In all cases the calculation is to be done in a linear colorspace.  If no
 *  gamma information is available to correct the encoding of the original RGB
 *  values this results in an implicit assumption that the original PNG RGB
 *  values were linear.
 *
 *  Other integer coefficents can be used via png_set_rgb_to_gray().  Because
 *  the API takes just red and green coefficients the blue coefficient is
 *  calculated to make the sum 32768.  This will result in different rounding
 *  to that used above.
 */
static int
png_do_rgb_to_gray(png_structrp png_ptr, png_row_infop row_info, png_bytep row)

{
   int rgb_error = 0;

   png_debug(1, "in png_do_rgb_to_gray");

   if ((row_info->color_type & PNG_COLOR_MASK_PALETTE) == 0 &&
       (row_info->color_type & PNG_COLOR_MASK_COLOR) != 0)
   {
      PNG_CONST png_uint_32 rc = png_ptr->rgb_to_gray_red_coeff;
      PNG_CONST png_uint_32 gc = png_ptr->rgb_to_gray_green_coeff;
      PNG_CONST png_uint_32 bc = 32768 - rc - gc;
      PNG_CONST png_uint_32 row_width = row_info->width;
      PNG_CONST int have_alpha =
         (row_info->color_type & PNG_COLOR_MASK_ALPHA) != 0;

      if (row_info->bit_depth == 8)
      {
#ifdef PNG_READ_GAMMA_SUPPORTED
         /* Notice that gamma to/from 1 are not necessarily inverses (if
          * there is an overall gamma correction).  Prior to 1.5.5 this code
          * checked the linearized values for equality; this doesn't match