zero_samples=0;
   png_set_write_user_transform_fn(write_ptr, count_zero_samples);
#endif

#if defined(PNG_READ_UNKNOWN_CHUNKS_SUPPORTED)
#  ifndef PNG_HANDLE_CHUNK_ALWAYS
#    define PNG_HANDLE_CHUNK_ALWAYS       3
#  endif
   png_set_keep_unknown_chunks(read_ptr, PNG_HANDLE_CHUNK_ALWAYS,
      png_bytep_NULL, 0);
#endif
#if defined(PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED)
#  ifndef PNG_HANDLE_CHUNK_IF_SAFE
#    define PNG_HANDLE_CHUNK_IF_SAFE      2
#  endif
   png_set_keep_unknown_chunks(write_ptr, PNG_HANDLE_CHUNK_IF_SAFE,
      png_bytep_NULL, 0);
#endif

   png_debug(0, "Reading info struct\n");
   png_read_info(read_ptr, read_info_ptr);

   png_debug(0, "Transferring info struct\n");
   {
      int interlace_type, compression_type, filter_type;

      if (png_get_IHDR(read_ptr, read_info_ptr, &width, &height, &bit_depth,
          &color_type, &interlace_type, &compression_type, &filter_type))
      {
         png_set_IHDR(write_ptr, write_info_ptr, width, height, bit_depth,
#if defined(PNG_WRITE_INTERLACING_SUPPORTED)
            color_type, interlace_type, compression_type, filter_type);
#else
            color_type, PNG_INTERLACE_NONE, compression_type, filter_type);
#endif
      }
   }
#if defined(PNG_FIXED_POINT_SUPPORTED)
#if defined(PNG_cHRM_SUPPORTED)
   {
      png_fixed_point white_x, white_y, red_x, red_y, green_x, green_y, blue_x,
         blue_y;
      if (png_get_cHRM_fixed(read_ptr, read_info_ptr, &white_x, &white_y, &red_x,
         &red_y, &green_x, &green_y, &blue_x, &blue_y))
      {
         png_set_cHRM_fixed(write_ptr, write_info_ptr, white_x, white_y, red_x,
            red_y, green_x, green_y, blue_x, blue_y);
      }
   }
#endif
#if defined(PNG_gAMA_SUPPORTED)
   {
      png_fixed_point gamma;

      if (png_get_gAMA_fixed(read_ptr, read_info_ptr, &gamma))
      {
         png_set_gAMA_fixed(write_ptr, write_info_ptr, gamma);
      }
   }
#endif
#else /* Use floating point versions */
#if defined(PNG_FLOATING_POINT_SUPPORTED)
#if defined(PNG_cHRM_SUPPORTED)
   {
      double white_x, white_y, red_x, red_y, green_x, green_y, blue_x,
         blue_y;
      if (png_get_cHRM(read_ptr, read_info_ptr, &white_x, &white_y, &red_x,
         &red_y, &green_x, &green_y, &blue_x, &blue_y))
      {
         png_set_cHRM(write_ptr, write_info_ptr, white_x, white_y, red_x,
            red_y, green_x, green_y, blue_x, blue_y);
      }
   }
#endif
#if defined(PNG_gAMA_SUPPORTED)
   {
      double gamma;

      if (png_get_gAMA(read_ptr, read_info_ptr, &gamma))
      {
         png_set_gAMA(write_ptr, write_info_ptr, gamma);
      }
   }
#endif
#endif /* floating point */
#endif /* fixed point */
#if defined(PNG_iCCP_SUPPORTED)
   {
      png_charp name;
      png_charp profile;
      png_uint_32 proflen;
      int compression_type;

      if (png_get_iCCP(read_ptr, read_info_ptr, &name, &compression_type,
                      &profile, &proflen))
      {
         png_set_iCCP(write_ptr, write_info_ptr, name, compression_type,
                      profile, proflen);
      }
   }
#endif
#if defined(PNG_sRGB_SUPPORTED)
   {
      int intent;

      if (png_get_sRGB(read_ptr, read_info_ptr, &intent))
      {
         png_set_sRGB(write_ptr, write_info_ptr, intent);
      }
   }
#endif
   {
      png_colorp palette;
      int num_palette;

      if (png_get_PLTE(read_ptr, read_info_ptr, &palette, &num_palette))
      {
         png_set_PLTE(write_ptr, write_info_ptr, palette, num_palette);
      }
   }
#if defined(PNG_bKGD_SUPPORTED)
   {
      png_color_16p background;

      if (png_get_bKGD(read_ptr, read_info_ptr, &background))
      {
         png_set_bKGD(write_ptr, write_info_ptr, background);
      }
   }
#endif
#if defined(PNG_hIST_SUPPORTED)
   {
      png_uint_16p hist;

      if (png_get_hIST(read_ptr, read_info_ptr, &hist))
      {
         png_set_hIST(write_ptr, write_info_ptr, hist);
      }
   }
#endif
#if defined(PNG_oFFs_SUPPORTED)
   {
      png_int_32 offset_x, offset_y;
      int unit_type;

      if (png_get_oFFs(read_ptr, read_info_ptr,&offset_x,&offset_y,&unit_type))
      {
         png_set_oFFs(write_ptr, write_info_ptr, offset_x, offset_y, unit_type);
      }
   }
#endif
#if defined(PNG_pCAL_SUPPORTED)
   {
      png_charp purpose, units;
      png_charpp params;
      png_int_32 X0, X1;
      int type, nparams;

      if (png_get_pCAL(read_ptr, read_info_ptr, &purpose, &X0, &X1, &type,
         &nparams, &units, &params))
      {
         png_set_pCAL(write_ptr, write_info_ptr, purpose, X0, X1, type,
            nparams, units, params);
      }
   }
#endif
#if defined(PNG_pHYs_SUPPORTED)
   {
      png_uint_32 res_x, res_y;
      int unit_type;

      if (png_get_pHYs(read_ptr, read_info_ptr, &res_x, &res_y, &unit_type))
      {
         png_set_pHYs(write_ptr, write_info_ptr, res_x, res_y, unit_type);
      }
   }
#endif
#if defined(PNG_sBIT_SUPPORTED)
   {
      png_color_8p sig_bit;

      if (png_get_sBIT(read_ptr, read_info_ptr, &sig_bit))
      {
         png_set_sBIT(write_ptr, write_info_ptr, sig_bit);
      }
   }
#endif
#if defined(PNG_sCAL_SUPPORTED)
#ifdef PNG_FLOATING_POINT_SUPPORTED
   {
      int unit;
      double scal_width, scal_height;

      if (png_get_sCAL(read_ptr, read_info_ptr, &unit, &scal_width,
         &scal_height))
      {
         png_set_sCAL(write_ptr, write_info_ptr, unit, scal_width, scal_height);
      }
   }
#else
#ifdef PNG_FIXED_POINT_SUPPORTED
   {
      int unit;
      png_charp scal_width, scal_height;

      if (png_get_sCAL_s(read_ptr, read_info_ptr, &unit, &scal_width,
          &scal_height))
      {
         png_set_sCAL_s(write_ptr, write_info_ptr, unit, scal_width, scal_height);
      }
   }
#endif
#endif
#endif
#if defined(PNG_TEXT_SUPPORTED)
   {
      png_textp text_ptr;
      int num_text;

      if (png_get_text(read_ptr, read_info_ptr, &text_ptr, &num_text) > 0)
      {
         png_debug1(0, "Handling %d iTXt/tEXt/zTXt chunks\n", num_text);
         png_set_text(write_ptr, write_info_ptr, text_ptr, num_text);
      }
   }
#endif
#if defined(PNG_tIME_SUPPORTED)
   {
      png_timep mod_time;

      if (png_get_tIME(read_ptr, read_info_ptr, &mod_time))
      {
         png_set_tIME(write_ptr, write_info_ptr, mod_time);
#if defined(PNG_TIME_RFC1123_SUPPORTED)
         /* we have to use png_strcpy instead of "=" because the string
            pointed to by png_convert_to_rfc1123() gets free'ed before
            we use it */
         png_strcpy(tIME_string,png_convert_to_rfc1123(read_ptr, mod_time));
         tIME_chunk_present++;
#endif /* PNG_TIME_RFC1123_SUPPORTED */
      }
   }
#endif
#if defined(PNG_tRNS_SUPPORTED)
   {
      png_bytep trans;
      int num_trans;
      png_color_16p trans_values;

      if (png_get_tRNS(read_ptr, read_info_ptr, &trans, &num_trans,
         &trans_values))
      {
         png_set_tRNS(write_ptr, write_info_ptr, trans, num_trans,
            trans_values);
      }
   }
#endif
#if defined(PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED)
   {
      png_unknown_chunkp unknowns;
      int num_unknowns = (int)png_get_unknown_chunks(read_ptr, read_info_ptr,
         &unknowns);
      if (num_unknowns)
      {
         png_size_t i;
         png_set_unknown_chunks(write_ptr, write_info_ptr, unknowns,
           num_unknowns);
         /* copy the locations from the read_info_ptr.  The automatically
            generated locations in write_info_ptr are wrong because we
            haven't written anything yet */
         for (i = 0; i < (png_size_t)num_unknowns; i++)
           png_set_unknown_chunk_location(write_ptr, write_info_ptr, i,
             unknowns[i].location);
      }
   }
#endif

#ifdef PNG_WRITE_SUPPORTED
   png_debug(0, "\nWriting info struct\n");

/* If we wanted, we could write info in two steps:
   png_write_info_before_PLTE(write_ptr, write_info_ptr);
 */
   png_write_info(write_ptr, write_info_ptr);
#endif

#ifdef SINGLE_ROWBUF_ALLOC
   png_debug(0, "\nAllocating row buffer...");
   row_buf = (png_bytep)png_malloc(read_ptr,
      png_get_rowbytes(read_ptr, read_info_ptr));
   png_debug1(0, "0x%08lx\n\n", (unsigned long)row_buf);
#endif /* SINGLE_ROWBUF_ALLOC */
   png_debug(0, "Writing row data\n");

#if defined(PNG_READ_INTERLACING_SUPPORTED) || \
  defined(PNG_WRITE_INTERLACING_SUPPORTED)
   num_pass = png_set_interlace_handling(read_ptr);
#  ifdef PNG_WRITE_SUPPORTED
   png_set_interlace_handling(write_ptr);
#  endif
#else
   num_pass=1;
#endif

#ifdef PNGTEST_TIMING
   t_stop = (float)clock();
   t_misc += (t_stop - t_start);
   t_start = t_stop;
#endif
   for (pass = 0; pass < num_pass; pass++)
   {
      png_debug1(0, "Writing row data for pass %d\n",pass);
      for (y = 0; y < height; y++)
      {
#ifndef SINGLE_ROWBUF_ALLOC
         png_debug2(0, "\nAllocating row buffer (pass %d, y = %ld)...", pass,y);
         row_buf = (png_bytep)png_malloc(read_ptr,
            png_get_rowbytes(read_ptr, read_info_ptr));
         png_debug2(0, "0x%08lx (%ld bytes)\n", (unsigned long)row_buf,
            png_get_rowbytes(read_ptr, read_info_ptr));
#endif /* !SINGLE_ROWBUF_ALLOC */
         png_read_rows(read_ptr, (png_bytepp)&row_buf, png_bytepp_NULL, 1);

#ifdef PNG_WRITE_SUPPORTED
#ifdef PNGTEST_TIMING
         t_stop = (float)clock();
         t_decode += (t_stop - t_start);
         t_start = t_stop;
#endif
         png_write_rows(write_ptr, (png_bytepp)&row_buf, 1);
#ifdef PNGTEST_TIMING
         t_stop = (float)clock();
         t_encode += (t_stop - t_start);
         t_start = t_stop;
#endif
#endif /* PNG_WRITE_SUPPORTED */

#ifndef SINGLE_ROWBUF_ALLOC
         png_debug2(0, "Freeing row buffer (pass %d, y = %ld)\n\n", pass, y);
         png_free(read_ptr, row_buf);
#endif /* !SINGLE_ROWBUF_ALLOC */
      }
   }

#if defined(PNG_READ_UNKNOWN_CHUNKS_SUPPORTED)
   png_free_data(read_ptr, read_info_ptr, PNG_FREE_UNKN, -1);
#endif
#if defined(PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED)
   png_free_data(write_ptr, write_info_ptr, PNG_FREE_UNKN, -1);
#endif

   png_debug(0, "Reading and writing end_info data\n");

   png_read_end(read_ptr, end_info_ptr);
#if defined(PNG_TEXT_SUPPORTED)
   {
      png_textp text_ptr;
      int num_text;

      if (png_get_text(read_ptr, end_info_ptr, &text_ptr, &num_text) > 0)
      {
         png_debug1(0, "Handling %d iTXt/tEXt/zTXt chunks\n", num_text);
         png_set_text(write_ptr, write_end_info_ptr, text_ptr, num_text);
      }
   }
#endif
#if defined(PNG_tIME_SUPPORTED)
   {
      png_timep mod_time;

      if (png_get_tIME(read_ptr, end_info_ptr, &mod_time))
      {
         png_set_tIME(write_ptr, write_end_info_ptr, mod_time);
#if defined(PNG_TIME_RFC1123_SUPPORTED)
         /* we have to use png_strcpy instead of "=" because the string
            pointed to by png_convert_to_rfc1123() gets free'ed before
            we use it */
         png_strcpy(tIME_string,png_convert_to_rfc1123(read_ptr, mod_time));
         tIME_chunk_present++;
#endif /* PNG_TIME_RFC1123_SUPPORTED */
      }
   }
#endif
#if defined(PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED)
   {
      png_unknown_chunkp unknowns;
      int num_unknowns;
      num_unknowns = (int)png_get_unknown_chunks(read_ptr, end_info_ptr,
         &unknowns);