text: 
  /* pngtrans.c - transforms the data in a row (used by both readers and writers)
   *
   * Copyright (c) 2018 Cosmin Truta
   * Copyright (c) 1998-2002,2004,2006-2018 Glenn Randers-Pehrson
   * Copyright (c) 1996-1997 Andreas Dilger
   * Copyright (c) 1995-1996 Guy Eric Schalnat, Group 42, Inc.
   *
   * This code is released under the libpng license.
   * For conditions of distribution and use, see the disclaimer
   * and license in png.h
   */
  
  #include "pngpriv.h"
  
  #if defined(PNG_READ_SUPPORTED) || defined(PNG_WRITE_SUPPORTED)
  
  #if defined(PNG_READ_BGR_SUPPORTED) || defined(PNG_WRITE_BGR_SUPPORTED)
  /* Turn on BGR-to-RGB mapping */
  void PNGAPI
  png_set_bgr(png_structrp png_ptr)
  {
     png_debug(1, "in png_set_bgr");
  
     if (png_ptr == NULL)
        return;
  
     png_ptr->transformations |= PNG_BGR;
  }
  #endif
  
  #if defined(PNG_READ_SWAP_SUPPORTED) || defined(PNG_WRITE_SWAP_SUPPORTED)
  /* Turn on 16-bit byte swapping */
  void PNGAPI
  png_set_swap(png_structrp png_ptr)
  {
     png_debug(1, "in png_set_swap");
  
     if (png_ptr == NULL)
        return;
  
     if (png_ptr->bit_depth == 16)
        png_ptr->transformations |= PNG_SWAP_BYTES;
  }
  #endif
  
  #if defined(PNG_READ_PACK_SUPPORTED) || defined(PNG_WRITE_PACK_SUPPORTED)
  /* Turn on pixel packing */
  void PNGAPI
  png_set_packing(png_structrp png_ptr)
  {
     png_debug(1, "in png_set_packing");
  
     if (png_ptr == NULL)
        return;
  
     if (png_ptr->bit_depth < 8)
     {
        png_ptr->transformations |= PNG_PACK;
  #     ifdef PNG_WRITE_SUPPORTED
           png_ptr->usr_bit_depth = 8;
  #     endif
     }
  }
  #endif
  
  #if defined(PNG_READ_PACKSWAP_SUPPORTED)||defined(PNG_WRITE_PACKSWAP_SUPPORTED)
  /* Turn on packed pixel swapping */
  void PNGAPI
  png_set_packswap(png_structrp png_ptr)
  {
     png_debug(1, "in png_set_packswap");
  
     if (png_ptr == NULL)
        return;
  
     if (png_ptr->bit_depth < 8)
        png_ptr->transformations |= PNG_PACKSWAP;
  }
  #endif
  
  #if defined(PNG_READ_SHIFT_SUPPORTED) || defined(PNG_WRITE_SHIFT_SUPPORTED)
  void PNGAPI
  png_set_shift(png_structrp png_ptr, png_const_color_8p true_bits)
  {
     png_debug(1, "in png_set_shift");
  
     if (png_ptr == NULL)
        return;
  
     png_ptr->transformations |= PNG_SHIFT;
     png_ptr->shift = *true_bits;
  }
  #endif
  
  #if defined(PNG_READ_INTERLACING_SUPPORTED) || \
      defined(PNG_WRITE_INTERLACING_SUPPORTED)
  int PNGAPI
  png_set_interlace_handling(png_structrp png_ptr)
  {
     png_debug(1, "in png_set_interlace handling");
  
     if (png_ptr != 0 && png_ptr->interlaced != 0)
     {
        png_ptr->transformations |= PNG_INTERLACE;
        return (7);
     }
  
     return (1);
  }
  #endif
  
  #if defined(PNG_READ_FILLER_SUPPORTED) || defined(PNG_WRITE_FILLER_SUPPORTED)
  /* Add a filler byte on read, or remove a filler or alpha byte on write.
   * The filler type has changed in v0.95 to allow future 2-byte fillers
   * for 48-bit input data, as well as to avoid problems with some compilers
   * that don't like bytes as parameters.
   */
  void PNGAPI
  png_set_filler(png_structrp png_ptr, png_uint_32 filler, int filler_loc)
  {
     png_debug(1, "in png_set_filler");
  
     if (png_ptr == NULL)
        return;
  
     /* In libpng 1.6 it is possible to determine whether this is a read or write
      * operation and therefore to do more checking here for a valid call.
      */
     if ((png_ptr->mode & PNG_IS_READ_STRUCT) != 0)
     {
  #     ifdef PNG_READ_FILLER_SUPPORTED
           /* On read png_set_filler is always valid, regardless of the base PNG
            * format, because other transformations can give a format where the
            * filler code can execute (basically an 8 or 16-bit component RGB or G
            * format.)
            *
            * NOTE: usr_channels is not used by the read code!  (This has led to
            * confusion in the past.)  The filler is only used in the read code.
            */
           png_ptr->filler = (png_uint_16)filler;
  #     else
           png_app_error(png_ptr, "png_set_filler not supported on read");
           PNG_UNUSED(filler) /* not used in the write case */
           return;
  #     endif
     }
  
     else /* write */
     {
  #     ifdef PNG_WRITE_FILLER_SUPPORTED
           /* On write the usr_channels parameter must be set correctly at the
            * start to record the number of channels in the app-supplied data.
            */
           switch (png_ptr->color_type)
           {
              case PNG_COLOR_TYPE_RGB:
                 png_ptr->usr_channels = 4;
                 break;
  
              case PNG_COLOR_TYPE_GRAY:
                 if (png_ptr->bit_depth >= 8)
                 {
                    png_ptr->usr_channels = 2;
                    break;
                 }
  
                 else
                 {
                    /* There simply isn't any code in libpng to strip out bits
                     * from bytes when the components are less than a byte in
                     * size!
                     */
                    png_app_error(png_ptr,
                        "png_set_filler is invalid for"
                        " low bit depth gray output");
                    return;
                 }
  
              default:
                 png_app_error(png_ptr,
                     "png_set_filler: inappropriate color type");
                 return;
           }
  #     else
           png_app_error(png_ptr, "png_set_filler not supported on write");
           return;
  #     endif
     }
  
     /* Here on success - libpng supports the operation, set the transformation
      * and the flag to say where the filler channel is.
      */
     png_ptr->transformations |= PNG_FILLER;
  
     if (filler_loc == PNG_FILLER_AFTER)
        png_ptr->flags |= PNG_FLAG_FILLER_AFTER;
  
     else
        png_ptr->flags &= ~PNG_FLAG_FILLER_AFTER;
  }
  
  /* Added to libpng-1.2.7 */
  void PNGAPI
  png_set_add_alpha(png_structrp png_ptr, png_uint_32 filler, int filler_loc)
  {
     png_debug(1, "in png_set_add_alpha");
  
     if (png_ptr == NULL)
        return;
  
     png_set_filler(png_ptr, filler, filler_loc);
     /* The above may fail to do anything. */
     if ((png_ptr->transformations & PNG_FILLER) != 0)
        png_ptr->transformations |= PNG_ADD_ALPHA;
  }
  
  #endif
  
  #if defined(PNG_READ_SWAP_ALPHA_SUPPORTED) || \
      defined(PNG_WRITE_SWAP_ALPHA_SUPPORTED)
  void PNGAPI
  png_set_swap_alpha(png_structrp png_ptr)
  {
     png_debug(1, "in png_set_swap_alpha");
  
     if (png_ptr == NULL)
        return;
  
     png_ptr->transformations |= PNG_SWAP_ALPHA;
  }
  #endif
  
  #if defined(PNG_READ_INVERT_ALPHA_SUPPORTED) || \
      defined(PNG_WRITE_INVERT_ALPHA_SUPPORTED)
  void PNGAPI
  png_set_invert_alpha(png_structrp png_ptr)
  {
     png_debug(1, "in png_set_invert_alpha");
  
     if (png_ptr == NULL)
        return;
  
     png_ptr->transformations |= PNG_INVERT_ALPHA;
  }
  #endif
  
  #if defined(PNG_READ_INVERT_SUPPORTED) || defined(PNG_WRITE_INVERT_SUPPORTED)
  void PNGAPI
  png_set_invert_mono(png_structrp png_ptr)
  {
     png_debug(1, "in png_set_invert_mono");
  
     if (png_ptr == NULL)
        return;
  
     png_ptr->transformations |= PNG_INVERT_MONO;
  }
  
  /* Invert monochrome grayscale data */
  void /* PRIVATE */
  png_do_invert(png_row_infop row_info, png_bytep row)
  {
     png_debug(1, "in png_do_invert");
  
    /* This test removed from libpng version 1.0.13 and 1.2.0:
     *   if (row_info->bit_depth == 1 &&
     */
     if (row_info->color_type == PNG_COLOR_TYPE_GRAY)
     {
        png_bytep rp = row;
        size_t i;
        size_t istop = row_info->rowbytes;
  
        for (i = 0; i < istop; i++)
        {
           *rp = (png_byte)(~(*rp));
           rp++;
        }
     }
  
     else if (row_info->color_type == PNG_COLOR_TYPE_GRAY_ALPHA &&
        row_info->bit_depth == 8)
     {
        png_bytep rp = row;
        size_t i;
        size_t istop = row_info->rowbytes;
  
        for (i = 0; i < istop; i += 2)
        {
           *rp = (png_byte)(~(*rp));
           rp += 2;
        }
     }
  
  #ifdef PNG_16BIT_SUPPORTED
     else if (row_info->color_type == PNG_COLOR_TYPE_GRAY_ALPHA &&
        row_info->bit_depth == 16)
     {
        png_bytep rp = row;
        size_t i;
        size_t istop = row_info->rowbytes;
  
        for (i = 0; i < istop; i += 4)
        {
           *rp = (png_byte)(~(*rp));
           *(rp + 1) = (png_byte)(~(*(rp + 1)));
           rp += 4;
        }
     }
  #endif
  }
  #endif
  
  #ifdef PNG_16BIT_SUPPORTED
  #if defined(PNG_READ_SWAP_SUPPORTED) || defined(PNG_WRITE_SWAP_SUPPORTED)
  /* Swaps byte order on 16-bit depth images */
  void /* PRIVATE */
  png_do_swap(png_row_infop row_info, png_bytep row)
  {
     png_debug(1, "in png_do_swap");
  
     if (row_info->bit_depth == 16)
     {
        png_bytep rp = row;
        png_uint_32 i;
        png_uint_32 istop= row_info->width * row_info->channels;
  
        for (i = 0; i < istop; i++, rp += 2)
        {
  #ifdef PNG_BUILTIN_BSWAP16_SUPPORTED
           /* Feature added to libpng-1.6.11 for testing purposes, not
            * enabled by default.
            */
           *(png_uint_16*)rp = __builtin_bswap16(*(png_uint_16*)rp);
  #else
           png_byte t = *rp;
           *rp = *(rp + 1);
           *(rp + 1) = t;
  #endif
        }
     }
  }
  #endif
  #endif
  
  #if defined(PNG_READ_PACKSWAP_SUPPORTED)||defined(PNG_WRITE_PACKSWAP_SUPPORTED)
  static const png_byte onebppswaptable[256] = {
     0x00, 0x80, 0x40, 0xC0, 0x20, 0xA0, 0x60, 0xE0,
     0x10, 0x90, 0x50, 0xD0, 0x30, 0xB0, 0x70, 0xF0,
     0x08, 0x88, 0x48, 0xC8, 0x28, 0xA8, 0x68, 0xE8,
     0x18, 0x98, 0x58, 0xD8, 0x38, 0xB8, 0x78, 0xF8,
     0x04, 0x84, 0x44, 0xC4, 0x24, 0xA4, 0x64, 0xE4,
     0x14, 0x94, 0x54, 0xD4, 0x34, 0xB4, 0x74, 0xF4,
     0x0C, 0x8C, 0x4C, 0xCC, 0x2C, 0xAC, 0x6C, 0xEC,
     0x1C, 0x9C, 0x5C, 0xDC, 0x3C, 0xBC, 0x7C, 0xFC,
     0x02, 0x82, 0x42, 0xC2, 0x22, 0xA2, 0x62, 0xE2,
     0x12, 0x92, 0x52, 0xD2, 0x32, 0xB2, 0x72, 0xF2,
     0x0A, 0x8A, 0x4A, 0xCA, 0x2A, 0xAA, 0x6A, 0xEA,
     0x1A, 0x9A, 0x5A, 0xDA, 0x3A, 0xBA, 0x7A, 0xFA,
     0x06, 0x86, 0x46, 0xC6, 0x26, 0xA6, 0x66, 0xE6,
     0x16, 0x96, 0x56, 0xD6, 0x36, 0xB6, 0x76, 0xF6,
     0x0E, 0x8E, 0x4E, 0xCE, 0x2E, 0xAE, 0x6E, 0xEE,
     0x1E, 0x9E, 0x5E, 0xDE, 0x3E, 0xBE, 0x7E, 0xFE,
     0x01, 0x81, 0x41, 0xC1, 0x21, 0xA1, 0x61, 0xE1,
     0x11, 0x91, 0x51, 0xD1, 0x31, 0xB1, 0x71, 0xF1,
     0x09, 0x89, 0x49, 0xC9, 0x29, 0xA9, 0x69, 0xE9,
     0x19, 0x99, 0x59, 0xD9, 0x39, 0xB9, 0x79, 0xF9,
     0x05, 0x85, 0x45, 0xC5, 0x25, 0xA5, 0x65, 0xE5,
     0x15, 0x95, 0x55, 0xD5, 0x35, 0xB5, 0x75, 0xF5,
     0x0D, 0x8D, 0x4D, 0xCD, 0x2D, 0xAD, 0x6D, 0xED,
     0x1D, 0x9D, 0x5D, 0xDD, 0x3D, 0xBD, 0x7D, 0xFD,
     0x03, 0x83, 0x43, 0xC3, 0x23, 0xA3, 0x63, 0xE3,
     0x13, 0x93, 0x53, 0xD3, 0x33, 0xB3, 0x73, 0xF3,
     0x0B, 0x8B, 0x4B, 0xCB, 0x2B, 0xAB, 0x6B, 0xEB,
     0x1B, 0x9B, 0x5B, 0xDB, 0x3B, 0xBB, 0x7B, 0xFB,
     0x07, 0x87, 0x47, 0xC7, 0x27, 0xA7, 0x67, 0xE7,
     0x17, 0x97, 0x57, 0xD7, 0x37, 0xB7, 0x77, 0xF7,
     0x0F, 0x8F, 0x4F, 0xCF, 0x2F, 0xAF, 0x6F, 0xEF,
     0x1F, 0x9F, 0x5F, 0xDF, 0x3F, 0xBF, 0x7F, 0xFF
  };
  
  static const png_byte twobppswaptable[256] = {
     0x00, 0x40, 0x80, 0xC0, 0x10, 0x50, 0x90, 0xD0,
     0x20, 0x60, 0xA0, 0xE0, 0x30, 0x70, 0xB0, 0xF0,
     0x04, 0x44, 0x84, 0xC4, 0x14, 0x54, 0x94, 0xD4,
     0x24, 0x64, 0xA4, 0xE4, 0x34, 0x74, 0xB4, 0xF4,
     0x08, 0x48, 0x88, 0xC8, 0x18, 0x58, 0x98, 0xD8,
     0x28, 0x68, 0xA8, 0xE8, 0x38, 0x78, 0xB8, 0xF8,
     0x0C, 0x4C, 0x8C, 0xCC, 0x1C, 0x5C, 0x9C, 0xDC,
     0x2C, 0x6C, 0xAC, 0xEC, 0x3C, 0x7C, 0xBC, 0xFC,
     0x01, 0x41, 0x81, 0xC1, 0x11, 0x51, 0x91, 0xD1,
     0x21, 0x61, 0xA1, 0xE1, 0x31, 0x71, 0xB1, 0xF1,
     0x05, 0x45, 0x85, 0xC5, 0x15, 0x55, 0x95, 0xD5,
     0x25, 0x65, 0xA5, 0xE5, 0x35, 0x75, 0xB5, 0xF5,
     0x09, 0x49, 0x89, 0xC9, 0x19, 0x59, 0x99, 0xD9,
     0x29, 0x69, 0xA9, 0xE9, 0x39, 0x79, 0xB9, 0xF9,
     0x0D, 0x4D, 0x8D, 0xCD, 0x1D, 0x5D, 0x9D, 0xDD,
     0x2D, 0x6D, 0xAD, 0xED, 0x3D, 0x7D, 0xBD, 0xFD,
     0x02, 0x42, 0x82, 0xC2, 0x12, 0x52, 0x92, 0xD2,
     0x22, 0x62, 0xA2, 0xE2, 0x32, 0x72, 0xB2, 0xF2,
     0x06, 0x46, 0x86, 0xC6, 0x16, 0x56, 0x96, 0xD6,
     0x26, 0x66, 0xA6, 0xE6, 0x36, 0x76, 0xB6, 0xF6,
     0x0A, 0x4A, 0x8A, 0xCA, 0x1A, 0x5A, 0x9A, 0xDA,
     0x2A, 0x6A, 0xAA, 0xEA, 0x3A, 0x7A, 0xBA, 0xFA,
     0x0E, 0x4E, 0x8E, 0xCE, 0x1E, 0x5E, 0x9E, 0xDE,
     0x2E, 0x6E, 0xAE, 0xEE, 0x3E, 0x7E, 0xBE, 0xFE,
     0x03, 0x43, 0x83, 0xC3, 0x13, 0x53, 0x93, 0xD3,
     0x23, 0x63, 0xA3, 0xE3, 0x33, 0x73, 0xB3, 0xF3,
     0x07, 0x47, 0x87, 0xC7, 0x17, 0x57, 0x97, 0xD7,
     0x27, 0x67, 0xA7, 0xE7, 0x37, 0x77, 0xB7, 0xF7,
     0x0B, 0x4B, 0x8B, 0xCB, 0x1B, 0x5B, 0x9B, 0xDB,
     0x2B, 0x6B, 0xAB, 0xEB, 0x3B, 0x7B, 0xBB, 0xFB,
     0x0F, 0x4F, 0x8F, 0xCF, 0x1F, 0x5F, 0x9F, 0xDF,
     0x2F, 0x6F, 0xAF, 0xEF, 0x3F, 0x7F, 0xBF, 0xFF
  };
  
  static const png_byte fourbppswaptable[256] = {
     0x00, 0x10, 0x20, 0x30, 0x40, 0x50, 0x60, 0x70,
     0x80, 0x90, 0xA0, 0xB0, 0xC0, 0xD0, 0xE0, 0xF0,
     0x01, 0x11, 0x21, 0x31, 0x41, 0x51, 0x61, 0x71,
     0x81, 0x91, 0xA1, 0xB1, 0xC1, 0xD1, 0xE1, 0xF1,
     0x02, 0x12, 0x22, 0x32, 0x42, 0x52, 0x62, 0x72,
     0x82, 0x92, 0xA2, 0xB2, 0xC2, 0xD2, 0xE2, 0xF2,
     0x03, 0x13, 0x23, 0x33, 0x43, 0x53, 0x63, 0x73,
     0x83, 0x93, 0xA3, 0xB3, 0xC3, 0xD3, 0xE3, 0xF3,
     0x04, 0x14, 0x24, 0x34, 0x44, 0x54, 0x64, 0x74,
     0x84, 0x94, 0xA4, 0xB4, 0xC4, 0xD4, 0xE4, 0xF4,
     0x05, 0x15, 0x25, 0x35, 0x45, 0x55, 0x65, 0x75,
     0x85, 0x95, 0xA5, 0xB5, 0xC5, 0xD5, 0xE5, 0xF5,
     0x06, 0x16, 0x26, 0x36, 0x46, 0x56, 0x66, 0x76,
     0x86, 0x96, 0xA6, 0xB6, 0xC6, 0xD6, 0xE6, 0xF6,
     0x07, 0x17, 0x27, 0x37, 0x47, 0x57, 0x67, 0x77,
     0x87, 0x97, 0xA7, 0xB7, 0xC7, 0xD7, 0xE7, 0xF7,
     0x08, 0x18, 0x28, 0x38, 0x48, 0x58, 0x68, 0x78,
     0x88, 0x98, 0xA8, 0xB8, 0xC8, 0xD8, 0xE8, 0xF8,
     0x09, 0x19, 0x29, 0x39, 0x49, 0x59, 0x69, 0x79,
     0x89, 0x99, 0xA9, 0xB9, 0xC9, 0xD9, 0xE9, 0xF9,
     0x0A, 0x1A, 0x2A, 0x3A, 0x4A, 0x5A, 0x6A, 0x7A,
     0x8A, 0x9A, 0xAA, 0xBA, 0xCA, 0xDA, 0xEA, 0xFA,
     0x0B, 0x1B, 0x2B, 0x3B, 0x4B, 0x5B, 0x6B, 0x7B,
     0x8B, 0x9B, 0xAB, 0xBB, 0xCB, 0xDB, 0xEB, 0xFB,
     0x0C, 0x1C, 0x2C, 0x3C, 0x4C, 0x5C, 0x6C, 0x7C,
     0x8C, 0x9C, 0xAC, 0xBC, 0xCC, 0xDC, 0xEC, 0xFC,
     0x0D, 0x1D, 0x2D, 0x3D, 0x4D, 0x5D, 0x6D, 0x7D,
     0x8D, 0x9D, 0xAD, 0xBD, 0xCD, 0xDD, 0xED, 0xFD,
     0x0E, 0x1E, 0x2E, 0x3E, 0x4E, 0x5E, 0x6E, 0x7E,
     0x8E, 0x9E, 0xAE, 0xBE, 0xCE, 0xDE, 0xEE, 0xFE,
     0x0F, 0x1F, 0x2F, 0x3F, 0x4F, 0x5F, 0x6F, 0x7F,
     0x8F, 0x9F, 0xAF, 0xBF, 0xCF, 0xDF, 0xEF, 0xFF
  };
  
  /* Swaps pixel packing order within bytes */
  void /* PRIVATE */
  png_do_packswap(png_row_infop row_info, png_bytep row)
  {
     png_debug(1, "in png_do_packswap");
  
     if (row_info->bit_depth < 8)
     {
        png_bytep rp;
        png_const_bytep end, table;
  
        end = row + row_info->rowbytes;
  
        if (row_info->bit_depth == 1)
           table = onebppswaptable;
  
        else if (row_info->bit_depth == 2)
           table = twobppswaptable;
  
        else if (row_info->bit_depth == 4)
           table = fourbppswaptable;
  
        else
           return;
  
        for (rp = row; rp < end; rp++)
           *rp = table[*rp];
     }
  }
  #endif /* PACKSWAP || WRITE_PACKSWAP */
  
  #if defined(PNG_WRITE_FILLER_SUPPORTED) || \
      defined(PNG_READ_STRIP_ALPHA_SUPPORTED)
  /* Remove a channel - this used to be 'png_do_strip_filler' but it used a
   * somewhat weird combination of flags to determine what to do.  All the calls
   * to png_do_strip_filler are changed in 1.5.2 to call this instead with the
   * correct arguments.
   *
   * The routine isn't general - the channel must be the channel at the start or
   * end (not in the middle) of each pixel.
   */
  void /* PRIVATE */
  png_do_strip_channel(png_row_infop row_info, png_bytep row, int at_start)
  {
     png_bytep sp = row; /* source pointer */
     png_bytep dp = row; /* destination pointer */
     png_bytep ep = row + row_info->rowbytes; /* One beyond end of row */
  
     /* At the start sp will point to the first byte to copy and dp to where
      * it is copied to.  ep always points just beyond the end of the row, so
      * the loop simply copies (channels-1) channels until sp reaches ep.
      *
      * at_start:        0 -- convert AG, XG, ARGB, XRGB, AAGG, XXGG, etc.
      *            nonzero -- convert GA, GX, RGBA, RGBX, GGAA, RRGGBBXX, etc.
      */
  
     /* GA, GX, XG cases */
     if (row_info->channels == 2)
     {
        if (row_info->bit_depth == 8)
        {
           if (at_start != 0) /* Skip initial filler */
              ++sp;
           else          /* Skip initial channel and, for sp, the filler */
           {
              sp += 2; ++dp;
           }
  
           /* For a 1 pixel wide image there is nothing to do */
           while (sp < ep)
           {
              *dp++ = *sp; sp += 2;
           }
  
           row_info->pixel_depth = 8;
        }
  
        else if (row_info->bit_depth == 16)
        {
           if (at_start != 0) /* Skip initial filler */
              sp += 2;
           else          /* Skip initial channel and, for sp, the filler */
           {
              sp += 4; dp += 2;
           }
  
           while (sp < ep)
           {
              *dp++ = *sp++; *dp++ = *sp; sp += 3;
           }
  
           row_info->pixel_depth = 16;
        }
  
        else
           return; /* bad bit depth */
  
        row_info->channels = 1;
  
        /* Finally fix the color type if it records an alpha channel */
        if (row_info->color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
           row_info->color_type = PNG_COLOR_TYPE_GRAY;
     }
  
     /* RGBA, RGBX, XRGB cases */
     else if (row_info->channels == 4)
     {
        if (row_info->bit_depth == 8)
        {
           if (at_start != 0) /* Skip initial filler */
              ++sp;
           else          /* Skip initial channels and, for sp, the filler */
           {
              sp += 4; dp += 3;
           }
  
           /* Note that the loop adds 3 to dp and 4 to sp each time. */
           while (sp < ep)
           {
              *dp++ = *sp++; *dp++ = *sp++; *dp++ = *sp; sp += 2;
           }
  
           row_info->pixel_depth = 24;
        }
  
        else if (row_info->bit_depth == 16)
        {
           if (at_start != 0) /* Skip initial filler */
              sp += 2;
           else          /* Skip initial channels and, for sp, the filler */
           {
              sp += 8; dp += 6;
           }
  
           while (sp < ep)
           {
              /* Copy 6 bytes, skip 2 */
              *dp++ = *sp++; *dp++ = *sp++;
              *dp++ = *sp++; *dp++ = *sp++;
              *dp++ = *sp++; *dp++ = *sp; sp += 3;
           }
  
           row_info->pixel_depth = 48;
        }
  
        else
           return; /* bad bit depth */
  
        row_info->channels = 3;
  
        /* Finally fix the color type if it records an alpha channel */
        if (row_info->color_type == PNG_COLOR_TYPE_RGB_ALPHA)
           row_info->color_type = PNG_COLOR_TYPE_RGB;
     }
  
     else
        return; /* The filler channel has gone already */
  
     /* Fix the rowbytes value. */
     row_info->rowbytes = (size_t)(dp-row);
  }
  #endif
  
  #if defined(PNG_READ_BGR_SUPPORTED) || defined(PNG_WRITE_BGR_SUPPORTED)
  /* Swaps red and blue bytes within a pixel */
  void /* PRIVATE */
  png_do_bgr(png_row_infop row_info, png_bytep row)
  {
     png_debug(1, "in png_do_bgr");
  
     if ((row_info->color_type & PNG_COLOR_MASK_COLOR) != 0)
     {
        png_uint_32 row_width = row_info->width;
        if (row_info->bit_depth == 8)
        {
           if (row_info->color_type == PNG_COLOR_TYPE_RGB)
           {
              png_bytep rp;
              png_uint_32 i;
  
              for (i = 0, rp = row; i < row_width; i++, rp += 3)
              {
                 png_byte save = *rp;
                 *rp = *(rp + 2);
                 *(rp + 2) = save;
              }
           }
  
           else if (row_info->color_type == PNG_COLOR_TYPE_RGB_ALPHA)
           {
              png_bytep rp;
              png_uint_32 i;
  
              for (i = 0, rp = row; i < row_width; i++, rp += 4)
              {
                 png_byte save = *rp;
                 *rp = *(rp + 2);
                 *(rp + 2) = save;
              }
           }
        }
  
  #ifdef PNG_16BIT_SUPPORTED
        else if (row_info->bit_depth == 16)
        {
           if (row_info->color_type == PNG_COLOR_TYPE_RGB)
           {
              png_bytep rp;
              png_uint_32 i;
  
              for (i = 0, rp = row; i < row_width; i++, rp += 6)
              {
                 png_byte save = *rp;
                 *rp = *(rp + 4);
                 *(rp + 4) = save;
                 save = *(rp + 1);
                 *(rp + 1) = *(rp + 5);
                 *(rp + 5) = save;
              }
           }
  
           else if (row_info->color_type == PNG_COLOR_TYPE_RGB_ALPHA)
           {
              png_bytep rp;
              png_uint_32 i;
  
              for (i = 0, rp = row; i < row_width; i++, rp += 8)
              {
                 png_byte save = *rp;
                 *rp = *(rp + 4);
                 *(rp + 4) = save;
                 save = *(rp + 1);
                 *(rp + 1) = *(rp + 5);
                 *(rp + 5) = save;
              }
           }
        }
  #endif
     }
  }
  #endif /* READ_BGR || WRITE_BGR */
  
  #if defined(PNG_READ_CHECK_FOR_INVALID_INDEX_SUPPORTED) || \
      defined(PNG_WRITE_CHECK_FOR_INVALID_INDEX_SUPPORTED)
  /* Added at libpng-1.5.10 */
  void /* PRIVATE */
  png_do_check_palette_indexes(png_structrp png_ptr, png_row_infop row_info)
  {
     if (png_ptr->num_palette < (1 << row_info->bit_depth) &&
        png_ptr->num_palette > 0) /* num_palette can be 0 in MNG files */
     {
        /* Calculations moved outside switch in an attempt to stop different
         * compiler warnings.  'padding' is in *bits* within the last byte, it is
         * an 'int' because pixel_depth becomes an 'int' in the expression below,
         * and this calculation is used because it avoids warnings that other
         * forms produced on either GCC or MSVC.
         */
        int padding = PNG_PADBITS(row_info->pixel_depth, row_info->width);
        png_bytep rp = png_ptr->row_buf + row_info->rowbytes - 1;
  
        switch (row_info->bit_depth)
        {
           case 1:
           {
              /* in this case, all bytes must be 0 so we don't need
               * to unpack the pixels except for the rightmost one.
               */
              for (; rp > png_ptr->row_buf; rp--)
              {
                if ((*rp >> padding) != 0)
                   png_ptr->num_palette_max = 1;
                padding = 0;
              }
  
              break;
           }
  
           case 2:
           {
              for (; rp > png_ptr->row_buf; rp--)
              {
                int i = ((*rp >> padding) & 0x03);
  
                if (i > png_ptr->num_palette_max)
                   png_ptr->num_palette_max = i;
  
                i = (((*rp >> padding) >> 2) & 0x03);
  
                if (i > png_ptr->num_palette_max)
                   png_ptr->num_palette_max = i;
  
                i = (((*rp >> padding) >> 4) & 0x03);
  
                if (i > png_ptr->num_palette_max)
                   png_ptr->num_palette_max = i;
  
                i = (((*rp >> padding) >> 6) & 0x03);
  
                if (i > png_ptr->num_palette_max)
                   png_ptr->num_palette_max = i;
  
                padding = 0;
              }
  
              break;
           }
  
           case 4:
           {
              for (; rp > png_ptr->row_buf; rp--)
              {
                int i = ((*rp >> padding) & 0x0f);
  
                if (i > png_ptr->num_palette_max)
                   png_ptr->num_palette_max = i;
  
                i = (((*rp >> padding) >> 4) & 0x0f);
  
                if (i > png_ptr->num_palette_max)
                   png_ptr->num_palette_max = i;
  
                padding = 0;
              }
  
              break;
           }
  
           case 8:
           {
              for (; rp > png_ptr->row_buf; rp--)
              {
                 if (*rp > png_ptr->num_palette_max)
                    png_ptr->num_palette_max = (int) *rp;
              }
  
              break;
           }
  
           default:
              break;
        }
     }
  }
  #endif /* CHECK_FOR_INVALID_INDEX */
  
  #if defined(PNG_READ_USER_TRANSFORM_SUPPORTED) || \
      defined(PNG_WRITE_USER_TRANSFORM_SUPPORTED)
  #ifdef PNG_USER_TRANSFORM_PTR_SUPPORTED
  void PNGAPI
  png_set_user_transform_info(png_structrp png_ptr, png_voidp
     user_transform_ptr, int user_transform_depth, int user_transform_channels)
  {
     png_debug(1, "in png_set_user_transform_info");
  
     if (png_ptr == NULL)
        return;
  
  #ifdef PNG_READ_USER_TRANSFORM_SUPPORTED
     if ((png_ptr->mode & PNG_IS_READ_STRUCT) != 0 &&
        (png_ptr->flags & PNG_FLAG_ROW_INIT) != 0)
     {
        png_app_error(png_ptr,
            "info change after png_start_read_image or png_read_update_info");
        return;
     }
  #endif
  
     png_ptr->user_transform_ptr = user_transform_ptr;
     png_ptr->user_transform_depth = (png_byte)user_transform_depth;
     png_ptr->user_transform_channels = (png_byte)user_transform_channels;
  }
  #endif
  
  /* This function returns a pointer to the user_transform_ptr associated with
   * the user transform functions.  The application should free any memory
   * associated with this pointer before png_write_destroy and png_read_destroy
   * are called.
   */
  #ifdef PNG_USER_TRANSFORM_PTR_SUPPORTED
  png_voidp PNGAPI
  png_get_user_transform_ptr(png_const_structrp png_ptr)
  {
     if (png_ptr == NULL)
        return (NULL);
  
     return png_ptr->user_transform_ptr;
  }
  #endif
  
  #ifdef PNG_USER_TRANSFORM_INFO_SUPPORTED
  png_uint_32 PNGAPI
  png_get_current_row_number(png_const_structrp png_ptr)
  {
     /* See the comments in png.h - this is the sub-image row when reading an
      * interlaced image.
      */
     if (png_ptr != NULL)
        return png_ptr->row_number;
  
     return PNG_UINT_32_MAX; /* help the app not to fail silently */
  }
  
  png_byte PNGAPI
  png_get_current_pass_number(png_const_structrp png_ptr)
  {
     if (png_ptr != NULL)
        return png_ptr->pass;
     return 8; /* invalid */
  }
  #endif /* USER_TRANSFORM_INFO */
  #endif /* READ_USER_TRANSFORM || WRITE_USER_TRANSFORM */
  #endif /* READ || WRITE */
  