/* pngrtran.c - transforms the data in a row for PNG readers
 *
 * Last changed in libpng 1.5.7 [December 15, 2011]
 * Copyright (c) 1998-2011 Glenn Randers-Pehrson
 * (Version 0.96 Copyright (c) 1996, 1997 Andreas Dilger)
 * (Version 0.88 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
 *
 * This file contains functions optionally called by an application
 * in order to tell libpng how to handle data when reading a PNG.
 * Transformations that are used in both reading and writing are
 * in pngtrans.c.
 */

#include "pngpriv.h"

#ifdef PNG_READ_SUPPORTED

/* Set the action on getting a CRC error for an ancillary or critical chunk. */
void PNGAPI
png_set_crc_action(png_structp png_ptr, int crit_action, int ancil_action)
{
   png_debug(1, "in png_set_crc_action");

   if (png_ptr == NULL)
      return;

   /* Tell libpng how we react to CRC errors in critical chunks */
   switch (crit_action)
   {
      case PNG_CRC_NO_CHANGE:                        /* Leave setting as is */
         break;

      case PNG_CRC_WARN_USE:                               /* Warn/use data */
         png_ptr->flags &= ~PNG_FLAG_CRC_CRITICAL_MASK;
         png_ptr->flags |= PNG_FLAG_CRC_CRITICAL_USE;
         break;

      case PNG_CRC_QUIET_USE:                             /* Quiet/use data */
         png_ptr->flags &= ~PNG_FLAG_CRC_CRITICAL_MASK;
         png_ptr->flags |= PNG_FLAG_CRC_CRITICAL_USE |
                           PNG_FLAG_CRC_CRITICAL_IGNORE;
         break;

      case PNG_CRC_WARN_DISCARD:    /* Not a valid action for critical data */
         png_warning(png_ptr,
            "Can't discard critical data on CRC error");
      case PNG_CRC_ERROR_QUIT:                                /* Error/quit */

      case PNG_CRC_DEFAULT:
      default:
         png_ptr->flags &= ~PNG_FLAG_CRC_CRITICAL_MASK;
         break;
   }

   /* Tell libpng how we react to CRC errors in ancillary chunks */
   switch (ancil_action)
   {
      case PNG_CRC_NO_CHANGE:                       /* Leave setting as is */
         break;

      case PNG_CRC_WARN_USE:                              /* Warn/use data */
         png_ptr->flags &= ~PNG_FLAG_CRC_ANCILLARY_MASK;
         png_ptr->flags |= PNG_FLAG_CRC_ANCILLARY_USE;
         break;

      case PNG_CRC_QUIET_USE:                            /* Quiet/use data */
         png_ptr->flags &= ~PNG_FLAG_CRC_ANCILLARY_MASK;
         png_ptr->flags |= PNG_FLAG_CRC_ANCILLARY_USE |
                           PNG_FLAG_CRC_ANCILLARY_NOWARN;
         break;

      case PNG_CRC_ERROR_QUIT:                               /* Error/quit */
         png_ptr->flags &= ~PNG_FLAG_CRC_ANCILLARY_MASK;
         png_ptr->flags |= PNG_FLAG_CRC_ANCILLARY_NOWARN;
         break;

      case PNG_CRC_WARN_DISCARD:                      /* Warn/discard data */

      case PNG_CRC_DEFAULT:
      default:
         png_ptr->flags &= ~PNG_FLAG_CRC_ANCILLARY_MASK;
         break;
   }
}

#ifdef PNG_READ_BACKGROUND_SUPPORTED
/* Handle alpha and tRNS via a background color */
void PNGFAPI
png_set_background_fixed(png_structp png_ptr,
    png_const_color_16p background_color, int background_gamma_code,
    int need_expand, png_fixed_point background_gamma)
{
   png_debug(1, "in png_set_background_fixed");

   if (png_ptr == NULL)
      return;

   if (background_gamma_code == PNG_BACKGROUND_GAMMA_UNKNOWN)
   {
      png_warning(png_ptr, "Application must supply a known background gamma");
      return;
   }

   png_ptr->transformations |= PNG_COMPOSE | PNG_STRIP_ALPHA;
   png_ptr->transformations &= ~PNG_ENCODE_ALPHA;
   png_ptr->flags &= ~PNG_FLAG_OPTIMIZE_ALPHA;

   png_memcpy(&(png_ptr->background), background_color,
      png_sizeof(png_color_16));
   png_ptr->background_gamma = background_gamma;
   png_ptr->background_gamma_type = (png_byte)(background_gamma_code);
   if (need_expand)
      png_ptr->transformations |= PNG_BACKGROUND_EXPAND;
   else
      png_ptr->transformations &= ~PNG_BACKGROUND_EXPAND;
}

#  ifdef PNG_FLOATING_POINT_SUPPORTED
void PNGAPI
png_set_background(png_structp png_ptr,
    png_const_color_16p background_color, int background_gamma_code,
    int need_expand, double background_gamma)
{
   png_set_background_fixed(png_ptr, background_color, background_gamma_code,
      need_expand, png_fixed(png_ptr, background_gamma, "png_set_background"));
}
#  endif  /* FLOATING_POINT */
#endif /* READ_BACKGROUND */

/* Scale 16-bit depth files to 8-bit depth.  If both of these are set then the
 * one that pngrtran does first (scale) happens.  This is necessary to allow the
 * TRANSFORM and API behavior to be somewhat consistent, and it's simpler.
 */
#ifdef PNG_READ_SCALE_16_TO_8_SUPPORTED
void PNGAPI
png_set_scale_16(png_structp png_ptr)
{
   png_debug(1, "in png_set_scale_16");

   if (png_ptr == NULL)
      return;

   png_ptr->transformations |= PNG_SCALE_16_TO_8;
}
#endif

#ifdef PNG_READ_STRIP_16_TO_8_SUPPORTED
/* Chop 16-bit depth files to 8-bit depth */
void PNGAPI
png_set_strip_16(png_structp png_ptr)
{
   png_debug(1, "in png_set_strip_16");

   if (png_ptr == NULL)
      return;

   png_ptr->transformations |= PNG_16_TO_8;
}
#endif

#ifdef PNG_READ_STRIP_ALPHA_SUPPORTED
void PNGAPI
png_set_strip_alpha(png_structp png_ptr)
{
   png_debug(1, "in png_set_strip_alpha");

   if (png_ptr == NULL)
      return;

   png_ptr->transformations |= PNG_STRIP_ALPHA;
}
#endif

#if defined(PNG_READ_ALPHA_MODE_SUPPORTED) || defined(PNG_READ_GAMMA_SUPPORTED)
static png_fixed_point
translate_gamma_flags(png_structp png_ptr, png_fixed_point output_gamma,
   int is_screen)
{
   /* Check for flag values.  The main reason for having the old Mac value as a
    * flag is that it is pretty near impossible to work out what the correct
    * value is from Apple documentation - a working Mac system is needed to
    * discover the value!
    */
   if (output_gamma == PNG_DEFAULT_sRGB ||
      output_gamma == PNG_FP_1 / PNG_DEFAULT_sRGB)
   {
      /* If there is no sRGB support this just sets the gamma to the standard
       * sRGB value.  (This is a side effect of using this function!)
       */
#     ifdef PNG_READ_sRGB_SUPPORTED
         png_ptr->flags |= PNG_FLAG_ASSUME_sRGB;
#     endif
      if (is_screen)
         output_gamma = PNG_GAMMA_sRGB;
      else
         output_gamma = PNG_GAMMA_sRGB_INVERSE;
   }

   else if (output_gamma == PNG_GAMMA_MAC_18 ||
      output_gamma == PNG_FP_1 / PNG_GAMMA_MAC_18)
   {
      if (is_screen)
         output_gamma = PNG_GAMMA_MAC_OLD;
      else
         output_gamma = PNG_GAMMA_MAC_INVERSE;
   }

   return output_gamma;
}

#  ifdef PNG_FLOATING_POINT_SUPPORTED
static png_fixed_point
convert_gamma_value(png_structp png_ptr, double output_gamma)
{
   /* The following silently ignores cases where fixed point (times 100,000)
    * gamma values are passed to the floating point API.  This is safe and it
    * means the fixed point constants work just fine with the floating point
    * API.  The alternative would just lead to undetected errors and spurious
    * bug reports.  Negative values fail inside the _fixed API unless they
    * correspond to the flag values.
    */
   if (output_gamma > 0 && output_gamma < 128)
      output_gamma *= PNG_FP_1;

   /* This preserves -1 and -2 exactly: */
   output_gamma = floor(output_gamma + .5);

   if (output_gamma > PNG_FP_MAX || output_gamma < PNG_FP_MIN)
      png_fixed_error(png_ptr, "gamma value");

   return (png_fixed_point)output_gamma;
}
#  endif
#endif /* READ_ALPHA_MODE || READ_GAMMA */

#ifdef PNG_READ_ALPHA_MODE_SUPPORTED
void PNGFAPI
png_set_alpha_mode_fixed(png_structp png_ptr, int mode,
   png_fixed_point output_gamma)
{
   int compose = 0;
   png_fixed_point file_gamma;

   png_debug(1, "in png_set_alpha_mode");

   if (png_ptr == NULL)
      return;

   output_gamma = translate_gamma_flags(png_ptr, output_gamma, 1/*screen*/);

   /* Validate the value to ensure it is in a reasonable range. The value
    * is expected to be 1 or greater, but this range test allows for some
    * viewing correction values.  The intent is to weed out users of this API
    * who use the inverse of the gamma value accidentally!  Since some of these
    * values are reasonable this may have to be changed.
    */
   if (output_gamma < 70000 || output_gamma > 300000)
      png_error(png_ptr, "output gamma out of expected range");

   /* The default file gamma is the inverse of the output gamma; the output
    * gamma may be changed below so get the file value first:
    */
   file_gamma = png_reciprocal(output_gamma);

   /* There are really 8 possibilities here, composed of any combination
    * of:
    *
    *    premultiply the color channels
    *    do not encode non-opaque pixels
    *    encode the alpha as well as the color channels
    *
    * The differences disappear if the input/output ('screen') gamma is 1.0,
    * because then the encoding is a no-op and there is only the choice of
    * premultiplying the color channels or not.
    *
    * png_set_alpha_mode and png_set_background interact because both use
    * png_compose to do the work.  Calling both is only useful when
    * png_set_alpha_mode is used to set the default mode - PNG_ALPHA_PNG - along
    * with a default gamma value.  Otherwise PNG_COMPOSE must not be set.
    */
   switch (mode)
   {
      case PNG_ALPHA_PNG:        /* default: png standard */
         /* No compose, but it may be set by png_set_background! */
         png_ptr->transformations &= ~PNG_ENCODE_ALPHA;
         png_ptr->flags &= ~PNG_FLAG_OPTIMIZE_ALPHA;
         break;

      case PNG_ALPHA_ASSOCIATED: /* color channels premultiplied */
         compose = 1;
         png_ptr->transformations &= ~PNG_ENCODE_ALPHA;
         png_ptr->flags &= ~PNG_FLAG_OPTIMIZE_ALPHA;
         /* The output is linear: */
         output_gamma = PNG_FP_1;
         break;

      case PNG_ALPHA_OPTIMIZED:  /* associated, non-opaque pixels linear */
         compose = 1;
         png_ptr->transformations &= ~PNG_ENCODE_ALPHA;
         png_ptr->flags |= PNG_FLAG_OPTIMIZE_ALPHA;
         /* output_gamma records the encoding of opaque pixels! */
         break;

      case PNG_ALPHA_BROKEN:     /* associated, non-linear, alpha encoded */
         compose = 1;
         png_ptr->transformations |= PNG_ENCODE_ALPHA;
         png_ptr->flags &= ~PNG_FLAG_OPTIMIZE_ALPHA;
         break;

      default:
         png_error(png_ptr, "invalid alpha mode");
   }

   /* Only set the default gamma if the file gamma has not been set (this has
    * the side effect that the gamma in a second call to png_set_alpha_mode will
    * be ignored.)
    */
   if (png_ptr->gamma == 0)
      png_ptr->gamma = file_gamma;

   /* But always set the output gamma: */
   png_ptr->screen_gamma = output_gamma;

   /* Finally, if pre-multiplying, set the background fields to achieve the
    * desired result.
    */
   if (compose)
   {
      /* And obtain alpha pre-multiplication by composing on black: */
      png_memset(&png_ptr->background, 0, sizeof png_ptr->background);
      png_ptr->background_gamma = png_ptr->gamma; /* just in case */
      png_ptr->background_gamma_type = PNG_BACKGROUND_GAMMA_FILE;
      png_ptr->transformations &= ~PNG_BACKGROUND_EXPAND;

      if (png_ptr->transformations & PNG_COMPOSE)
         png_error(png_ptr,
            "conflicting calls to set alpha mode and background");

      png_ptr->transformations |= PNG_COMPOSE;
   }

   /* New API, make sure apps call the correct initializers: */
   png_ptr->flags |= PNG_FLAG_DETECT_UNINITIALIZED;
}

#  ifdef PNG_FLOATING_POINT_SUPPORTED
void PNGAPI
png_set_alpha_mode(png_structp png_ptr, int mode, double output_gamma)
{
   png_set_alpha_mode_fixed(png_ptr, mode, convert_gamma_value(png_ptr,
      output_gamma));
}
#  endif
#endif

#ifdef PNG_READ_QUANTIZE_SUPPORTED
/* Dither file to 8-bit.  Supply a palette, the current number
 * of elements in the palette, the maximum number of elements
 * allowed, and a histogram if possible.  If the current number
 * of colors is greater then the maximum number, the palette will be
 * modified to fit in the maximum number.  "full_quantize" indicates
 * whether we need a quantizing cube set up for RGB images, or if we
 * simply are reducing the number of colors in a paletted image.
 */

typedef struct png_dsort_struct
{
   struct png_dsort_struct FAR * next;
   png_byte left;
   png_byte right;
} png_dsort;
typedef png_dsort FAR *       png_dsortp;
typedef png_dsort FAR * FAR * png_dsortpp;

void PNGAPI
png_set_quantize(png_structp png_ptr, png_colorp palette,
    int num_palette, int maximum_colors, png_const_uint_16p histogram,
    int full_quantize)
{
   png_debug(1, "in png_set_quantize");

   if (png_ptr == NULL)
      return;

   png_ptr->transformations |= PNG_QUANTIZE;

   if (!full_quantize)
   {
      int i;

      png_ptr->quantize_index = (png_bytep)png_malloc(png_ptr,
          (png_uint_32)(num_palette * png_sizeof(png_byte)));
      for (i = 0; i < num_palette; i++)
         png_ptr->quantize_index[i] = (png_byte)i;
   }

   if (num_palette > maximum_colors)
   {
      if (histogram != NULL)
      {
         /* This is easy enough, just throw out the least used colors.
          * Perhaps not the best solution, but good enough.
          */

         int i;

         /* Initialize an array to sort colors */
         png_ptr->quantize_sort = (png_bytep)png_malloc(png_ptr,
             (png_uint_32)(num_palette * png_sizeof(png_byte)));

         /* Initialize the quantize_sort array */
         for (i = 0; i < num_palette; i++)
            png_ptr->quantize_sort[i] = (png_byte)i;

         /* Find the least used palette entries by starting a
          * bubble sort, and running it until we have sorted
          * out enough colors.  Note that we don't care about
          * sorting all the colors, just finding which are
          * least used.
          */

         for (i = num_palette - 1; i >= maximum_colors; i--)
         {
            int done; /* To stop early if the list is pre-sorted */
            int j;

            done = 1;
            for (j = 0; j < i; j++)
            {
               if (histogram[png_ptr->quantize_sort[j]]
                   < histogram[png_ptr->quantize_sort[j + 1]])
               {
                  png_byte t;

                  t = png_ptr->quantize_sort[j];
                  png_ptr->quantize_sort[j] = png_ptr->quantize_sort[j + 1];
                  png_ptr->quantize_sort[j + 1] = t;
                  done = 0;
               }
            }

            if (done)
               break;
         }

         /* Swap the palette around, and set up a table, if necessary */
         if (full_quantize)
         {
            int j = num_palette;

            /* Put all the useful colors within the max, but don't
             * move the others.
             */
            for (i = 0; i < maximum_colors; i++)
            {
               if ((int)png_ptr->quantize_sort[i] >= maximum_colors)
               {
                  do
                     j--;
                  while ((int)png_ptr->quantize_sort[j] >= maximum_colors);

                  palette[i] = palette[j];
               }
            }
         }
         else
         {
            int j = num_palette;

            /* Move all the used colors inside the max limit, and
             * develop a translation table.
             */
            for (i = 0; i < maximum_colors; i++)
            {
               /* Only move the colors we need to */
               if ((int)png_ptr->quantize_sort[i] >= maximum_colors)
               {
                  png_color tmp_color;

                  do
                     j--;
                  while ((int)png_ptr->quantize_sort[j] >= maximum_colors);

                  tmp_color = palette[j];
                  palette[j] = palette[i];
                  palette[i] = tmp_color;
                  /* Indicate where the color went */
                  png_ptr->quantize_index[j] = (png_byte)i;
                  png_ptr->quantize_index[i] = (png_byte)j;
               }
            }

            /* Find closest color for those colors we are not using */
            for (i = 0; i < num_palette; i++)
            {
               if ((int)png_ptr->quantize_index[i] >= maximum_colors)
               {
                  int min_d, k, min_k, d_index;

                  /* Find the closest color to one we threw out */
                  d_index = png_ptr->quantize_index[i];
                  min_d = PNG_COLOR_DIST(palette[d_index], palette[0]);
                  for (k = 1, min_k = 0; k < maximum_colors; k++)
                  {
                     int d;

                     d = PNG_COLOR_DIST(palette[d_index], palette[k]);

                     if (d < min_d)
                     {
                        min_d = d;
                        min_k = k;
                     }
                  }
                  /* Point to closest color */
                  png_ptr->quantize_index[i] = (png_byte)min_k;
               }
            }
         }
         png_free(png_ptr, png_ptr->quantize_sort);
         png_ptr->quantize_sort = NULL;
      }
      else
      {
         /* This is much harder to do simply (and quickly).  Perhaps
          * we need to go through a median cut routine, but those
          * don't always behave themselves with only a few colors
          * as input.  So we will just find the closest two colors,
          * and throw out one of them (chosen somewhat randomly).
          * [We don't understand this at all, so if someone wants to
          *  work on improving it, be our guest - AED, GRP]
          */
         int i;
         int max_d;
         int num_new_palette;
         png_dsortp t;
         png_dsortpp hash;

         t = NULL;

         /* Initialize palette index arrays */
         png_ptr->index_to_palette = (png_bytep)png_malloc(png_ptr,
             (png_uint_32)(num_palette * png_sizeof(png_byte)));
         png_ptr->palette_to_index = (png_bytep)png_malloc(png_ptr,
             (png_uint_32)(num_palette * png_sizeof(png_byte)));

         /* Initialize the sort array */
         for (i = 0; i < num_palette; i++)
         {
            png_ptr->index_to_palette[i] = (png_byte)i;
            png_ptr->palette_to_index[i] = (png_byte)i;
         }

         hash = (png_dsortpp)png_calloc(png_ptr, (png_uint_32)(769 *
             png_sizeof(png_dsortp)));

         num_new_palette = num_palette;

         /* Initial wild guess at how far apart the farthest pixel
          * pair we will be eliminating will be.  Larger
          * numbers mean more areas will be allocated, Smaller
          * numbers run the risk of not saving enough data, and
          * having to do this all over again.
          *
          * I have not done extensive checking on this number.
          */
         max_d = 96;

         while (num_new_palette > maximum_colors)
         {
            for (i = 0; i < num_new_palette - 1; i++)
            {
               int j;

               for (j = i + 1; j < num_new_palette; j++)
               {
                  int d;

                  d = PNG_COLOR_DIST(palette[i], palette[j]);

                  if (d <= max_d)
                  {

                     t = (png_dsortp)png_malloc_warn(png_ptr,
                         (png_uint_32)(png_sizeof(png_dsort)));

                     if (t == NULL)
                         break;

                     t->next = hash[d];
                     t->left = (png_byte)i;
                     t->right = (png_byte)j;
                     hash[d] = t;
                  }
               }
               if (t == NULL)
                  break;
            }

            if (t != NULL)
            for (i = 0; i <= max_d; i++)
            {
               if (hash[i] != NULL)
               {
                  png_dsortp p;

                  for (p = hash[i]; p; p = p->next)
                  {
                     if ((int)png_ptr->index_to_palette[p->left]
                         < num_new_palette &&
                         (int)png_ptr->index_to_palette[p->right]
                         < num_new_palette)
                     {
                        int j, next_j;

                        if (num_new_palette & 0x01)
                        {
                           j = p->left;
                           next_j = p->right;
                        }
                        else
                        {
                           j = p->right;
                           next_j = p->left;
                        }

                        num_new_palette--;
                        palette[png_ptr->index_to_palette[j]]
                            = palette[num_new_palette];
                        if (!full_quantize)
                        {
                           int k;

                           for (k = 0; k < num_palette; k++)
                           {
                              if (png_ptr->quantize_index[k] ==
                                  png_ptr->index_to_palette[j])
                                 png_ptr->quantize_index[k] =
                                     png_ptr->index_to_palette[next_j];

                              if ((int)png_ptr->quantize_index[k] ==
                                  num_new_palette)
                                 png_ptr->quantize_index[k] =
                                     png_ptr->index_to_palette[j];
                           }
                        }

                        png_ptr->index_to_palette[png_ptr->palette_to_index
                            [num_new_palette]] = png_ptr->index_to_palette[j];

                        png_ptr->palette_to_index[png_ptr->index_to_palette[j]]
                            = png_ptr->palette_to_index[num_new_palette];

                        png_ptr->index_to_palette[j] =
                            (png_byte)num_new_palette;

                        png_ptr->palette_to_index[num_new_palette] =
                            (png_byte)j;
                     }
                     if (num_new_palette <= maximum_colors)
                        break;
                  }
                  if (num_new_palette <= maximum_colors)
                     break;
               }
            }

            for (i = 0; i < 769; i++)
            {
               if (hash[i] != NULL)
               {
                  png_dsortp p = hash[i];
                  while (p)
                  {
                     t = p->next;
                     png_free(png_ptr, p);
                     p = t;
                  }
               }
               hash[i] = 0;
            }
            max_d += 96;
         }
         png_free(png_ptr, hash);
         png_free(png_ptr, png_ptr->palette_to_index);
         png_free(png_ptr, png_ptr->index_to_palette);
         png_ptr->palette_to_index = NULL;
         png_ptr->index_to_palette = NULL;
      }
      num_palette = maximum_colors;
   }
   if (png_ptr->palette == NULL)
   {
      png_ptr->palette = palette;
   }
   png_ptr->num_palette = (png_uint_16)num_palette;

   if (full_quantize)
   {
      int i;
      png_bytep distance;
      int total_bits = PNG_QUANTIZE_RED_BITS + PNG_QUANTIZE_GREEN_BITS +
          PNG_QUANTIZE_BLUE_BITS;
      int num_red = (1 << PNG_QUANTIZE_RED_BITS);
      int num_green = (1 << PNG_QUANTIZE_GREEN_BITS);
      int num_blue = (1 << PNG_QUANTIZE_BLUE_BITS);
      png_size_t num_entries = ((png_size_t)1 << total_bits);

      png_ptr->palette_lookup = (png_bytep)png_calloc(png_ptr,
          (png_uint_32)(num_entries * png_sizeof(png_byte)));

      distance = (png_bytep)png_malloc(png_ptr, (png_uint_32)(num_entries *
          png_sizeof(png_byte)));

      png_memset(distance, 0xff, num_entries * png_sizeof(png_byte));

      for (i = 0; i < num_palette; i++)
      {
         int ir, ig, ib;
         int r = (palette[i].red >> (8 - PNG_QUANTIZE_RED_BITS));
         int g = (palette[i].green >> (8 - PNG_QUANTIZE_GREEN_BITS));
         int b = (palette[i].blue >> (8 - PNG_QUANTIZE_BLUE_BITS));

         for (ir = 0; ir < num_red; ir++)
         {
            /* int dr = abs(ir - r); */
            int dr = ((ir > r) ? ir - r : r - ir);
            int index_r = (ir << (PNG_QUANTIZE_BLUE_BITS +
                PNG_QUANTIZE_GREEN_BITS));

            for (ig = 0; ig < num_green; ig++)
            {
               /* int dg = abs(ig - g); */
               int dg = ((ig > g) ? ig - g : g - ig);
               int dt = dr + dg;
               int dm = ((dr > dg) ? dr : dg);
               int index_g = index_r | (ig << PNG_QUANTIZE_BLUE_BITS);

               for (ib = 0; ib < num_blue; ib++)
               {
                  int d_index = index_g | ib;
                  /* int db = abs(ib - b); */
                  int db = ((ib > b) ? ib - b : b - ib);
                  int dmax = ((dm > db) ? dm : db);
                  int d = dmax + dt + db;

                  if (d < (int)distance[d_index])
                  {
                     distance[d_index] = (png_byte)d;
                     png_ptr->palette_lookup[d_index] = (png_byte)i;
                  }
               }
            }
         }
      }

      png_free(png_ptr, distance);
   }
}
#endif /* PNG_READ_QUANTIZE_SUPPORTED */

#ifdef PNG_READ_GAMMA_SUPPORTED
void PNGFAPI
png_set_gamma_fixed(png_structp png_ptr, png_fixed_point scrn_gamma,
   png_fixed_point file_gamma)
{
   png_debug(1, "in png_set_gamma_fixed");

   if (png_ptr == NULL)
      return;

   /* New in libpng-1.5.4 - reserve particular negative values as flags. */
   scrn_gamma = translate_gamma_flags(png_ptr, scrn_gamma, 1/*screen*/);
   file_gamma = translate_gamma_flags(png_ptr, file_gamma, 0/*file*/);

#if PNG_LIBPNG_VER >= 10600
   /* Checking the gamma values for being >0 was added in 1.5.4 along with the
    * premultiplied alpha support; this actually hides an undocumented feature
    * of the previous implementation which allowed gamma processing to be
    * disabled in background handling.  There is no evidence (so far) that this
    * was being used; however, png_set_background itself accepted and must still
    * accept '0' for the gamma value it takes, because it isn't always used.
    *
    * Since this is an API change (albeit a very minor one that removes an
    * undocumented API feature) it will not be made until libpng-1.6.0.
    */
   if (file_gamma <= 0)
      png_error(png_ptr, "invalid file gamma in png_set_gamma");

   if (scrn_gamma <= 0)
      png_error(png_ptr, "invalid screen gamma in png_set_gamma");
#endif

   /* Set the gamma values unconditionally - this overrides the value in the PNG
    * file if a gAMA chunk was present.  png_set_alpha_mode provides a
    * different, easier, way to default the file gamma.
    */
   png_ptr->gamma = file_gamma;
   png_ptr->screen_gamma = scrn_gamma;
}

#  ifdef PNG_FLOATING_POINT_SUPPORTED
void PNGAPI
png_set_gamma(png_structp png_ptr, double scrn_gamma, double file_gamma)
{
   png_set_gamma_fixed(png_ptr, convert_gamma_value(png_ptr, scrn_gamma),
      convert_gamma_value(png_ptr, file_gamma));
}
#  endif /* FLOATING_POINT_SUPPORTED */
#endif /* READ_GAMMA */

#ifdef PNG_READ_EXPAND_SUPPORTED
/* Expand paletted images to RGB, expand grayscale images of
 * less than 8-bit depth to 8-bit depth, and expand tRNS chunks
 * to alpha channels.
 */
void PNGAPI
png_set_expand(png_structp png_ptr)
{
   png_debug(1, "in png_set_expand");

   if (png_ptr == NULL)
      return;

   png_ptr->transformations |= (PNG_EXPAND | PNG_EXPAND_tRNS);
   png_ptr->flags &= ~PNG_FLAG_ROW_INIT;
}

/* GRR 19990627:  the following three functions currently are identical
 *  to png_set_expand().  However, it is entirely reasonable that someone
 *  might wish to expand an indexed image to RGB but *not* expand a single,
 *  fully transparent palette entry to a full alpha channel--perhaps instead
 *  convert tRNS to the grayscale/RGB format (16-bit RGB value), or replace
 *  the transparent color with a particular RGB value, or drop tRNS entirely.
 *  IOW, a future version of the library may make the transformations flag
 *  a bit more fine-grained, with separate bits for each of these three
 *  functions.
 *
 *  More to the point, these functions make it obvious what libpng will be
 *  doing, whereas "expand" can (and does) mean any number of things.
 *
 *  GRP 20060307: In libpng-1.2.9, png_set_gray_1_2_4_to_8() was modified
 *  to expand only the sample depth but not to expand the tRNS to alpha
 *  and its name was changed to png_set_expand_gray_1_2_4_to_8().
 */

/* Expand paletted images to RGB. */
void PNGAPI
png_set_palette_to_rgb(png_structp png_ptr)
{
   png_debug(1, "in png_set_palette_to_rgb");

   if (png_ptr == NULL)
      return;

   png_ptr->transformations |= (PNG_EXPAND | PNG_EXPAND_tRNS);
   png_ptr->flags &= ~PNG_FLAG_ROW_INIT;
}

/* Expand grayscale images of less than 8-bit depth to 8 bits. */
void PNGAPI
png_set_expand_gray_1_2_4_to_8(png_structp png_ptr)
{
   png_debug(1, "in png_set_expand_gray_1_2_4_to_8");

   if (png_ptr == NULL)
      return;

   png_ptr->transformations |= PNG_EXPAND;
   png_ptr->flags &= ~PNG_FLAG_ROW_INIT;
}



/* Expand tRNS chunks to alpha channels. */
void PNGAPI
png_set_tRNS_to_alpha(png_structp png_ptr)
{
   png_debug(1, "in png_set_tRNS_to_alpha");

   png_ptr->transformations |= (PNG_EXPAND | PNG_EXPAND_tRNS);
   png_ptr->flags &= ~PNG_FLAG_ROW_INIT;
}
#endif /* defined(PNG_READ_EXPAND_SUPPORTED) */

#ifdef PNG_READ_EXPAND_16_SUPPORTED
/* Expand to 16-bit channels, expand the tRNS chunk too (because otherwise
 * it may not work correctly.)
 */
void PNGAPI
png_set_expand_16(png_structp png_ptr)
{
   png_debug(1, "in png_set_expand_16");

   if (png_ptr == NULL)
      return;

   png_ptr->transformations |= (PNG_EXPAND_16 | PNG_EXPAND | PNG_EXPAND_tRNS);
   png_ptr->flags &= ~PNG_FLAG_ROW_INIT;

   /* New API, make sure apps call the correct initializers: */
   png_ptr->flags |= PNG_FLAG_DETECT_UNINITIALIZED;
}
#endif

#ifdef PNG_READ_GRAY_TO_RGB_SUPPORTED
void PNGAPI
png_set_gray_to_rgb(png_structp png_ptr)
{
   png_debug(1, "in png_set_gray_to_rgb");

   if (png_ptr != NULL)
   {
      /* Because rgb must be 8 bits or more: */
      png_set_expand_gray_1_2_4_to_8(png_ptr);
      png_ptr->transformations |= PNG_GRAY_TO_RGB;
      png_ptr->flags &= ~PNG_FLAG_ROW_INIT;
   }
}
#endif

#ifdef PNG_READ_RGB_TO_GRAY_SUPPORTED
void PNGFAPI
png_set_rgb_to_gray_fixed(png_structp png_ptr, int error_action,
    png_fixed_point red, png_fixed_point green)
{
   png_debug(1, "in png_set_rgb_to_gray");

   if (png_ptr == NULL)
      return;

   switch(error_action)
   {
      case PNG_ERROR_ACTION_NONE:
         png_ptr->transformations |= PNG_RGB_TO_GRAY;
         break;

      case PNG_ERROR_ACTION_WARN:
         png_ptr->transformations |= PNG_RGB_TO_GRAY_WARN;
         break;

      case PNG_ERROR_ACTION_ERROR:
         png_ptr->transformations |= PNG_RGB_TO_GRAY_ERR;
         break;

      default:
         png_error(png_ptr, "invalid error action to rgb_to_gray");
         break;
   }
   if (png_ptr->color_type == PNG_COLOR_TYPE_PALETTE)
#ifdef PNG_READ_EXPAND_SUPPORTED
      png_ptr->transformations |= PNG_EXPAND;
#else
   {
      png_warning(png_ptr,
        "Cannot do RGB_TO_GRAY without EXPAND_SUPPORTED");

      png_ptr->transformations &= ~PNG_RGB_TO_GRAY;
   }
#endif
   {
      if (red >= 0 && green >= 0 && red + green <= PNG_FP_1)
      {
         png_uint_16 red_int, green_int;

         /* NOTE: this calculation does not round, but this behavior is retained
          * for consistency, the inaccuracy is very small.  The code here always
          * overwrites the coefficients, regardless of whether they have been
          * defaulted or set already.
          */
         red_int = (png_uint_16)(((png_uint_32)red*32768)/100000);
         green_int = (png_uint_16)(((png_uint_32)green*32768)/100000);

         png_ptr->rgb_to_gray_red_coeff   = red_int;
         png_ptr->rgb_to_gray_green_coeff = green_int;
         png_ptr->rgb_to_gray_coefficients_set = 1;
      }

      else
      {
         if (red >= 0 && green >= 0)
            png_warning(png_ptr,
               "ignoring out of range rgb_to_gray coefficients");

         /* Use the defaults, from the cHRM chunk if set, else the historical
          * values which are close to the sRGB/HDTV/ITU-Rec 709 values.  See
          * png_do_rgb_to_gray for more discussion of the values.  In this case
          * the coefficients are not marked as 'set' and are not overwritten if
          * something has already provided a default.
          */
         if (png_ptr->rgb_to_gray_red_coeff == 0 &&
            png_ptr->rgb_to_gray_green_coeff == 0)
         {
            png_ptr->rgb_to_gray_red_coeff   = 6968;
            png_ptr->rgb_to_gray_green_coeff = 23434;
            /* png_ptr->rgb_to_gray_blue_coeff  = 2366; */
         }
      }
   }
}

#ifdef PNG_FLOATING_POINT_SUPPORTED
/* Convert a RGB image to a grayscale of the same width.  This allows us,
 * for example, to convert a 24 bpp RGB image into an 8 bpp grayscale image.
 */

void PNGAPI
png_set_rgb_to_gray(png_structp png_ptr, int error_action, double red,
   double green)
{
   if (png_ptr == NULL)
      return;

   png_set_rgb_to_gray_fixed(png_ptr, error_action,
      png_fixed(png_ptr, red, "rgb to gray red coefficient"),
      png_fixed(png_ptr, green, "rgb to gray green coefficient"));
}
#endif /* FLOATING POINT */

#endif

#if defined(PNG_READ_USER_TRANSFORM_SUPPORTED) || \
    defined(PNG_WRITE_USER_TRANSFORM_SUPPORTED)
void PNGAPI
png_set_read_user_transform_fn(png_structp png_ptr, png_user_transform_ptr
    read_user_transform_fn)
{
   png_debug(1, "in png_set_read_user_transform_fn");

   if (png_ptr == NULL)
      return;

#ifdef PNG_READ_USER_TRANSFORM_SUPPORTED
   png_ptr->transformations |= PNG_USER_TRANSFORM;
   png_ptr->read_user_transform_fn = read_user_transform_fn;
#endif
}
#endif

#ifdef PNG_READ_TRANSFORMS_SUPPORTED
#ifdef PNG_READ_GAMMA_SUPPORTED
/* In the case of gamma transformations only do transformations on images where
 * the [file] gamma and screen_gamma are not close reciprocals, otherwise it
 * slows things down slightly, and also needlessly introduces small errors.
 */
static int /* PRIVATE */
png_gamma_threshold(png_fixed_point screen_gamma, png_fixed_point file_gamma)
{
   /* PNG_GAMMA_THRESHOLD is the threshold for performing gamma
    * correction as a difference of the overall transform from 1.0
    *
    * We want to compare the threshold with s*f - 1, if we get
    * overflow here it is because of wacky gamma values so we
    * turn on processing anyway.
    */
   png_fixed_point gtest;
   return !png_muldiv(&gtest, screen_gamma, file_gamma, PNG_FP_1) ||
       png_gamma_significant(gtest);
}
#endif

/* Initialize everything needed for the read.  This includes modifying
 * the palette.
 */

/*For the moment 'png_init_palette_transformations' and
 * 'png_init_rgb_transformations' only do some flag canceling optimizations.
 * The intent is that these two routines should have palette or rgb operations
 * extracted from 'png_init_read_transformations'.
 */
static void /* PRIVATE */
png_init_palette_transformations(png_structp png_ptr)
{
   /* Called to handle the (input) palette case.  In png_do_read_transformations
    * the first step is to expand the palette if requested, so this code must
    * take care to only make changes that are invariant with respect to the
    * palette expansion, or only do them if there is no expansion.
    *
    * STRIP_ALPHA has already been handled in the caller (by setting num_trans
    * to 0.)
    */
   int input_has_alpha = 0;
   int input_has_transparency = 0;

   if (png_ptr->num_trans > 0)
   {
      int i;

      /* Ignore if all the entries are opaque (unlikely!) */
      for (i=0; i<png_ptr->num_trans; ++i)
         if (png_ptr->trans_alpha[i] == 255)
            continue;
         else if (png_ptr->trans_alpha[i] == 0)
            input_has_transparency = 1;
         else
            input_has_alpha = 1;
   }

   /* If no alpha we can optimize. */
   if (!input_has_alpha)
   {
      /* Any alpha means background and associative alpha processing is
       * required, however if the alpha is 0 or 1 throughout OPTIIMIZE_ALPHA
       * and ENCODE_ALPHA are irrelevant.
       */
      png_ptr->transformations &= ~PNG_ENCODE_ALPHA;
      png_ptr->flags &= ~PNG_FLAG_OPTIMIZE_ALPHA;

      if (!input_has_transparency)
         png_ptr->transformations &= ~(PNG_COMPOSE | PNG_BACKGROUND_EXPAND);
   }

#if defined(PNG_READ_EXPAND_SUPPORTED) && defined(PNG_READ_BACKGROUND_SUPPORTED)
   /* png_set_background handling - deals with the complexity of whether the
    * background color is in the file format or the screen format in the case
    * where an 'expand' will happen.
    */

   /* The following code cannot be entered in the alpha pre-multiplication case
    * because PNG_BACKGROUND_EXPAND is cancelled below.
    */
   if ((png_ptr->transformations & PNG_BACKGROUND_EXPAND) &&
       (png_ptr->transformations & PNG_EXPAND))
   {
      {
         png_ptr->background.red   =
             png_ptr->palette[png_ptr->background.index].red;
         png_ptr->background.green =
             png_ptr->palette[png_ptr->background.index].green;
         png_ptr->background.blue  =
             png_ptr->palette[png_ptr->background.index].blue;

#ifdef PNG_READ_INVERT_ALPHA_SUPPORTED
        if (png_ptr->transformations & PNG_INVERT_ALPHA)
        {
           if (!(png_ptr->transformations & PNG_EXPAND_tRNS))
           {
              /* Invert the alpha channel (in tRNS) unless the pixels are
               * going to be expanded, in which case leave it for later
               */
              int i, istop = png_ptr->num_trans;

              for (i=0; i<istop; i++)
                 png_ptr->trans_alpha[i] = (png_byte)(255 -
                    png_ptr->trans_alpha[i]);
           }
        }
#endif /* PNG_READ_INVERT_ALPHA_SUPPORTED */
      }
   } /* background expand and (therefore) no alpha association. */
#endif /* PNG_READ_EXPAND_SUPPORTED && PNG_READ_BACKGROUND_SUPPORTED */
}

static void /* PRIVATE */
png_init_rgb_transformations(png_structp png_ptr)
{
   /* Added to libpng-1.5.4: check the color type to determine whether there
    * is any alpha or transparency in the image and simply cancel the
    * background and alpha mode stuff if there isn't.
    */
   int input_has_alpha = (png_ptr->color_type & PNG_COLOR_MASK_ALPHA) != 0;
   int input_has_transparency = png_ptr->num_trans > 0;

   /* If no alpha we can optimize. */
   if (!input_has_alpha)
   {
      /* Any alpha means background and associative alpha processing is
       * required, however if the alpha is 0 or 1 throughout OPTIIMIZE_ALPHA
       * and ENCODE_ALPHA are irrelevant.
       */
#     ifdef PNG_READ_ALPHA_MODE_SUPPORTED
         png_ptr->transformations &= ~PNG_ENCODE_ALPHA;
         png_ptr->flags &= ~PNG_FLAG_OPTIMIZE_ALPHA;
#     endif

      if (!input_has_transparency)
         png_ptr->transformations &= ~(PNG_COMPOSE | PNG_BACKGROUND_EXPAND);
   }

#if defined(PNG_READ_EXPAND_SUPPORTED) && defined(PNG_READ_BACKGROUND_SUPPORTED)
   /* png_set_background handling - deals with the complexity of whether the
    * background color is in the file format or the screen format in the case
    * where an 'expand' will happen.
    */

   /* The following code cannot be entered in the alpha pre-multiplication case
    * because PNG_BACKGROUND_EXPAND is cancelled below.
    */
   if ((png_ptr->transformations & PNG_BACKGROUND_EXPAND) &&
       (png_ptr->transformations & PNG_EXPAND) &&
       !(png_ptr->color_type & PNG_COLOR_MASK_COLOR))
       /* i.e., GRAY or GRAY_ALPHA */
   {
      {
         /* Expand background and tRNS chunks */
         int gray = png_ptr->background.gray;
         int trans_gray = png_ptr->trans_color.gray;

         switch (png_ptr->bit_depth)
         {
            case 1:
               gray *= 0xff;
               trans_gray *= 0xff;
               break;

            case 2:
               gray *= 0x55;
               trans_gray *= 0x55;
               break;

            case 4:
               gray *= 0x11;
               trans_gray *= 0x11;
               break;

            default:

            case 8:
               /* Already 8 bits, fall through */

            case 16:
               /* Already a full 16 bits */
               break;
         }

         png_ptr->background.red = png_ptr->background.green =
            png_ptr->background.blue = (png_uint_16)gray;

         if (!(png_ptr->transformations & PNG_EXPAND_tRNS))
         {
            png_ptr->trans_color.red = png_ptr->trans_color.green =
               png_ptr->trans_color.blue = (png_uint_16)trans_gray;
         }
      }
   } /* background expand and (therefore) no alpha association. */
#endif /* PNG_READ_EXPAND_SUPPORTED && PNG_READ_BACKGROUND_SUPPORTED */
}

void /* PRIVATE */
png_init_read_transformations(png_structp png_ptr)
{
   png_debug(1, "in png_init_read_transformations");

   /* This internal function is called from png_read_start_row in pngrutil.c
    * and it is called before the 'rowbytes' calculation is done, so the code
    * in here can change or update the transformations flags.
    *
    * First do updates that do not depend on the details of the PNG image data
    * being processed.
    */

#ifdef PNG_READ_GAMMA_SUPPORTED
   /* Prior to 1.5.4 these tests were performed from png_set_gamma, 1.5.4 adds
    * png_set_alpha_mode and this is another source for a default file gamma so
    * the test needs to be performed later - here.  In addition prior to 1.5.4
    * the tests were repeated for the PALETTE color type here - this is no
    * longer necessary (and doesn't seem to have been necessary before.)
    */
   {
      /* The following temporary indicates if overall gamma correction is
       * required.
       */
      int gamma_correction = 0;

      if (png_ptr->gamma != 0) /* has been set */
      {
         if (png_ptr->screen_gamma != 0) /* screen set too */
            gamma_correction = png_gamma_threshold(png_ptr->gamma,
               png_ptr->screen_gamma);

         else
            /* Assume the output matches the input; a long time default behavior
             * of libpng, although the standard has nothing to say about this.
             */
            png_ptr->screen_gamma = png_reciprocal(png_ptr->gamma);
      }

      else if (png_ptr->screen_gamma != 0)
         /* The converse - assume the file matches the screen, note that this
          * perhaps undesireable default can (from 1.5.4) be changed by calling
          * png_set_alpha_mode (even if the alpha handling mode isn't required
          * or isn't changed from the default.)
          */
         png_ptr->gamma = png_reciprocal(png_ptr->screen_gamma);

      else /* neither are set */
         /* Just in case the following prevents any processing - file and screen
          * are both assumed to be linear and there is no way to introduce a
          * third gamma value other than png_set_background with 'UNIQUE', and,
          * prior to 1.5.4
          */
         png_ptr->screen_gamma = png_ptr->gamma = PNG_FP_1;

      /* Now turn the gamma transformation on or off as appropriate.  Notice
       * that PNG_GAMMA just refers to the file->screen correction.  Alpha
       * composition may independently cause gamma correction because it needs
       * linear data (e.g. if the file has a gAMA chunk but the screen gamma
       * hasn't been specified.)  In any case this flag may get turned off in
       * the code immediately below if the transform can be handled outside the
       * row loop.
       */
      if (gamma_correction)
         png_ptr->transformations |= PNG_GAMMA;

      else
         png_ptr->transformations &= ~PNG_GAMMA;
   }
#endif

   /* Certain transformations have the effect of preventing other
    * transformations that happen afterward in png_do_read_transformations,
    * resolve the interdependencies here.  From the code of
    * png_do_read_transformations the order is:
    *
    *  1) PNG_EXPAND (including PNG_EXPAND_tRNS)
    *  2) PNG_STRIP_ALPHA (if no compose)
    *  3) PNG_RGB_TO_GRAY
    *  4) PNG_GRAY_TO_RGB iff !PNG_BACKGROUND_IS_GRAY
    *  5) PNG_COMPOSE
    *  6) PNG_GAMMA
    *  7) PNG_STRIP_ALPHA (if compose)
    *  8) PNG_ENCODE_ALPHA
    *  9) PNG_SCALE_16_TO_8
    * 10) PNG_16_TO_8
    * 11) PNG_QUANTIZE (converts to palette)
    * 12) PNG_EXPAND_16
    * 13) PNG_GRAY_TO_RGB iff PNG_BACKGROUND_IS_GRAY
    * 14) PNG_INVERT_MONO
    * 15) PNG_SHIFT
    * 16) PNG_PACK
    * 17) PNG_BGR
    * 18) PNG_PACKSWAP
    * 19) PNG_FILLER (includes PNG_ADD_ALPHA)
    * 20) PNG_INVERT_ALPHA
    * 21) PNG_SWAP_ALPHA
    * 22) PNG_SWAP_BYTES
    * 23) PNG_USER_TRANSFORM [must be last]
    */
#ifdef PNG_READ_STRIP_ALPHA_SUPPORTED
   if ((png_ptr->transformations & PNG_STRIP_ALPHA) &&
      !(png_ptr->transformations & PNG_COMPOSE))
   {
      /* Stripping the alpha channel happens immediately after the 'expand'
       * transformations, before all other transformation, so it cancels out
       * the alpha handling.  It has the side effect negating the effect of
       * PNG_EXPAND_tRNS too:
       */
      png_ptr->transformations &= ~(PNG_BACKGROUND_EXPAND | PNG_ENCODE_ALPHA |
         PNG_EXPAND_tRNS);
      png_ptr->flags &= ~PNG_FLAG_OPTIMIZE_ALPHA;

      /* Kill the tRNS chunk itself too.  Prior to 1.5.4 this did not happen
       * so transparency information would remain just so long as it wasn't
       * expanded.  This produces unexpected API changes if the set of things
       * that do PNG_EXPAND_tRNS changes (perfectly possible given the
       * documentation - which says ask for what you want, accept what you
       * get.)  This makes the behavior consistent from 1.5.4:
       */
      png_ptr->num_trans = 0;
   }
#endif /* STRIP_ALPHA supported, no COMPOSE */

#ifdef PNG_READ_ALPHA_MODE_SUPPORTED
   /* If the screen gamma is about 1.0 then the OPTIMIZE_ALPHA and ENCODE_ALPHA
    * settings will have no effect.
    */
   if (!png_gamma_significant(png_ptr->screen_gamma))
   {
      png_ptr->transformations &= ~PNG_ENCODE_ALPHA;
      png_ptr->flags &= ~PNG_FLAG_OPTIMIZE_ALPHA;
   }
#endif

#if defined(PNG_READ_EXPAND_SUPPORTED) && \
   defined(PNG_READ_BACKGROUND_SUPPORTED) && \
   defined(PNG_READ_GRAY_TO_RGB_SUPPORTED)
   /* Detect gray background and attempt to enable optimization for
    * gray --> RGB case.
    *
    * Note:  if PNG_BACKGROUND_EXPAND is set and color_type is either RGB or
    * RGB_ALPHA (in which case need_expand is superfluous anyway), the
    * background color might actually be gray yet not be flagged as such.
    * This is not a problem for the current code, which uses
    * PNG_BACKGROUND_IS_GRAY only to decide when to do the
    * png_do_gray_to_rgb() transformation.
    *
    * TODO: this code needs to be revised to avoid the complexity and
    * interdependencies.  The color type of the background should be recorded in
    * png_set_background, along with the bit depth, then the code has a record
    * of exactly what color space the background is currently in.
    */
   if (png_ptr->transformations & PNG_BACKGROUND_EXPAND)
   {
      /* PNG_BACKGROUND_EXPAND: the background is in the file color space, so if
       * the file was grayscale the background value is gray.
       */
      if (!(png_ptr->color_type & PNG_COLOR_MASK_COLOR))
         png_ptr->mode |= PNG_BACKGROUND_IS_GRAY;
   }

   else if (png_ptr->transformations & PNG_COMPOSE)
   {
      /* PNG_COMPOSE: png_set_background was called with need_expand false,
       * so the color is in the color space of the output or png_set_alpha_mode
       * was called and the color is black.  Ignore RGB_TO_GRAY because that
       * happens before GRAY_TO_RGB.
       */
      if (png_ptr->transformations & PNG_GRAY_TO_RGB)
      {
         if (png_ptr->background.red == png_ptr->background.green &&
             png_ptr->background.red == png_ptr->background.blue)
         {
            png_ptr->mode |= PNG_BACKGROUND_IS_GRAY;
            png_ptr->background.gray = png_ptr->background.red;
         }
      }
   }
#endif /* PNG_READ_GRAY_TO_RGB_SUPPORTED (etc) */

   /* For indexed PNG data (PNG_COLOR_TYPE_PALETTE) many of the transformations
    * can be performed directly on the palette, and some (such as rgb to gray)
    * can be optimized inside the palette.  This is particularly true of the
    * composite (background and alpha) stuff, which can be pretty much all done
    * in the palette even if the result is expanded to RGB or gray afterward.
    *
    * NOTE: this is Not Yet Implemented, the code behaves as in 1.5.1 and
    * earlier and the palette stuff is actually handled on the first row.  This
    * leads to the reported bug that the palette returned by png_get_PLTE is not
    * updated.
    */
   if (png_ptr->color_type == PNG_COLOR_TYPE_PALETTE)
      png_init_palette_transformations(png_ptr);

   else
      png_init_rgb_transformations(png_ptr);

#if defined(PNG_READ_BACKGROUND_SUPPORTED) && \
   defined(PNG_READ_EXPAND_16_SUPPORTED)
   if ((png_ptr->transformations & PNG_EXPAND_16) &&
      (png_ptr->transformations & PNG_COMPOSE) &&
      !(png_ptr->transformations & PNG_BACKGROUND_EXPAND) &&
      png_ptr->bit_depth != 16)
   {
      /* TODO: fix this.  Because the expand_16 operation is after the compose
       * handling the background color must be 8, not 16, bits deep, but the
       * application will supply a 16-bit value so reduce it here.
       *
       * The PNG_BACKGROUND_EXPAND code above does not expand to 16 bits at
       *…