text: /*
   * PNG image format
   * Copyright (c) 2003 Fabrice Bellard
   *
   * This file is part of Libav.
   *
   * Libav is free software; you can redistribute it and/or
   * modify it under the terms of the GNU Lesser General Public
   * License as published by the Free Software Foundation; either
   * version 2.1 of the License, or (at your option) any later version.
   *
   * Libav is distributed in the hope that it will be useful,
   * but WITHOUT ANY WARRANTY; without even the implied warranty of
   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   * Lesser General Public License for more details.
   *
   * You should have received a copy of the GNU Lesser General Public
   * License along with Libav; if not, write to the Free Software
   * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
   */
  #include "avcodec.h"
  #include "bytestream.h"
  #include "png.h"
  
  const uint8_t ff_pngsig[8] = { 137, 80, 78, 71, 13, 10, 26, 10 };
  const uint8_t ff_mngsig[8] = { 138, 77, 78, 71, 13, 10, 26, 10 };
  
  /* Mask to determine which y pixels are valid in a pass */
  const uint8_t ff_png_pass_ymask[NB_PASSES] = {
      0x80, 0x80, 0x08, 0x88, 0x22, 0xaa, 0x55,
  };
  
  /* minimum x value */
  static const uint8_t ff_png_pass_xmin[NB_PASSES] = {
      0, 4, 0, 2, 0, 1, 0
  };
  
  /* x shift to get row width */
  static const uint8_t ff_png_pass_xshift[NB_PASSES] = {
      3, 3, 2, 2, 1, 1, 0
  };
  
  /* Mask to determine which pixels are valid in a pass */
  const uint8_t ff_png_pass_mask[NB_PASSES] = {
      0x80, 0x08, 0x88, 0x22, 0xaa, 0x55, 0xff
  };
  
  void *ff_png_zalloc(void *opaque, unsigned int items, unsigned int size)
  {
      return av_mallocz_array(items, size);
  }
  
  void ff_png_zfree(void *opaque, void *ptr)
  {
      av_free(ptr);
  }
  
  int ff_png_get_nb_channels(int color_type)
  {
      int channels;
      channels = 1;
      if ((color_type & (PNG_COLOR_MASK_COLOR | PNG_COLOR_MASK_PALETTE)) ==
          PNG_COLOR_MASK_COLOR)
          channels = 3;
      if (color_type & PNG_COLOR_MASK_ALPHA)
          channels++;
      return channels;
  }
  
  /* compute the row size of an interleaved pass */
  int ff_png_pass_row_size(int pass, int bits_per_pixel, int width)
  {
      int shift, xmin, pass_width;
  
      xmin = ff_png_pass_xmin[pass];
      if (width <= xmin)
          return 0;
      shift      = ff_png_pass_xshift[pass];
      pass_width = (width - xmin + (1 << shift) - 1) >> shift;
      return (pass_width * bits_per_pixel + 7) >> 3;
  }
  